Calorimetric and Fourier transform infrared spectroscopic study of solid proteins immersed in low water organic solvents

Calorimetric heat effects and structural rearrangements assessed by means o f Fourier transform infrared (FTIR) amide I spectra were followed by immers ing dry human serum albumin and bovine pancreatic a-chymotrypsin in low wat er organic solvents and in pure water at 298 K. Enthalpy changes upon immer sion of the proteins in different media are in a good linear correlation wi th the corresponding IR absorbance changes. Based on calorimetric and FTIR data the solvents were divided into two groups. The first group includes ca rbon tetrachloride, benzene, nitromethane, acetonitrile, 1,4-dioxane, n-but anol, n-propanol and pyridine where no significant heat evolution and struc tural changes were found during protein immersion. Due to kinetic reasons n o significant protein-solvent interactions are expected in such systems. Th e second group of solvents includes dimethyl sulfoxide, methanol, ethanol, and water. Immersion of proteins in these media results in protein swelling and involves significant exothermic heat evolution and structural changes in the protein. Dividing of different media in the two groups is in a quali tative correlation with the soh ent hydrophilicity defined as partial exces s molar Gibbs free energy of water at infinite dilution in a given solvent. The first group includes the solvents with hydrophilicity exceeding 2.7 kJ /mol. More hydrophilic second group solvents have this energy values less t han 2.3 kJ/mol. The hydrogen bond donating ability of the solvents also ass ists in protein swelling. Hydrogen bonding between protein and solvent is a ssumed to be a main factor controlling the swelling of dry solid proteins i n the studied solvents. (C) 2001 Elsevier Science B.V. All rights reserved.