On the nature of the temperature-induced transition from the molten globule to the unfolded state of globular proteins

In this paper we try to perform a thermodynamic analysis of the temperature -induced transition from the molten globule to the unfolded state of globul ar proteins. A series of calorimetric investigations showed that this proce ss is not associated with an excess heat capacity absorption peak, and cann ot be regarded as a first-order phase transition. This result contrasts wit h the well-established conclusion that the thermal unfolding of the native tertiary structure of globular proteins is a first-order phase transition. First, the theoretical approach developed by Ikegami is outlined to emphasi ze that a second-order or gradual transition induced by temperature is expe cted for globular proteins when the various secondary structure elements do not interact cooperatively. Secondly, a simple thermodynamic model is pres ented which, taking into account the independence of the secondary structur e elements among each other, is able to rationalize the shape of the experi mental DSC profiles.