Partial molar volumes of l-alanine, dl-serine, dl-threonine, l-histidine, glycine, and glycylglycine in water, NaCl, and DMSO aqueous solutions at T = 298.15 K

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Abstract

The apparent molar volumes of l-alanine, dl-serine, dl-threonine, l-histidine, glycine, and glycylglycine in water and in the aqueous solutions of NaCl and DMSO with various concentrations at T = 298.15 K have been measured by the precise vibrating-tube digital densimeter. The calculated partial molar volumes at infinite dilution have been used to obtain corresponding transfer volumes from water to various solutions. The experimental results show that the standard partial molar volumes of the above amino acids and peptide at the dilute DMSO aqueous solutions are very close to those in water. However, the volumes show several types of variations with the increase of the concentrations of DMSO due to different types of side chain of amino acids, which should be discussed specifically. The NaCl changes considerably the infinite dilution standard partial molar volumes of the above amino acids and peptide in the aqueous solutions. The infinite dilution standard partial molar volumes of the each amino acids and peptide increase with the concentrations of NaCl. The experimental results have been rationalized by a cosphere overlap model.

Introduction

Most of the biochemical processes occur in aqueous solutions, so the studies on the thermodynamic properties of biological molecules in aqueous solutions are important. The partial molar volumes of five amino acids and one peptide in water, DMSO, NaCl, and DMSO/NaCl aqueous solutions at different temperatures were reported by our group [1]. In the present paper, the concentration effect of DMSO and NaCl in aqueous solutions on partial molar volumes of the above amino acids and peptide has been studied.

Many researchers have explored the concentration effect of NaCl on partial molar volumes of amino acids in aqueous solutions. For example, Bhat and Ahluwalia [2] have determined partial molar volumes of 12 amino acids in NaCl solution with different concentrations. Wadi and Ramasami [3] and Ramesh and Rma [4] have also determined the partial molar volumes of amino acids in NaCl and KSCN solutions with different concentrations. Those experiments show that the standard partial molar volumes of the each amino acid and peptide increase with the increase in the concentrations of NaCl [5], [6]. Our research has also verified the above conclusion.

DMSO is widely used as a polar aprotic solvent. It mixes with water over the whole molarity range. At a high mole fraction (about 0.75) and in pure DMSO, proteins are completely unfolded while at low fractions, DMSO shows no detectable effect on the secondary structure of proteins [7]. In order to understand the behavior of proteins in DMSO solution, one way is to simulate proteins by their model molecules, amino acids. Therefore, in this research we determined partial molar volumes of several amino acids in DMSO aqueous solution with various concentrations so that we can analyze the interaction between DMSO molecules and amino acids.

Section snippets

Experimental

The amino acids and one peptide used in this study, namely l-alanine, dl-serine, dl-threonine, l-histidine, glycine, and glycylglycine were BR grade reagents purchased from Beijing Chemical Reagents Company. The purity of all the reagents was more than 99 percent and their content of water was less than 0.5 percent. The deionized water was further purified by distilling it from KMnO4 solution and was used for all the measurements. All the mixed solvents were prepared by molarity at T = 298.15 K.

Results

The experimental densities ρ and apparent molar volumes VΦ of l-alanine, dl-serine, dl-threonine, l-histidine, glycine, and glycylglycine in NaCl aqueous solutions at T = 298.15 K are listed in TABLE 1, TABLE 2, TABLE 3, TABLE 4, TABLE 5, TABLE 6, and the densities ρ and apparent molar volumes VΦ of the above amino acids and peptide in DMSO aqueous solutions at T = 298.15 K are listed in TABLE 7, TABLE 8, TABLE 9, TABLE 10, TABLE 11, TABLE 12. The standard partial molar volumes VΦ of the five

Influence of NaCl

As many experiments showed [2], [3], [4], [8], [9], the ΔtrVΦ values of the amino acids and peptides studied from water to 3.0 mol · dm3 sodium chloride were found to be positive. Based on the Frank cosphere theory [10], [11], the ΔtrVΦ can be divided into three parts as follows:ΔtrVΦ=VVW+VV+VS,where VVW is van der Waals volume, VV is the volume associated with void or empty space, and VVW + VV is transfer intrinsic volume of solute from water to aqueous NaCl solvent. The VVW and VV are not

Acknowledgements

This project was supported by the National Natural Science Foundation of China Grant No. 20133030. We are also grateful to Ms. Bing Wu and Ms. Feng-Juan Chou for their earlier experimental exploration to this study.

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