Volumetric and calorimetric investigations of molecular interactions in some amino acids and peptides in the combined presence of surfactants and glycine betaine

The values of apparent molar volume V-2,V-phi and apparent molar adiabatic compressibility K-S,(2,phi) of amino acids glycine, L-alanine, DL-alpha-amino-n-butyric acid (ABA), L-valine, L-leucine and peptides diglycine and triglycine have been determined in the aqueous solutions of surfactants and glycine betaine by means of density and sound velocity measurements. The heat (Q) evolved or absorbed during the course of interactions of amino acids and peptides with the aqueous solution of surfactants and glycine betaine were determined by the calorimetric method at T = 298 K. The values of standard partial molar volume V-2,m(0) and standard partial molar adiabatic compressibility K-S,2,m(0) at infinite dilution were calculated from V-2,V-phi and K-S,(2,phi), and the values of limiting enthalpy of dilution Delta H-dil(0) were calculated from the heat evolved or absorbed during the calorimetric experiments. The transfer values of partial molar volume Delta V-tr(2,m)0, partial molar adiabatic compressibility Delta K-tr(S,2,m)0 and limiting heat of dilution Delta(tr)Delta H-dil(0) of amino acids and peptides from water to aqueous solution of surfactants and glycine betaine, in general, demonstrated a dominance of polar interactions. Furthermore, a specific trend in the involvement of different types of interactions was observed which varied as a function of size and hydrophobicity of different amino acids and peptides. The present study indicated that glycine betaine primarily exhibits polar interactions with the zwitterionic centres of the amino acids and peptide bonds of the peptides but might enhance the overall solvent structure in the presence of amino acids with bulkier alkyl groups. On the whole, the findings of volumetric and calorimetric studies in the present work precisely correlate and discuss the molecular mechanism on how glycine betaine might impart stability in the unfolded proteins induced by surfactants. (C) 2012 Elsevier Ltd. All rights reserved.