Thermodynamics of proton transfer in phenol-acetate hydrogen bonds with large proton polarizability and the conversion of light energy into chemical energy in bacteriorhodopsin

Abstract

Phenol-acetate solutions in CCl/sub 4/ are studied by IR spectroscopy as a function of the pK/sub a/ of the phenols. The (I) Ar-OH.../sup -/OC in equilibrium Ar-O/sup -/...HOC (II) hydrogen bonds formed show large proton polarizability as indicated by continua in the IR spectra. The percent proton transfer (PT) increases from the p-cresol-acetate to the pentachlorophenol-acetate system from 0% to 56%. The Gibbs free energy, ..delta..G/sup 0//sub PT/, values of the PT equilibria at 295 K are determined as well as the standard enthalpy values, ..delta..H/sup 0//sub PT/, and the standard entropy values, ..delta..S/sup 0//sub PT. The shape of the intensity of the continuum as a function of the ..delta..H/sup 0//sub PT/ value changes. In the classical approximation the average difference between the two minima of the proton double-minimum potential is given by ..delta..H/sup 0//sub PT/. With the decreasing amount of ..delta..H/sup 0//sub PT/, i.e., decreasing degree of asymmetry, the intensity of the continua decreases at higher and increases at lower wavenumbers. This result is in good agreement with the predictions from calculated line spectra. In the photocycle of bacteriorhodopsin a tyrosine-aspartate hydrogen bond is probably of importance for the conversion of light energy into chemical energy. On the basismore » of the obtained data it is shown that 9.5 kJ/mol can be converted into chemical energy due to a proton transfer induced by a local electrical field in a Tyr-Asp hydrogen bond. Furthermore, if the Ar-O/sup -/...HOC structure is broken afterwards by a conformation change, at least 25 kJ/mol of conformation energy is converted. Thus, altogether 34.5 kJ/mol of Gibbs free energy may be converted into chemical energy and stored by these processes.« less