Differential scanning calorimetric study of heat-set gels of globular protein

Abstract

A calorimetric study of concentrated solutions of globular protein lysozyme has been carried out in the temperature range 20–140°C. The effects of pH, ionic strength of the solution, and protein concentration on thermotropic gelation have been studied. Calorimetric studies were complemented by the visual control of solution state and properties in identical heating conditions. The results obtained have shown that two types of gel can be formed within the pH range 1.7–9.0: a clear meltable gel (type I) and a turbid one (type II), which does not melt. The thremograms of the type I gel heating demonstrate that, in addition to the denaturation heat absorption, there also exists a high temperature maximum (HTM), which is the calorimetric manifestation of cooperative transformation accompanying the melting of this gel. The thermodynamic melting parameters of heat-set lysozyme gel are obtained. The incipient stage of type I gel formation is observed at critical values of pH, ionic strength and protein concentration. It is shown that by adjusting the protein aggregation in solution by the use of these parameters, one can expand considerably the pH range in which heat-gel is formed, and provide such changes in the structure of branched gel hich bring up its melting. This results in the appearance of HTM in the thermograms of such a gel.