Partial Molar Volumes of Amino Acids and Peptides in Aqueous Salt Solutions at 25 ° C and a Correlation with Stability of Proteins in the Presence of Salts

Abstract

Partial molar volumes for a homologous series of amino acids and peptides have been measured in aqueous 1M sodium acetate, sodium thiocyanate, and sodium sulfate at 25°C. These data have been utilized in conjunction with the data in water to deduce partial molar volumes of transfer V2,m0(tr) from water to these aqueous salt solutions. The volumes of transfer for the amino acids and peptides are found to be positive. The interpretation is that this result arises from the dominant interaction of the sodium salts with the charged centers of amino acids and peptides. Thermal denaturation of the structurally homologous proteins lysozyme and α-lactalbumin has been studied in the presence of these salts. Significant thermal stabilization of hen egg-white lysozyme has been observed in the presence of sodium acetate and sodium sulfate. However, the thermal stabilization observed for α-lactalbumin is very small in the presence of these salts and sodium thiocyanate leads to a lowering of its thermal denaturation temperature. The rise in the surface tension of aqueous salt solutions with salt concentration has been correlated with the calorimetric and volumetric measurements. The results show that V2,m0(tr) depends less on the type of electrolyte than on the ionic strength of the solution. The V2,m0(tr) values correlate very well with the increase in the surface tension of aqueous salt solutions, indicating significant role of surface tension in interactions of amino acids, peptides, or protein with the salts.