Thermodynamic study of solvation of some amino acids, diglycine and lysozyme in aqueous and mixed aqueous solutions

Abstract

Apparent molar adiabatic compressibilities and viscosities of glycine, dl-α-alanine, dl-α-amino- n-butyric acid, l-valine, l-leucine and diglycine have been determined in aqueous and mixed aqueous solutions of m B =1.0, 2.0, 3.0, 4.0 and 5.0 aqueous n-propanol solutions at 298.15 K. From these data the partial molar adiabatic compressibilities and viscosity B-coefficients have been evaluated to calculate the corresponding transfer functions. The partial molar adiabatic compressibilities of transfer at infinite dilution Δ tr K° 2,S for all the studied model compounds are positive and increase with the concentration of n-propanol. Positive and negative B-coefficients of transfer Δ tr B have been observed for the studied amino acids in lower and in higher concentration of n-propanol, respectively. The activation free energy for viscous flow in aqueous and mixed aqueous n-propanol solutions has been calculated from B-coefficient and partial molar volume data. Hydration numbers and interaction coefficients have also been calculated from these data. These parameters have been discussed in terms of solute–cosolvent interactions. Thermal denaturation of lysozyme has also been studied using UV-visible spectrophotometer in aqueous and in mixed aqueous solutions of n-propanol, 1,2-propandiol and glycerol. The thermodynamic parameters accompanying the thermal denaturation have been evaluated. The results have been explained on the basis of competing patterns of interactions of the cosolvents with the native↔denatured reaction. The preferential interaction parameters have been calculated from these thermodynamic data and by correlating the surface tension data of n-propanol and 1,2-propandiol to the surface area of the protein. Some parallelism in the patterns of interactions has been observed for the studied model compounds and protein in the aqueous solutions of these solvents.