Physical nature of the phase transition in globular proteins. Calorimetric study of human alpha-lactalbumin.

The guanidine hydrochloride-induced unfolding of human alpha-lactalbumin has been studied by isothermal calorimetry. It has been shown that a cooperative transition takes place only in the concentration interval of the denaturant between 0.3 and 2 mol X l-1. The cooperative transition coincides with the transition detected by circular dichroism in the near-ultraviolet region which reflects the destruction of the specific environment of aromatic side groups. According to scanning calorimetric investigations, the transition disappears in the acid form of the protein where circular dichroism of aromatic side groups is practically absent. At higher concentrations of guanidine hydrochloride, where destruction of the secondary structure and unfolding of the chain are observed, there is no cooperative heat absorption.