HEAT-CAPACITY OF HYDRATED AND DEHYDRATED GLOBULAR-PROTEINS - DENATURATION INCREMENT OF HEAT-CAPACITY

Experimental evidence has been obtained suggesting that, in contrast t o generally accepted notions, thermal motion of the protein globule pr edominates in the denaturation increment of heat capacity. Temperature dependences of the absolute values of the heat capacities of four hyd rated proteins (11S globulin, ovalbumin, ribonuclease A, and lysozyme) were measured with a differential scanning calorimeter. Denaturation of these proteins in the absence of free water was found to lead to an increase in heat capacity. Depending on temperature and water content , denatured proteins can exist in both hyperelastic and glass states. Heat capacities of these states differ by 0.55 J.(-1).K-o(-1). Specifi c heat capacities in the hyperelastic and glass states are the same fo r all the proteins studied and depend only on temperature. Glass trans ition is also observed in native proteins. The denaturation increment of heat capacity for native proteins at low water content, as well as the heat capacity step associated with glass transition in denatured p roteins, was shown to result from the appearance of additional degrees of freedom in globule thermal motion.