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

Journal Article · Thu Nov 20 00:00:00 EST 1986 · J. Phys. Chem.; (United States)

DOI:https://doi.org/10.1021/j100282a024· OSTI ID:7038686

Merz, H; Tangermann, U; Zundel, G

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 basis 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.

• Research Organization: Universitaet Muenchen, West Germany
• OSTI ID: 7038686
• Journal Information: J. Phys. Chem.; (United States), Vol. 90:24
• Country of Publication: United States
• Language: English

Related Subjects

14 SOLAR ENERGY
59 BASIC BIOLOGICAL SCIENCES
ACETATES
THERMODYNAMIC PROPERTIES
PHENOL
RHODOPSIN
PHOTOSYNTHESIS
CHEMICAL BONDS
EXPERIMENTAL DATA
HYDROGEN TRANSFER
INFRARED SPECTRA
PH VALUE
POLARIZATION
RADIANT FLUX DENSITY
THERMODYNAMICS
AROMATICS
CARBOXYLIC ACID SALTS
CHEMICAL REACTIONS
DATA
FLUX DENSITY
HYDROXY COMPOUNDS
INFORMATION
NUMERICAL DATA
ORGANIC COMPOUNDS
PHENOLS
PHOTOCHEMICAL REACTIONS
PHYSICAL PROPERTIES
PIGMENTS
PROTEINS
SPECTRA
SYNTHESIS
140505* - Solar Energy Conversion- Photochemical
Photobiological
& Thermochemical Conversion- (1980-)
550500 - Metabolism