Dehydration of glycine in mixed solvents

Abstract

The analysis of changes in the partial volume of glycine 0 V 'in solutions of substances that have a different effect on water structure is presented. For glycine in mixtures of water with glycerol and ethylene glycol, we derive a single equation for the 0 V ' dependence on the volume fraction of alcohol. The addition of tert-butyl alcohol, ethylene glycol, glycerol, and urea to water leads to a decrease in the hydration number of the amino acid (glycine dehydrates). In 1m solutions the losses of hydration water are 3.2%, 4.5%, 5.7%, and 7.6% respectively. In a 4m solution of tert-butyl alcohol, glycine loses 44% of hydration water, the same as in a 15m urea solution and a 20m glycerol solution. A contribution of the structural dehydration of glycine is observed in dilute aqueous solutions of t-BuOH. In more concentrated solutions, intermolecular interactions in the binary mixed solvent counteract dehydration. These interactions compensate for 15-22% of water lost by glycine in a 20m solution of urea, glycerol, and ethylene glycol and a 4m solution of t-BuOH. The partial volumes are also discussed within preferential solvation concepts.Hydration numbers of substances are the important characteristics of aqueous solutions [1-9]. They can be regarded as a link between the elements of the triad: thermodynamic property, hydration of the solute, and solution structure. In this context, the hydration number is essentially "a thermodynamic hydration number" [10]*.The aim of this paper is to study the hydration numbers of glycine in solutions of substances that have a different effect on the water structure. Glycine itself is known to disrupt the structure of water [11]. One of the ways to find the desired characteristics is to use partial molar volumes. The standard partial volume of an amino acid in an aqueous solution 0 W V can be written as [1-3]where V in is the intrinsic (effective) volume of a particle in the solution; n W is the hydration number of the amino acid in water; V h is the molar volume of water in the hydration shell of a particle; and V W is the molar volume of water in bulk. We rewrite (1) with respect to the partial volume of an amino acid in a mixed solvent [12]where n is the hydration number of a particle in a mixed solvent; W V can be taken equal to the partial volume of water in a *In our opinion, the value obtained from thermodynamic characteristics is best defined by the term "aquation (aquatic) number."