Hydration of α-chymotrypsin: Excess partial enthalpies of water and enzyme

A novel method has been developed for studying simultaneously the excess partial enthalpies of water and the enzyme in the entire range of water content. Bovine pancreatic α-chymotrypsin was used as a model enzyme. The proposed method includes the measurements of the enthalpies of solution of the dried and hydrated enzyme in water at 25 °C. From these thermochemical data the excess partial enthalpies of water and α-chymotrypsin were calculated. The partial quantities are very sensitive to the changes in the state of water and α-chymotrypsin. A transition from the glassy to the flexible state of α-chymotrypsin is accompanied by significant changes in the excess partial enthalpies of water and α-chymotrypsin. This transition appears at water weight fraction (w1) of 0.06 when charged groups of α-chymotrypsin are covered. Excess partial quantities reach their fully hydrated values at w1 > 0.4 when coverage of both polar and weakly interacting surface elements is complete.