FTIR spectroscopic studies with thermo acoustical parameters in binary and ternary liquid mixtures of amino acid and saccharide in aqueous medium

Abstract

The interactions between amino acids and saccharides in aqueous environments are fascinating and have significant implications for various fields. These interactions can provide valuable insights into physiological processes, drug targeting, and delivery systems. To comprehend the synergy between saccharide (l-arabinose$/$d-xylose) and non-essential amino acid (l-aspartic acid; Asp) in an aqueous system, ultrasonic velocity ($U$) at 293.15 K–313.15 K (with 5 K interval) and at experimental pressure P = 101 kPa were measured using a digital ultrasonic interferometer. The solution density,$\uprho$ and the propagation of sound waves through the experimental solutions are directly correlated with the weak and strong molecular interactions that take place between the solution’s constituents. $\uprho$ and $U$ data was utilised to compute the following acoustic parameters isentropic compressibility ${K}_{s}$, apparent molar isentropic compressibility ${\text{K}}_{{{\text{s}},\upphi }} ,$ free volume ${V}_{f}$, free length ${L}_{f}$, internal pressure ${\pi }_{i}$, acoustic impedance $Z$, surface tension $\gamma$ and relative association ${R}_{A}$. Positive ${\text{K}}_{\text{s}}^{0}$ values make ion–solvent interactions stronger than ion-ion interactions. Positive values of ${\text{K}}_{{{\text{s}},\upphi ,{\text{ tr}}}}^{0}$ imply greater interactions between the polar segments of l-arabinose/d-xylose and the zwitterionic groups of Asp. The solvation mechanisms of Asp result in the reconstruction of the water structure. The FTIR technique was used to verify the results of the acoustic study. The presence of intermolecular hydrogen bonding and intramolecular hydrogen bonding is shown by the broadening of the absorption band. The system under research exhibits predominant ion-hydrophilic$/$hydrophilic interactions as confirmed by FTIR analysis. Understanding how Asp in aqueous environment interacts with saccharides such as l-arabinose and d-xylose might help one better understand how these molecules behave in biological systems.

Highlights

• Acoustic properties were evaluated for Asp in aqueous saccharide solution

• ${K}_{s}$ decreases as Asp and saccharide (s) concentrations rise

• In the presence of saccharides, Asp serves as a structure maker

• An FTIR analysis shows that saccharide enhances the H-bonded network of water by Asp

Introduction

The interactions between amino acids and saccharides in aqueous environments are of great significance due to their impact on protein stability, biological processes, and fundamental research. These interactions involve non-covalent bonds such as hydrophobic and electrostatic interactions, and hydrogen bonding, which play a crucial role in maintaining protein structure and stabilizing biological macromolecules. Study of amino acid-saccharide interactions enables the development of new materials, drugs, environmental solutions, food products, and much more, ultimately contributing to technological progress and improving quality of life. As an extension of our earlier research [1,2,3,4,5,6], we have attempted to investigate the interactions between Asp and saccharides in an aqueous media using spectroscopic and acoustic measurement. Since amino acids serve as the foundation of all living organisms, their physicochemical and thermodynamic characteristics are of great importance. These properties provide important insights that help us understand protein in aqueous medium better [7, 8]. Though there has always been a lot of interest in studies on protein stability, low molecular weight molecules (amino acids) are often used for these studies due to the challenges that arise from proteins’ huge molecular weights and complicated structures. In order to understand the mechanism of molecular interactions that occur in various biochemical processes in the human body, it is crucial to examine the physicochemical characteristics of amino acids in mixed aqueous solutions [9,10,11,12,13]. Asp and l-arabinose$/$d-xylose are now taken into consideration as part of our ongoing research on the interaction of amino acids and carbohydrates having biological significance. In the central nervous system, Asp functions as an excitatory neurotransmitter. It plays a role in the nerve impulse transmission process, especially in the brain, where it aids in the communication of nerve cells. Asp plays a crucial role in the citric acid cycle, which is the primary metabolic process for producing adenosine triphosphate (ATP) from proteins, lipids, and carbohydrates. This cycle is also referred to as the Krebs cycle or TCA cycle. The presence of different chemicals like sugars, ionic salts, chaotropic agents, detergents, reducing agents, metal ions, in aqueous protein solutions is known to alter the structure of proteins [14,15,16]. Additives such as polyhydroxy alcohols, carbohydrates and alcohols reduce the denaturation of proteins. Carbohydrates, in particular, aid in maintaining the original shape of globular proteins. Hydrophilic and hydrophobic groups coexist in monosaccharides, making them multifunctional solutes. Although, there has already been a lot of research published in the literature [7, 8, 10,11,12,13] on molecular interactions between various biomolecules such as amino acids, carbohydrates, and enzymes, utilizing physicochemical, spectroscopic, and computational techniques, the major objectives of the present investigation is to understand and analyse how the molecular interactions are affected due to the conformational modifications of the saccharides in aqueous medium. These molecules exhibit a minimal percentage of linear form in aqueous solutions, but at equilibrium, they exist in pyranose and furanose-like structures with the –OH groups in distinct orientations (axial and equatorial) [10, 12]. These carbohydrates’ conformational structures control the hydration behaviour. Pentoses called l-arabinose and d-xylose are the carbohydrates, widely found in the cell walls of lignocellulosic plants and are the primary building blocks of xylans. In our earlier report we published the acoustic and spectroscopic studies of l-glutamic acid (Glu) in aqueous carbohydrates solutions [6]. In addition, we have published the volumetric and viscometric characteristics of Asp in aqueous solutions of l-arabinose$/$d-xylose analyzing the interactions among ionic, polar and nonpolar sites of amino acid and saccharide molecule [2]. This article represents an extension work for the acoustic properties and FTIR investigation of Asp in water and in aqueous l-arabinose$/$d-xylose solution (2.0 wt %, 2.5 wt %, 3.0 wt %) at T = 293.15 K to 313.15 K. This study focused on the volumetric and viscometric properties to explore the interactions between Asp and saccharides in aqueous medium across the temperature range of T = (293.15–313.15 K). The outcomes of this research provided a strong foundation for understanding such systems, still there remains a need for deeper insight into the molecular and thermodynamic aspects of these interactions using advanced methodologies. The current study shifts focus to Asp, another essential amino acid with distinct structural and functional properties. This study aims to extend the exploration by employing acoustic, FT-IR spectroscopy, and surface tension techniques. Our earlier investigation [6] between Glu and saccharides in aqueous medium using a combination of acoustic, FT-IR, and surface tension techniques provided valuable insights into the physicochemical behavior of Asp in the presence of saccharides, l-arabinose$/$d-xylose (2.0 wt %, 2.5 wt %, 3.0 wt %) in water.

Experimental

Chemicals

All chemicals were used exactly as received, without any further purification. Table 1 gives details about the chemicals that were used, including information on their purity.

Table 1 Specification of chemicals

Solution setup

Solutions of l-arabinose/d-xylose were prepared at three different concentrations: 2.0 wt %, 2.5 wt %, and 3.0 wt %. Deionized water was used (with a specific conductance of $(2.89 - 3.26)\upmu {\text{Scm}}^{ - 1} ).$ Different molal concentrations of Asp (0.01 mol kg⁻¹ to 0.05 mol kg⁻¹) were prepared using the aqueous l-arabinose/d-xylose stock solutions. Weights were measured using a digital balance with an accuracy of ± 0.0001 g (Model: Citizon-Synchronics Electronics Pvt. Ltd.). Solutions were made by mass. All solutions were prepared at room temperature ($T$=298.15 K). The experimental pressure was maintained at ($P$=101 kPa). The molal uncertainty of the solutions was found to be ${\pm 1\times 10}^{-3}$ mol kg^−1.

Methodology

Density and viscosity measurement

The density ($\uprho$) of freshly prepared Asp solutions at various concentrations (0.01 mol kg⁻¹ to 0.05 mol kg⁻¹) was measured using an Anton Paar DMA™ 5000 M density analyzer, which was acquired from Austria. The measurements were conducted in pure water as well as in aqueous solutions containing 2.0 wt %, 2.5 wt %, and 3.0 wt % l-arabinose or d-xylose. We already covered equipment calibration in our earlier article [16]. Prior to measuring $\uprho$ data, the density analyser was calibrated at atmospheric pressure using deionized distilled water and dry air. Distilled water and then anhydrous ${\text{C}}_{2}{\text{H}}_{5}\text{OH}$ were used to rinse the vibrator following each measurement [17]. To make sure the device was working as it should, $\uprho$ of deionized water was first tested. The device (DMA™ 5000 M) has an integrated Peltier thermostat that helps it keep its temperature constant. The sensitivity of the device is 0.000007 g/cm³, which is equivalent to density precision. The measured density values are reported in our earlier article [2] and are also listed in the current article. A 25 mL Ostwald viscometer was used to measure the solutions’ viscosities [18]. The viscometer was calibrated at experimental temperatures with deionized water. Using a thermostatic water bath, the temperature around the viscometer was controlled within ± 0.01 K. Three recordings of the flow timings were made using a digital stopwatch with an accuracy of ± 0.01 s. To make the computation easier, the data were averaged. The viscosity measurements’ standard deviation was found to be within ± 1%.

Assessment of ultrasonic velocity

Using an ultrasonic interferometer, the velocity of sound waves in a liquid medium was determined. The wavelength ($\lambda /$ms)) of sound waves in the experimental liquid medium was measured using a single frequency variable path ultrasonic interferometer (Model: F-05) from Mittal Enterprises, New Delhi, India. The apparatus is made up of ultrasonic measuring cell (10 mL capacity) with a quartz crystal base and a double-walled, premium steel frame with a chromium-coated inner surface. The quartz crystal creates ultrasonic vibrations at f = 2 MHz (a set frequency) when it comes into contact with an anodic current produced by a piezoelectric generator. The value for $\lambda /2$ is obtained from the interferometer's micrometer display. An electrostatically heated water bath was used to maintain the experimental medium's temperature at 298.15 K. The ultrasonic velocity ($U/{\text{ms}}^{-1}$) was calculated using the Eq. 1. The uncertainty of the ultrasonic velocity measurement was determined to be ± 0.2 ${\text{ms}}^{-1}$.

$$U = f \times {\uplambda }$$

(1)

Surface tension calculation

The presence of functional groups –NH₂, –COOH, –OH and >C=O in l-arabinose$/$d-xylose and Asp can lead to a variety of intermolecular interactions in water, which might affect solution behaviour. Temperature and concentration are additional variables that modify these interactions. Surface tension is one such feature of a solution. In this investigation, surface tension ($\gamma$) is computed using the Auerbach relation (Eq. 2) [19] using experimental density ($\rho$) and ultrasonic velocity ($U$) data.

$$\gamma = 6.4 \times 10^{ - 10} \times \rho \times U^{{{\raise0.7ex\hbox{$3$} \!\mathord{\left/ {\vphantom {3 2}}\right.\kern-0pt} \!\lower0.7ex\hbox{$2$}}}}$$

(2)

Fourier Transform Infrared (FTIR) measurement

FTIR is commonly used to identify functional groups and chemical bonds in sample solutions. In this study, an FTIR spectrophotometer (JASCO, FTIR-4600) with computer-based software for device control and data processing was utilized to report the solutions’ infrared spectra. For optimal FTIR spectra and maximum signal strength, the sample was positioned close to the Attenuated Total Reflectance (ATR) evanescent wave. ATR involves a change in a strongly internally reflected infrared beam when it interacts with the sample. The FTIR spectrophotometer operated within the range of 4000–400 cm⁻¹ at standard atmospheric temperature and pressure.

Results and discussion

Ultrasonic properties

Ultrasonic properties are the key indicators of molecular interactions within solutions, revealing insights into hydrogen bonding, van der Waals forces, and ionic interactions. Changes in ultrasonic properties also highlight structural variations in solutes and solvents caused by factors like temperature or concentration. Additionally, Ultrasonic properties help deduce thermodynamic properties such as density, viscosity, and thermal behaviour, making it a valuable tool for applications in fields like material science, medicine, and biochemistry. Understanding the behavior of amino acids in mixed solvent systems by ultrasonic properties analysis, can have applications in biochemistry and food science. Using ultrasonic properties analysis to understand how amino acids behave in mixed solvent systems can be useful in food science and biology.

Sound velocity

The ultrasonic velocity or sound velocity, U of a solution, refers to the speed at which ultrasonic waves propagate through that solution. The behaviour of intra- and intermolecular interactions, complex formation, and related structural changes taking place inside the liquid solutions are all studied with great benefit by using ultrasonic speed measurements. The speed of sound for solutions of Asp + l-arabinose$/$d-xylose + water having varied concentrations (0.01 mol kg⁻¹ to 0.05 mol kg⁻¹) was experimentally determined across different temperatures (293.15 K to 313.15 K) under constant pressure, $P$ =101 kPa. The calculated ultrasonic velocity values are presented in Table 2. Additionally, a literature review on the ultrasonic values of Asp solutions in water was conducted. Comparisons were done between the experimental and comparable values in the literature, and there were very few cases where the concentrations of the solutions matched those of our experimental solutions [20,21,22,23]. As can be seen from the Fig. 1, the experimental data is very close to the literature data (except Ref. [20]) at almost all concentrations. The experimental ultrasonic velocity values obtained in this study, as well as those reported in one of the literature sources, are presented [20], differ from one another. However, the deviations of current results with the literature value may be due to several factors. Experimental conditions like variations in temperature, pressure, or concentration during experiments can significantly affect ultrasonic velocity. Impurities in the solute or solvent can alter molecular interactions, leading to discrepancies. Calibration issues, sensitivity of the ultrasonic equipment or measurement inaccuracies can contribute to deviations. Because the experimental results correspond well with the other papers that were highlighted. For a clear comparison, a graphical depiction is displayed. The variation in $\text{U}$ and the resultant parameters, as influenced by solvent composition and concentration, are qualitatively examined. The observed trends are consistent with values reported in the existing literature. In a similar manner, the solvents’ (aqueous l-arabinose/d-xylose) ultrasonic velocities are compared to the data from the literature and found to be consistent [24]. The ultrasonic velocity increases with the rise in Asp concentration (0.01 mol kg⁻¹ to 0.05 mol kg⁻¹). The increase in the ultrasonic velocity, i.e. speed of sound in an aqueous solution can be attributed to the complex interplay of hydrogen bonding interactions: (i) the three-dimensional hydrogen bonding network in water itself, (ii) intermolecular hydrogen bonds between water and solute molecules and (iii) hydrogen bonding occurring inside the solute molecules. These interactions collectively contribute to increasing the density and elasticity of the mixture, which in turn increases the speed at which sound propagates through it [25]. The ultrasonic velocity for Asp is larger in aqueous saccharide (s) than in pure water, as Table 2 clearly shows. This suggests that when cosolute l-arabinose/d-xylose is present, more molecular interactions take place for aqueous Asp solutions. Due to the addition of cosolute the bonds break between water and Asp, Table 2 also shows that the speed of sound increases when the composition of l-arabinose/d-xylose in an aqueous medium increases. The observed variations in sound speed values can be attributed to this intricate interaction between hydrogen bond formation and rupture. Comparing the $\text{U}$ values of Asp + l-arabinose/d-xylose + water solutions with that of Glu + l-arabinose/d-xylose + water solutions [6] it is observed that $\text{U}$ values are less in the former system than that of the later. The difference in $\text{U}$ values between Asp and Glu in the same solvent can be attributed to their molecular structures and interactions with the solvent. Asp has a shorter side chain compared to Glu, which results in different solvation behaviors. The shorter side chain of Asp leads to a more compact structure, allowing for stronger interactions with the solvent molecules. This increased interaction with the solvent molecules results in higher compressibility and lower ultrasonic velocity. On the other hand, Glu has a longer side chain, which makes its structure more extended. This extended structure leads to weaker interactions with the solvent molecules, resulting in lower compressibility and higher ultrasonic velocity. Correspondingly, the other properties of the Asp solutions can be compared with the properties of Glu solutions, focusing on their molecular structures and solvent interactions.

Table 2 Density ($\uprho$), ultrasonic velocity ($U$), viscosity ($\eta ),$ and compressibility parameters $\left( {K_{s} ,K_{s, \upphi } } \right)$ of Asp in water and aqueous l-arabinose/d-xylose at various temperatures and concentrations, and experimental pressure $P$ = 101 kPa

Fig. 1

A comparison of experimental and literature ultrasonic velocity values ($U$) of aqueous Asp at 298.15 K

Additionally, Table 2 demonstrates that aqueous l-arabinose solutions have greater ultrasonic velocity values of Asp than aqueous d-xylose solutions. A similar observation was also obtained for Glu in aqueous l-arabinose$/$d-xylose [6]. These features result from the stereochemical differences between xylose and arabinose, even though both are pentoses. In water, the main conformation of l-arabinose is 1e2e3e4a, while that of d-xylose is 1e2e3e4e [26]. Due to this, d-xylose better comply with water than l-arabinose. Compared to the latter, the former has stronger hydrogen bonds with water. Hence, greater molecular association on the hydration of Asp as well as the associative connection between ions (${-\text{COO}}^{-}$, ${-\text{NH}}_{3}^{+}$) of Asp and hydrophilic regions of l-arabinose in water are indicated by a rising trend in ultrasonic velocity readings than in D-xylose. The change of U with respect to Asp molality in L-arabinose$/$d-xylose + water at 298.15 K is shown graphically in Fig. 2. A similar trend is followed at all other temperatures.

Fig. 2

Ultrasonic velocity, $U$ of Asp in aqueous l-arabinose and d-xylose at 298.15 K

Compressibility parameters

Isentropic compressibility ${(K}_{s}$ ) and isothermal compressibility ${(K}_{T}$ )

The term compressibility, often known as the coefficient of compressibility, in thermodynamics describes how a system’s volume changes proportionately when pressure changes. Isentropic compressibility is the thermodynamic property that characterizes a substance’s relative change in volume in response to a change in pressure while maintaining a constant entropy. Isothermal compressibility, on the other hand, refers to the relative change in volume of a material in reaction to a change in pressure while keeping the temperature constant. These two parameters have the following mathematical expressions (Eqs. 3 & 4) [27]:

$$K_{S} = - \frac{1}{V}\left( {\frac{\partial V}{{\partial P}}} \right)_{S}$$

(3)

$$K_{T} = - \frac{1}{V}\left( {\frac{\partial V}{{\partial P}}} \right)_{T}$$

(4)

where, T, P, V denotes the temperature, pressure, volume, and $s$ denotes that the process is isentropic (adiabatic and reversible).

${K}_{s}$ can also be calculated using the Newton–Laplace Eq. (5) [28].

$$K_{s} = 1/\left( {\rho U^{2} } \right)$$

(5)

where $U$ and $\rho$ represents the speed of sound and density of the medium. The values for isentropic compressibility ${K}_{s}$ are listed in Table 2. The uncertainty in ${K}_{s}$ was determined to be $\pm 0.004{\times 10}^{-10}$ Pa⁻¹ using Eq. (6).

$$\partial K_{s} = K_{s} \times \left[ {(2\partial u/u)^{2} + (\partial \rho /\rho^{2} } \right]^{1/2}$$

(6)

It is assumed that in electrolyte solutions the isentropic compressibility is equal to the sum of the solute’s and solvent’s intrinsic isentropic compressibilities, i.e. $K_{{s\left( {solute\;intrinsic} \right)}}$ and $K_{{s\left( {solvent\;intrinsic} \right)}} ,$ respectively. The term “intrinsic compressibility” describes a solvent’s innate capacity to compress under pressure without being considerably impacted by outside variables, such as dissolved solutes, whereas the molecular structure, conformational flexibility, and intermolecular interactions of a solute can all have an impact on its intrinsic compressibility. Solvent intrinsic compressibility originates from the compression of water's three-dimensional network structure. In contrast, solute intrinsic compressibility arises from the compression of ions’ hydration shells. Since the hydration shells exhibit minimal compressibility, the solvent intrinsic compressibility is considered the primary contributing factor [29].

More molecules of solvent get involved in the development of solvation shells as the solute concentration rises, where they are subjected to severe electrostriction provided by the trapped ions. The solvation shells are, therefore, compressed and their capacity to compress further is diminished. This effect is demonstrated by the pattern of ${K}_{s}$ falling as Asp and saccharide(s) content rise. A graphical presentation is given (Fig. 3) for the variation of ${K}_{s}$ with concentration of Asp and solvent (aqueous l-arabinose/d-xylose) compositions of 2.0 wt %, 2.5 wt % and 3.0 wt % at 298.15 K. Figure 3 illustrates that as temperatures increase, water molecule’s 3D structure is disrupted, leading to greater hydration of Asp molecules. This observation aligns with the results of volumetric measurements [2]. Same pattern of ${K}_{s}$ for Asp in all the compositions of saccharides is also obtained at other temperatures.

Fig. 3

Isentropic compressibility, ${K}_{s}$ vs square root of molality, $\surd m$ for Asp in aqueous a 2.0, 2.5, 3.0 wt % arabinose, b 2.0, 2.5, 3.0 wt % xylose at 298.15 K

The linear relation between the isentropic compressibility and solute's molality is expressed by Eq. (7).

$$K_{s} = K_{s}^{0} + S_{K} \sqrt m$$

(7)

The ${K}_{s}$ vs. $\surd m$ figure can be extended to the Y-axis to determine the limiting or partial isentropic compressibility, ${K}_{s}^{0}$. The isentropic compressibility at infinite dilution is represented by these values. The slope of the line, which represents ion pair interactions, indicates another crucial parameter, ${S}_{K}$. The interactions between individual solute ions are minimal under infinite dilution circumstances because a sizable volume of solvent effectively separates the solute particles. The solvent molecules and the Asp ions interact most frequently in aqueous Asp solutions.

It is impossible to ignore connections among saccharides’ polar moieties and the zwitterionic ions (${-COO}^{-}$, ${-NH}_{3}^{+}$) and the side chain carboxylate ion (${-COO}^{-}$) of Asp, even at infinite dilution with saccharides in solution. The saccharide(s) concentration causes ion-ion interactions to rise while ion–solvent interactions progressively decrease. Table 3 displays the values for ${K}_{s}^{0}$ and ${S}_{K}$. The stronger ion–solvent interactions than ion–ion interactions are demonstrated by the greater and positive values of ${K}_{s}^{0}$ in comparison to the lower and negative values of ${S}_{K}$ for all of the solutions studied.

Table 3 Values of $\left( {K_{s}^{0} } \right)$, $\left( {S_{K} } \right)$ $\left( {K_{s,\upphi }^{0} } \right)$, $\left( {S_{K,\upphi } } \right)$ and $\left( {K_{s,\upphi , tr}^{0} } \right)$ for Asp in water, aqueous l-arabinose$/$d-xylose at 293.15 K, 298.15 K, 303.15, 308.15 K, 313.15 K and experimental pressure P = 101 kPa

Compressibility under isothermal conditions ${K}_{T}$ is a key parameter that provides insights into the physical, mechanical, and thermodynamic properties of substances across a wide range of disciplines, from material science to geophysics to biology. In addition, studying isothermal compressibility ${K}_{T}$ is valuable for unravelling the intricate interplay between different segments present in solutions, particularly in systems with diverse interactions, such as those involving ionic, hydrophilic, and hydrophobic components. The following Eq. (8) is used to calculate ${K}_{T}$ [30].

$$K_{T} = \left( {171/(T^{4/9} \rho^{4/3} U^{2} } \right)$$

(8)

Table 2 documents isentropic compressibility values, ${K}_{s}$, and isothermal compressibility values, ${K}_{T}$. The table illustrates how ${K}_{T}$ varies with Asp and l-arabinose/d-xylose concentrations, which often follow the similar pattern as ${K}_{s}$. Because of the strong ion–solvent interactions and the electrostriction of the hydration shells, both of the compressibility parameters encourage the loss of compressibility.

Apparent molar isentropic compressibility $\left( {K_{s,\upphi } } \right)$

The apparent molar isentropic compressibility, denoted as $K_{s,\upphi }$, is a thermodynamic property that describes the volume change (compressibility) of a solution with each mole of solute added while entropy is constant. $\upphi$ usually refers to the species or solute component in the solution. It is a measurement of how the addition of solute molecules alters the solution’s compressibility while taking solvent–solvent and solute–solvent interactions into consideration. When solute ions are dissolved in an aqueous solution, water molecules play a part in the process. By forming solvation layers surrounding the ions, water molecules ensnare the ions of solute. The water molecules that are subjected to structural deformation inside the ions'electrostatic field are strongly electrostrictively pulled by these ions. Near the ions, the water molecules are almost noncompressible because they are closely packed. Conversely, bulk water molecules, or those outside the ions'electrostatic field, are not constrained by electrostriction and can compress. Apparent molar isentropic compressibility refers to the relative compressibility of water molecules in these two distinct settings [31]. $K_{s,\upphi }$ values for Asp in aqueous saccharide(s) solutions at T = 293.15 K–313.15 K are calculated using the relation given below (Eq. 9) [32], and the values are listed in Table 2.

$$K_{s,\upphi } = \left( {K_{s} \rho_{0} - K_{s}^{0} \rho } \right)/m\rho \rho_{0} + MK_{s} /\rho$$

(9)

where M and m stand for the solute’s molecular mass and molality, while $\rho$ and ${\rho }_{0}$ for the densities of the solute and solvent. Using the following Eq. (10) $\partial K_{s,\upphi }$ was determined to be 0.082 ${\times 10}^{-14}{\text{m}}^{3}{\text{mol}}^{-1}{\text{Pa}}^{-1}$ at a lower molality range [33].

$$\begin{aligned} \partial K_{s,\upphi } & = \left\{ {\left[ {\left( {MK_{s} /\rho ) - K_{s,\upphi } } \right)\left( {\partial m/m} \right)} \right]^{2} + \left[ {\left( {K_{s,\upphi } + K_{s}^{0} /m\rho_{0} } \right)\left( {\partial \rho /\rho } \right)} \right]^{2} } \right. \\ & \quad \left. { + \left[ {\left. {K_{s,\upphi } + K_{s}^{0} /m\rho_{0} } \right)\left( {\partial K_{s} /K_{s} } \right)} \right]^{2} } \right\}^{1/2} \\ \end{aligned}$$

(10)

For a given concentration of saccharide(s), the negative $K_{s,\upphi }$ value diminishes as the molality of Asp increases. However, this negativity for $K_{s,\upphi }$ value of Asp in pure water increases with temperature, whereas it diminishes when l-arabinose/d-xylose is present. It can be noticed that the values of $K_{s,\upphi }$ for Asp is less than of Glu in aqueous l-arabinose$/$d-xylose [6]. The reason may be, Asp with its shorter side chain, tends to form stronger and more compact hydration shells, leading to lower compressibility. On the other hand, Glu, with its longer side chain, may have a more extended hydration structure, resulting in higher compressibility.

The size and shape of the zwitterion, the kind of solvent, and the temperature and pressure can all affect the intermolecular free space surrounding organic anions. In some cases, the arrangement of solvent molecules around the organic anion may result in relatively large spaces between them, allowing for the accommodation of additional solvent molecules. This phenomenon is particularly relevant in solution chemistry and has implications for various properties and behaviours of solutions containing organic anions [34]. Throughout the whole concentration range of Asp and l-arabinose/d-xylose, negative values of $K_{s,\upphi }$ suggest solvent penetration into the intra-ionic free spaces of organic molecules. Another potential reason for the negative value of $K_{s,\upphi }$ is that water molecules in the hydration shells are less compressible than those in the bulk. If we compare the $K_{s,\upphi }$ values of Asp in aqueous l-arabinose and aqueous d-xylose, the negative values are higher in l-arabinose than d-xylose, i.e. the compressibility is higher in aqueous d-xylose medium. As discussed, d-xylose can better fit into the water molecules than l-arabinose, which leads to greater compressibility of the water molecules in the hydration shells compared to those in the bulk. As the concentration of Asp increases, more hydration shells are formed, leading to the loss of a greater number of water molecules from the bulk. The compressibility of the bulk drops as a result of water molecules being lost, and the gap between the compressibility of water near the solute and in the bulk narrows. Table 2 shows the variation of $K_{s,\upphi }$ with Asp concentration. The properties like homochirality, stereoselectivity, and coordinating abilities, and presence of hydrophilic hydroxyl (–OH) groups in sacharides contribute to their significant role in various biochemical processes and their wide range of applications in different fields. That’s why saccharides are considered a fundamental family of chemicals. The solubility of saccharides in polar solvents like water is enhanced by their hydrophilic groups, such as hydroxyl $(-OH$), carbonyl $(>C=O)$, and amino (${-NH}_{2}$) groups. When L-arabinose or d-xylose (types of saccharides) interact with water, their polar sites attract water molecules and become hydrated, forming hydration shells around them. As a result, these water molecules are no longer available to hydrate the Asp (aspartic acid) molecules. Consequently, the primary hydration shells that initially surrounded the Asp molecules lose some of their water molecules, leading to a reduction in their hydration. In essence, the hydration of l-arabinose or d-xylose causes a dehydration effect on the Asp molecules. This results in less negative $K_{s,\upphi }$ values due to the increased compressibility and reduced electrostriction of water molecules in the outer solvation shells [36]. When l-arabinose or d-xylose interact with Asp, they form ion pairs with the polar groups of Asp, such as the carboxyl $(-\text{COOH}$) and amino (${-\text{NH}}_{2}$) groups, as well as with the zwitterionic centers ${(-\text{COO}}^{-}$ and ${\text{NH}}_{3}^{+})$. This pairing means that the ions from Asp are more attracted to the saccharides rather than to water molecules. Because of this, the electrostatic interactions between Asp ions and water molecules are reduced. As a result, water molecules become more loosely packed, increasing their compressibility. Essentially, the presence of saccharides changes the way water molecules interact with each other and with Asp, making the water molecules more flexible and easier to compress.

Equation (11) is used to derive the limiting apparent molar isentropic compression,

$$K_{s,\upphi } = K_{s,\upphi }^{0} + S_{k,\upphi } \sqrt m$$

(11)

The intercept of the above linear equation, called limiting apparent molar isentropic compression yields $K_{s,\upphi }^{0}$ and the slope, called experimental slope for isentropic compression, yields $S_{k,\upphi }$. Table 3 shows the values of $K_{s,\upphi }^{0}$ and $S_{k,\upphi }$ along with the standard errors. Ion-hydrophilic interactions between the solute ions and the solvent are shown by the magnitude of $K_{s,\upphi }^{0}$$,$ whereas ion-ion, ion-hydrophilic, and hydrophilic –hydrophilic interactions between the solute and cosolute ions and polar groups are indicated by $S_{k,\upphi }$. The hydrophobic-hydrophobic interactions between (hydrophobic) carbon skeleton of l-arabinose$/$d-xylose’s and Asp’s could be the cause of $K_{s,\upphi }^{0}$ values that are negative. It is noteworthy that all values of $K_{s,\upphi }$ are negative, and that the negativity seems to diminish as the temperature and the composition of the co-solute, l-arabinose/d-xylose, increase. Water molecules that are liberated from the bulk due to a reduction in electrostriction demonstrate the existence of strong, attractive interactions between Asp and water. In this case, the behaviour of $K_{s,\upphi }^{0}$ values in the presence of l-arabinose$/$d-xylose can be explained using the Kirkwood model [35].

This model states that because electrostriction water has a more compact structure than bulk water, it is highly compressible. Therefore, when the contacts between Asp and saccharides increase, the electrostriction connections between water molecules and Asp decrease. In doing so, the solute's compressibility is further enhanced by driving electrostriction water back into the bulk through the solvation layer [36].

Partial molar isentropic compression of transfer $\left( {K_{s,\upphi , tr}^{0} } \right)$

The transfer parameter of the limiting apparent molar isentropic compressibility provides insight into how the compressibility of a solution changes as solute molecules are added. Positive values indicate that the compressibility increases upon adding solute, while negative values indicate a decrease in compressibility. The values of partial molar isentropic compression of transfer, $K_{s,\upphi , tr}^{0}$ for the Asp solution can be obtained using the relation (Eq. 12).

$$K_{{s,\upphi , tr \left( {{\text{aqueous saccharide}} \to {\text{water}}} \right)}}^{0} = K_{s,\upphi }^{0} \left( {\text{in aqueous saccharide}} \right) - K_{s,\upphi }^{0} \left( {\text{in water}} \right)$$

(12)

The values of $K_{s,\upphi , tr}^{0}$ for Asp at every composition of supplied saccharides are positive and are indexed in Table 3. Based on the co-sphere overlap model of ternary mixtures, the solute and co-solute species in water may interact in different ways. [39, 40]. Ion-ion interactions are formed by ions such as zwitterionic $({-\text{COO}}^{-}\text{ and }{-\text{NH}}_{3}^{+}$), ${-\text{COO}}^{-}$ of Asp and ions of l-arabinose/d-xylose in water (figure S1, supplementary material). The polar $-OH$ and $-{\text{NH}}_{2}$ groups of Asp, along with the above mentioned ions, produce hydrophilic-hydrophilic and ion-hydrophilic interactions. Ions create ion-hydrophobic and hydrophilic-hydrophobic interactions with polar groups and hydrocarbon segments of Asp, L-arabinose, and d-xylose. l-arabinose/d-xylose and Asp's alkyl backbones interact hydrophobically with one another. A study on the enthalpies of mixing aqueous solutions of glycine, l-alanine, and l-serine with aqueous solutions of fructose and sorbose, as well as the corresponding enthalpies of dilution, revealed that different $-OH$ groups of the same saccharide molecule can interact with the zwitterionic and $-OH$ groups of the l-serine molecule. In this case, the hydrophobic contacts are strengthened and the nonpolar groups of l-serine and saccharide get closer [37].

The following types of interactions can be emphasized while extrapolating the current interactions. The following interactions are listed in order of preference: Groups that are (a) nonpolar and nonpolar, (b) polar and nonpolar, (c) polar and polar, and (d) polar and ionic. $K_{s,\upphi , tr}^{0}$ values would be positive for interactions of the forms (a) and (b), whereas $K_{s,\upphi , tr}^{0}$ values would be negative for interactions of the other types, based on these concepts. $K_{s,\upphi , tr}^{0}$ results indexed in Table 3 show that the (a) and (b) types of interactions between the polar groups of saccharides and the ions of Asp are prevalent in this scenario. It can also be inferred that as saccharide composition increases, $K_{s,\upphi , tr}^{0}$ values for Asp increase as well and with increasing temperature at all concentrations of supplied saccharides $K_{s,\upphi , tr}^{0}$ values drop. The structure forming tendency of ions is indicated by the preponderance of connections between the zwitterionic centre of Asp and polar parts of l-arabinose and d-xylose, as indicated by the positive values of $K_{s,\upphi , tr}^{0}$. As the saccharide concentration increases, the zwitterionic centre of Asp interacts with saccharides more. As the concentration of saccharide increases, electrostriction decreases, and the propensity to form ions in structures increases. Because of this, the compressibility of electrostricted water is much less than that of bulk water, and it gets much less as the saccharide composition increases. This implies that a large number of solvent molecules have bonded to a large number of solute molecules, boosting the ions’ interactions with each other [38]. Overall, the study demonstrates that the interactions between the zwitterion of Asp and polar sites of saccharide (s) cause the positive values of $K_{s,\upphi , tr}^{0}$ to indicate the structure-making behaviour of Asp in aqueous saccharide (s) [39]. However, a study on Asp’s interactions with d-glucose, d-galactose, and d-fructose using densimetry, calorimetry of dissolution, and isothermal titration calorimetry reveals Asp as a structure breaker [40].

In this study, Asp functions as a structure maker, contributing to the formation and stabilization of specific structures within the solution. Its interactions with other molecules, such as saccharides, help maintain the organization and integrity of the system being studied. The report in our recent publication [3], i.e. the negative values of the temperature derivative of the B-coefficient ${\partial B}_{J}/\partial T$, and the positive values of the Hepler’s constant ${({\partial }^{2}{V}_{\varnothing }^{0}/\partial {T}^{2})}_{p}$ and free energy of activation $(\Delta {\mu }_{1}^{0\#}$ and ${\Delta \mu }_{2}^{0,\#}$) supports this.

Interaction co-efficients

The Eq. 13 shown below allowed for the calculation of the pair and triplet interaction coefficient of isentropic compression.

$$K_{s,\upphi , tr}^{0} = 2K_{AB} m_{B} + 3K_{ABB} m_{B}^{2} + \cdots$$

(13)

Asp is represented by A, saccharide (s) by B, and ${m}_{B}$ represents the molal concentration of saccharide (s).

${K}_{AB}$ is the pair interaction co-efficient and ${K}_{ABB}$ is the triplet interaction co-efficient of isentropic compression. To calculate and index the pair and triplet coefficients in table S1 (supplementary material), McMillan–Mayer theory was applied [41]. This theory fits the partial molar property values for $K_{s,\upphi , tr}^{0}$ to find the coefficients, ${K}_{AB}$ and ${K}_{ABB}$. The interactions between molecules in triplets and pairs are examined using the McMillan–Mayer theory. The pair interaction coefficient, ${K}_{AB}$ in the context of isentropic compression, denotes the energy of interaction between pairs of particles during the compression of the material. Similarly, the triplet interaction coefficient, ${K}_{ABB}$ extends the concept of interaction to groups of three particles in the context of isentropic compression. Further information on this subject can be found in the works of Friedman and Krishan [42] and Franks et al. [43] for a more thorough comprehension. Pair-wise interactions are important for the current study, as Table S1 demonstrates that the mixture of Asp + saccharide (s) + water has higher positive values for pair interaction coefficients than triplet interaction coefficients. Furthermore, compared to aqueous d-xylose, the pairwise interactions for the amino acid are more pronounced in aqueous l-arabinose.

Molecular separation (${L}_{f})$

Usually, when discussing a solution, the term molecular separation or intermolecular free length, ${L}_{f}$ refers to the average distance between molecules. Jacobson’s technique was used in accordance with Eq. (14) [44] to calculate ${L}_{f}$, and the results are displayed in Table 4.

$$L_{f} = kK_{s}^{1/2}$$

(14)

k is a temperature-dependent constant and is calculated as per the following relation (Eq. 15) [47].

$$k = \left( {93.875{ } + { }0.375{ }T} \right) \times 10^{ - 8}$$

(15)

Table 4 Values of internal pressure $({\pi }_{i})$, relative ion association ${(R}_{A})$, acoustic impedance (Z), free length (${L}_{f})$, free volume (${V}_{f})$ and surface tension $(\gamma$) as functions of molality of Asp in water, aqueous l-arabinose$/$d-xylose media at 293.15 K, 298.15 K, 303.15 K, 308.15 K, 313.15 K and experimental pressure P = 101 kPa

The experimental temperature, T, for this study, is equal to 293.15 K, 298.15 K, 303.15 K, 308.15 K and 313.15 K.

In this instance, intra- and intermolecular interactions have a direct impact on the ${L}_{f}$ values, which are associated with the isentropic compressibility values. The presence of hydrophilic groups $-OH$ and $-O-$ in saccharide (s) and ${-NH}_{2}\text{ and }-COOH$ in Asp is what causes the intermolecular and intramolecular interactions. Eyring’s liquid state model explains the behavior and properties of liquids by considering molecular interactions and structures. According to this model, symmetrical systems tend to have low values of ${L}_{f}$. The parameter ${L}_{f}$ is influenced by three key factors: symmetry, free volume, and molecular form. Symmetrical molecules are more uniform in shape and size, allowing them to pack more efficiently in a liquid state. This efficient packing reduces the available free volume and contributes to lower ${L}_{f}$ values. Free volume refers to the space within the liquid that is not occupied by molecules. Thus, in symmetrical systems, the efficient packing of molecules leaves less free volume, further lowering ${L}_{f}$. The shape and structure of molecules also impact ${L}_{f}$. Symmetrical molecules tend to have simpler and more regular shapes, which facilitate efficient packing and minimize the free volume [45].

It can be observed from Table 4 that in an aqueous solution, Asp has lower values ${L}_{f}$, but in saccharides, ${L}_{f}$ has higher values. The amino and hydroxyl functionalities in Asp and saccharides result in hydrogen-bonded big and symmetrical structures in solvents; when Asp concentration rises, ${L}_{f}$ decreases in as per Eyring's liquid state modalities. Additionally, the presence of clathrate-like structures [46] traps the Asp molecules, creating incompressible structures that lower ${K}_{s}$ and, in accordance with Eq. 5, lower ${L}_{f}$. ${L}_{f}$ values fall as saccharide composition rises as well, suggesting enhanced amino acid-saccharide interaction. ${L}_{f}$ values, on the other hand, rise with temperature, suggesting that the potential energy of interactions among the solute and solvent molecules decreases as the distance between them increases [47]. Furthermore, the trend towards decreasing free length suggests that the solute molecule is acting in a way that promotes structure. For a clear understanding of the variation of ${L}_{f}$ with the concentration of Asp in aqueous l-arabinose$/$d-xylose at 298.15 K, a graphic representation is shown in figure S2.

Free volume (${V}_{f})$ and internal pressure (${\pi }_{i})$

In a solution, the concept of “free volume” refers to the unoccupied space within the solvent where solute molecules can move freely. It represents the volume not occupied by solvent molecules or solute particles. Equation (16), which is the expression used for the evaluation of free volume, ${V}_{f}$,

$$V_{f} = \left\{ {\left( {\bar{M}U/\left( {k \times \eta } \right)} \right.} \right\}^{3/2}$$

(16)

$\bar{M}$ The average molar mass (m) of the solution can be computed using the Eq. (17) that follows [48]

$$\bar{M} = \mathop \sum \limits_{i = 1}^{3} x_{i} M_{i}$$

(17)

A value of ${4.28\times 10}^{9}$ is assigned to the temperature-independent constant term k, whereas η represents the dynamic viscosity of the solutions being studied, whose obtained values in N s m² are converted into Kgm⁻¹ s⁻¹ [24] and are recorded in Table 2. According to the average free volume concept, hard-core spheres of solvated particles arise as a result of strong attraction forces between solute and solvent molecules. As stronger interactions are anticipated at higher solute and cosolute concentrations, the solvation spheres shrink in volume and fit neatly inside the solvent’s interstitial spaces. As a result, as seen in Table 4, the free volume decreases when Asp and saccharide (s) content ascend. The solute–solvent interaction is more prominent in the Asp and aqueous saccharide systems, according to the results of adiabatic compressibility and intermolecular free path length, which were found to decrease with concentration and increase with temperature, whereas ultrasonic velocity and viscosity were found to increase with concentration and increase with temperature. This was further confirmed by the analysis of another important acoustic parameter, i.e. internal pressure. The outcome of the forces of attraction and repulsion between molecules in the interior of the solution is the internal pressure (${\pi }_{i}$). ${\pi }_{i}$ can be deduced using the Eq. (18) given below,

$$\pi_{i} = \left( {\alpha T/K_{T} } \right) - P$$

(18)

Table 4 lists the values of ${\pi }_{i}$. The experiment was conducted at atmospheric pressure ($\text{P}$), which is not taken into account in the calculations that are shown.

Associative solute–solvent interactions put structural and chemical pressure on the solute. The internal pressure of a solution thus increases above that of a pure solvent. Strong, cohesive contacts between the polar groups of Asp, saccharides, and the ionic components at increasing concentrations promote the clustering process, which causes internal pressure to rise with concentration. Table 4 illustrates that in both aqueous l-arabinose and aqueous d-xylose systems, free volume dropped and internal pressure rose for Asp. Once more, this supported the notion that interactions between solutes and solvents exist in the system under study.

Relative association ${(R}_{A}$ )

In the context of solutions, it often refers to the tendency of solute molecules to associate with each other or with solvent molecules, altering their behaviour compared to what would be expected based solely on their individual properties. In other words, a crucial acoustical characteristic that sheds light on the kinds of interactions that occur between like-like and unlike parts is relative association, or ${R}_{A}$. It provides insight into the degree of correlation that exists among the constituents. The densities and sound velocities of solutions and solvents are employed in accordance with the following relation (Eq. 19) to evaluate the ${R}_{A}$ parameter [49].

$$R_{A} = \left( {\rho /\rho_{o} ) \times (U_{0} /U} \right)^{1/3}$$

(19)

In Table 4, we saw an increasing trend in ${R}_{A}$ variation with Asp and saccharide (s) concentrations. In the context of ion-ion and ion-hydrophilic attractive forces between ${-\text{NH}}_{3}^{+}\text{ and}$ ${-\text{COO}}^{-}$ on the Asp chain ends, polar parts of saccharides and the polar water molecules, an explanation of this trend has been put forth. Two things affect the ${R}_{A}$ values: (i) the solute molecules’ hydration; (ii) The molecules of the solute break the solvent’s structure. Component (i) raises the ${R}_{A}$ values, whereas component (ii) lowers them. Table 4 demonstrates that for binary and ternary Asp solutions, the hydration of solute molecules dominates over the solvent structure breaking with an increase in solute concentration at both temperatures. An increase in ${R}_{A}$ values with an increase in Asp concentration for each composition of saccharides under investigation suggests the presence of significant interionic and solute–solvent interactions. That is the values of ${R}_{A}$ increase with an increase in the composition of l-arabinose/d-xylose, suggesting a strong interaction between the amino acids and saccharides. As the temperature increases, the interionic attraction between the solute and solvent molecules weakens. This means that the forces holding the solute ions and solvent molecules together become less strong. This weakening of attraction is reflected in the decrease in the ${R}_{A}$ values (a parameter that indicates the strength of these interactions) as the temperature rises. In simpler terms, higher temperatures reduce the strength of the interaction between solute and solvent molecules, which is evidenced by the lower ${R}_{A}$ values observed at elevated temperatures.

Acoustic impedance ( $Z$ )

Using density ($\uprho$) and ultrasonic velocity ($\text{U}$) data, the acoustic impedance ($\text{Z}$) has been calculated (Eq. 20) [50]. It is the medium’s resistance to the motion of the longitudinal waves. The more molecules interact with one another, the stronger the opposition will be, raising the values. The Table 4 for $Z$ values of each system makes this clear.

$$Z = \rho \times U$$

(20)

The ratio of the effective sound pressure on a particle in the medium to the effective particle velocity is typically used to determine the medium’s acoustic impedance (Z). When a sound wave travels through a medium, its inertial and elastic qualities cause various medium particles to experience varying sound pressures. Therefore, the total rise in $Z$ values can be attributed to an increase in the effective sound pressure at any given solution molecule. The Z values in the systems under investigation have increased due to the rise in solute (Asp) and co-solute (l-arabinose/d-xylose) concentrations (Table 4). This suggests that the solvent and solute molecules have strong molecular interactions. A visual depiction is given in figure S3 for a comprehensive grasp on the variation in $Z$ values with a change in concentration of Asp in different compositions of saccharides at 298.15 K. The stronger cohesive molecular connections are predicted as the solution's concentration rises. Sound waves travel by vibrating particles in a medium (like air, water, or a solid). The closer the particles are packed together, the more efficiently the sound can travel through that medium. Impedance in this context is a measure of how much the medium resists the passage of sound waves. When particles are closely packed (denser media), sound waves encounter more resistance because there are more particles to vibrate. When solute (like salt) or cosolutes (another substance) is added to a solution, it increases the density of the solution because the added molecules fill in the spaces between the solvent molecules (like water). Hence, when the concentration of solutes or cosolutes is increased, the medium becomes denser, thereby increasing the impedance (Z value) because there are more particles for the sound waves to interact with [51]. Stereochemistry deals with the 3D arrangement of atoms in molecules. l-arabinose and D-xylose, despite being similar sugars, have different spatial arrangements. This affects how they interact with other molecules, including Asp. A number of studies have emphasized the critical function that the spatial orientation of the $-OH$ group at C4 plays in the interactions between proteins and molecular ligands as well as carbohydrates. The interactions between Asp and certain isomeric monosaccharides (d-glucose, d-galactose, and Dfructose) in aqueous solution are investigated by G. A. Kulikova et al. following densimetry, calorimetry of dissolution and isothermal titration calorimetry approach. It was observed that the axial orientation of the $-OH$ group at C4 in d-galactose molecule prevented the establishment of the interaction between this monosaccharide and Asp [40]. d-xylose fits better into the water structure, meaning it forms more stable interactions with water molecules. Because of this, D-xylose is less available to interact with the polar sites on Asp. l-arabinose doesn’t fit into the water structure as well as d-xylose does. For the current study, it can be observed that $Z$ values are higher for Asp in aqueous l-arabinose than aqueous d-xylose (Table 4). This means l-arabinose is more available to interact with Asp’s polar sites, leading to stronger interactions and higher Z values in the solution [1, 2]. Thus, the molecular packing of the medium [52], the molecular structure of the particles, the solvation process, the caging effect of the solvent, and the solute's structure-making/breaking properties all affect the pattern of sound waves. Pressure changes from one particle to another, which happens as a sound wave passes through a medium. The medium's elastic quality is a key component that controls this aspect.

Surface tension ( $\gamma$ )

A physical characteristic of matter, surface tension, or σ, is connected to the potential energy of intermolecular interactions and the liquid interfacial microstructure [53]. At low and moderate temperatures and pressures, the surface tension can be measured with excellent accuracy; at high temperatures and pressures, computer simulations are used to determine the values [54]. As a result of molecules moving more quickly at higher temperatures, which diminishes cohesive forces, surface tension typically decreases with temperature. On the other hand, as the temperature rose in the current investigation, surface tension increased. Although it’s not common, there are instances in which surface tension rises with temperature; these are less common and often happen with particular compounds or under particular conditions. The variability can be attributed to a number of reasons, including (i) particular molecular interactions, (ii) temperature-dependent structural changes, (iii) the degree of non-ideality, etc. Saccharides are the fundamental family of chemicals due to their hydrophilic hydroxyl ($-OH$) groups, homochirality, stereoselectivity, and coordinating abilities. Every sugar that is crystalline-free has a distinct stereoisomer. Many sugars, like l-arabinose and d-xylose, can form a ring structure known as a hemiacetal. This ring can open and close when the sugar dissolves in water. The opening and closing of the sugar rings in water (mutarotation) leads to multiple ring structures of different shapes and sizes at a specific carbon atom called the anomeric center. Due to mutarotation, l-arabinose and d-xylose exhibit various potential structures in water. They don’t just stay in one form but constantly switch between multiple forms. Less than 1% of reducing monosaccharides are found in the acyclic state in solution. Most of them exist in cyclic (ring) forms. These sugars exist in a dynamic balance between their cyclic and acyclic forms. The dynamic nature of sugars like l-arabinose and d-xylose in water, due to mutarotation and the balance between cyclic and acyclic forms, leads to variations in intermolecular interactions. These variations influence the arrangement of molecules at the liquid's surface, ultimately affecting the surface tension. It is seen from Table 4 that surface tension rose as temperature increased. This may be due to the strong intermolecular interactions that occur between l-arabinose/d-xylose and Asp's polar sites in aqueous conditions.

The initial drop in surface tension when introducing a small amount of solute is due to the disruption of the uniform solvent–solvent interactions at the surface, which reduces the cohesive forces among the solvent molecules. This makes it easier to increase the surface area of the liquid, hence the lower surface tension. Because of the creation of aggregates or intricate structures at the surface, increased solute concentration might result in strong solute–solvent interactions and potentially raise surface tension. With concentration, these intricate structures have the ability to change the surface's characteristics and cause surface tension to increase [55]. The surface tension increasing as the amount of Asp in the solution increases (Table 4) reveals this. Along with the rise in saccharide composition in the aqueous medium, the surface tension values also rose. The increased surface tension data for aqueous l-arabinose across all compositions lends additional credence to the idea that Asp interacts with l-arabinose more frequently than d-xylose. Xueli Zhou et al. reported the surface tension data for N, N-diethylethanolamine (DEEA) in aqueous 2-amino-2-methyl-1-propanol (AMP)/piperazine (PZ). Surface tension values for pure components decreased with temperature in all systems under study, following the order H2O>AMP/PZ>DEEA. The surface tension values are nearly in the same range as in the current study [56]. The literature also reports, with increase in amino alcohol (monoethanolamine (MEA)/2-Amino-2-methyl-1-propanol (AMP)) concentration in aqueous medium surface tension gets increased. However, the alteration in surface tension due to a specific variation in alkanolamine concentration is more pronounced at lower concentrations compared to higher concentrations [57].

FTIR spectral analysis

An additional crucial tool that offers details on the interactions between various groups is FTIR spectroscopy. It depends on the vibrations of the atoms within a molecule. These vibrations give insight into the chemical interactions within the system [58]. The FTIR spectra of Asp in a 2.0 wt % aqueous solution of l-arabinose or d-xylose were recorded in the range of wavenumbers from 4000 cm⁻¹ to 400 cm⁻¹. One major peak appears in all the spectral data, corresponding to the O–H stretching band. This is a characteristic absorption due to the vibration of O–H bonds (like those in water or alcohol groups). Between 1625 cm⁻¹ and 1660 cm⁻¹, a distinct stretching band is observed for the aliphatic carboxylic groups ($>\text{C}=\text{O}$) in all amino acid solutions. This corresponds to the carbonyl group vibrations in the amino acid. The absorption peaks between 3300 cm⁻¹ and 3650 cm⁻¹ are sensitive to hydrogen bonding. They correspond to O–H and N–H stretching vibrations. When molecules form hydrogen bonds, these vibrations shift, indicating changes in the molecular environment. The broad absorption band shifts due to intermolecular hydrogen bonding, suggesting ion-hydrophilic (water-attracting) or hydrophobic (water-repelling) interactions in the solution. This means that the interactions between different parts of the molecules are changing the way they vibrate and absorb IR light. As the concentration of Aspartic acid in the solution increases, there is more hydrogen bonding between water molecules and the polar groups $\left({NH}_{3}^{+}, {COO}^{-}\right)$ of the amino acid [59, 60]. This increased hydrogen bonding causes O–H and N–H stretching bands to shift to a lower frequency in the IR spectrum. Lower frequency means that the bonds are more relaxed or stretched due to the interactions [61]. It signifies that the H-bonded network of water by Asp is strengthened by the presence of l-arabinose and d-xylose. So it may be interpreted that a change in concentration of amino acid causes a shift in wave numbers, which indicates some structural changes, which are also predicted from the acoustic study. A graphic representation of the spectra of Asp in pure water and aqueous l-arabinose$/$d-xylose (2.0 wt % composition) is displayed in Fig. 4.

Fig. 4

Asp’s FTIR spectra in (a) water, (b) 2.0 weight percent aqueous l-arabinose, and (c) 2.0 weight percent d-xylose

Based on acoustic and FTIR studies, as well as thermodynamic analysis and transfer parameters from the volumetric and viscometric study that was recently published [2],this study challenges that molecular interactions of the types hydrophilic-hydrophilic and hydrophilic-ionic are easily observed and efficient among the axial and equatorial ${-}{\text{OH}}$ groups of l-arabinose$/$d-xylose, and the zwitterions of Asp in water. Temperature-dependent changes in the strength or extent of these connections among Asp and l-arabinose$/$d-xylose affect the solution’s overall stability and behaviour. Therefore, the system being studied and the methodology selected provide a fresh viewpoint, allowing for the prediction of interaction patterns or complexes that may not be easily detectable through experimental methods. These insights go beyond what traditional experimental methods might capture, offering a deeper understanding of the system's behaviour. In addition, the present work investigates these interactions emphasizing the thermodynamic and structural differences between Asp and Glu in saccharide-rich environments. By employing similar physicochemical and spectroscopic techniques, this study enables a comparative analysis that builds upon the findings of the earlier work.

Conclusions

The current investigation has examined the degree of molecular interaction between the solute (Asp), co-solute (l-arabinose/d-xylose), and solvent (water) using acoustic and spectroscopic methods and their characteristics at five distinct temperatures (293.15 K to 313.15 K). Increased ultrasonic velocity is the result of stronger particle interactions caused by higher concentrations of Asp and l-arabinose/d-xylose. The decreasing trends in compressibility metrics (${\text{K}}_{\text{s}}$, ${\text{K}}_{\text{T}}$ and ${\text{K}}_{{{\text{s}},\upphi }}$) with concentration suggest that bulk water molecules are much more compressible than the electrostricted water molecules in the hydration shells around Asp and the polar regions of saccharides. In evaluated solutions, positive values of ${\text{K}}_{{{\text{s}},\upphi ,{\text{ tr}}}}^{0}$ indicate Asp's structure-making activity. Its pair and triplet interaction coefficient indicates that Asp has more pairwise interactions with water + l-arabinose than with watery d-xylose. The increase in acoustic impedance (Z), internal pressure (${\uppi }_{\text{i}}$), relative association (${\text{R}}_{\text{A}}$), surface tension ($\upgamma$), and decrease in free volume (${\text{V}}_{\text{f}}$) and free length (${\text{L}}_{\text{f}}$) reveal that the interactions between hydrophilic parts and ions are stronger than those between hydrophobic moities. l-arabinose molecules'axial-equatorial $-OH$ groups prefer to bind with Asp molecules over d-xylose’s equatorial $-OH$ groups. FTIR analysis reveals that Asp strengthens the H-bonded network of water in the presence of l-arabinose and d-xylose by shifting the ${-}O{-}H$ and ${-}N{-}H$ stretching bands to a lower frequency range. Further studies on different saccharides and amino acids are strongly advised, with consideration given to factors like mutarotation, anomeric effect, solubility and stereochemistry of saccharides, which affect how the different moieties interact with one another in the solution. The results of this investigation can be utilized to quantitatively investigate the interactions between Asp and l-arabinose/d-xylose. Using the FTIR spectroscopy analysis, the chemical structures and functional groups involved in interactions can be further examined. Experimental results can be enhanced by computational methods like quantum chemistry computations and molecular dynamics simulations. research offers exciting future prospects across various fields. It can contribute significantly to drug design by providing insights into saccharide-based therapeutics and efficient drug delivery systems. The findings on molecular interaction may also advance biotechnology by aiding in the development of biocompatible materials and enhancing enzymatic processes involving amino acids and saccharides. In nutritional science, this study could inspire innovations in dietary supplements and functional food formulations. Additionally, the research has potential applications in material science for creating advanced biomaterials tailored for medical and industrial uses.