PREDICTION AND CHARACTERIZATION OF THE WATER SORPTION ISOTHERM FOR BOVINE SOMATOTROPIN

The sorption of water by recombinant bovine somatotropin (rbSt) solids was described by a sigmoidal-shaped isotherm. Isotherm predictions based on rbst's primary sequence and the H2O binding capacity of various functional groups provided reasonable estimates of the isotherm, depending on the rbSt salt used. The isotherms were described mathematically by the BET and GAB equations, resulting in ''monolayer'' values of 5-8 g of H2O/100 g of protein (60-100 mol of H2O/mol of protein), depending on the salt of rbSt. The isotherms were independent of particle surface area, and H2O/N2 surface area ratios were greater than 1, both consistent with the penetration of water into the solid. The hysteresis in the sorpion-desorption isotherms was consistent with kinetically metastable states typically observed in amorphous polymeric systems. The importance of moisture on rbSt stability was demonstrated and discussed with respect to potential plasticization of the solid by water.