Heat capacity of hydrated and dehydrated globular proteins. Denaturation increment of heat capacity

Abstract

The experimental results obtained indicate that the dominating contribution to the denaturation increment of heat capacity is connected with thermal motion in a protein globule, which is in contrast to commonly accepted ideas. Absolute values of heat capacity have been studied as a function of temperature by differential scanning calorimetry (DSC) method for four hydrated globular proteins: IIS-globulin, ovalbumin, ribonuclease A, and lysozyme (45% ≥ H 2O ≥ 0). It has been shown that for such proteins with ‘bound’ water, denaturation is accompanied by a heat capacity increase, as is the case for protein solutions. Depending on temperature and humidity, the denatured proteins can be in either a highly elastic (‘rubber’) or glass state. Δ C p between these states is equal to 0.55 J/g.K. Specific heat capacities for these two states have the same values for all proteins studied and depend only on temperature. Glass transition has also been observed in the native proteins. Our results indicate that the denaturation increment of heat capacity for native protein at low humidity, as well as the heat capacity increment at glass transition for denatured protein, is connected with the appearance of additional degrees of freedom of thermal motion in the protein globule.