Solvent facilitated proton transfer in hydrogen bonded complexes of proton donor and acceptor groups in model protein systems

Abstract

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 p K a in the range 3–10, can participate in proton transfer complexes with basic groups whose conjugate acids possess a greater p K a, 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.