Differential scanning calorimetric study of the interactions of some stabilizing amino acids and oligopeptides with hen egg white lysozyme

Abstract

Thermal denaturation of lysozyme has been studied at pH 2 in aqueous mixtures of glycine, alanine, leucine, lysine, diglycine, triglycine, tetraglycine, pentaglycine, glycylleucine and glycyl-glycyl-leucine by high-sensitivity differential scanning calorimetry (DSC). The most obvious effect of all the amino acids and peptides was to raise the temperature of denaturation t_1/2. Both the calorimetric and van't Hoff enthalpies of denaturation increased in the presence of these systems, the ratio of the two enthalpies being nearly unity in each case, indicating the validity of the two-state approximation for the unfolding of lysozyme in these cosolute systems. The reversibility of the denaturation has been demonstrated by the reversibility of the DSC curves. It has been observed that the peptides provide more thermal stabilization than the corresponding free amino acids. The quantitative thermodynamic parameters accompanying the thermal denaturation have been evaluated. The observed stabilization of the protein in the presence of these cosolutes has been explained on the basis of preferential hydration of the protein and the attenuation of this effect by the hydrophobic and H-bonding groups of the amino acids and peptides.