SOLVENT FACILITATED PROTON TRANSFER IN HYDROGEN BONDED COMPLEXES OF PROTON DONOR AND ACCEPTOR GROUPS IN MODEL PROTEIN SYSTEMS

The tautomeric equilibrium between a hydrogen bonded complex and a proton transfer complex (hydrogen bonded ion pair) in a number of phenol-amine systems, is dependent, among other factors, on the polarity of the solvent. The formation of a proton transfer complex is accompanied by a decrease in free energy of 2-6 kcal/mole. Based on spectrophotometric studies of the base dependent ionization of several phenol derivatives in aqueous dioxane solutions, it is likely that proton donor sidechain groups possessing a pKa in the range 3-10, can participate in proton transfer complexes with basic groups whose conjugate acids possess a greater pKa, in nonpolar and partially polar regions of a protein. It is suggested, that the decrease in free energy accompanying the formation of a proton transfer complex can affect the direction of a protein conformational transition. © 1969.