Hydration numbers of α-alanine in an aqueous urea solution

Abstract

Literature data on the apparent molar volumes φ of alanine in water and aqueous urea solutions at 298 K are analyzed. It is shown that the slope of the φ dependence on the alanine concentration is not dependent on the urea concentration. The standard partial volume of alanine increases linearly with the increase in the urea concentration (wt.%). The structural characteristics of hydrated complexes of alanine (hydration number, molar volume of water inside and outside the hydration sphere, and proper volume of alanine in solution) are given. The hydration number of alanine decreases by a factor of two in passing from water to a saturated (20m) urea solution. The effects of urea additions on the hydration numbers of alanine and glycine are compared.The thermodynamic characteristics of aqueous solutions are of fundamental importance for gaining new knowledge about such a phenomenon as hydration of substances and studying interparticle interactions and the structure of solutions. Thus, partial molar volumes "keep track" of the changes that occur with the unique structure of liquid water after dissolution of hydrophilic or hydrophobic substances as well as changes in the concentration of the solution and system parameters. They contain information about the state of water in a hydrate complex and allow one to calculate, within the current approaches, their structural characteristics such as the hydration number.Of particular interest, including in the structural aspect, are ternary systems, in which there is a wide range of particle-particle interactions. The purpose of this study is to analyze the data on apparent volumes of α-alanine in water and aqueous urea solutions, obtain an analytical expression for the dependence of the partial volume of alanine on the urea concentration, and calculate the hydration numbers of alanine. This paper continues the study of hydration numbers of substances in aqueous urea solutions [1,2].It should be mentioned that the properties and structure of aqueous urea solutions continue to be in focus of researchers [3][4][5][6][7][8][9]. Urea is most often referred to as a destroyer of water structure, but there are other points of view (see [10,11] and references therein). The analysis in [12] based on the criterion [13] allowed us to characterize both urea and amino acids studied in [12] (including α-alanine) as destroyers of water structure.The standard partial molar volumes of alanine in an aqueous urea solution at the molality m of 12.6 mol/kg water or less were determined in [14][15][16][17]. These characteristics are derived by extrapolating the apparent volumes φ to a zero