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On the molecular mechanisms of solar energy storage during the photocycle of the other photosynthetic system in nature, bacteriorhodopsin
Author(s) -
ElSayed M. A.
Publication year - 1988
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.560340841
Subject(s) - bacteriorhodopsin , deprotonation , chemistry , photochemistry , protonation , chromophore , proton , halobacteriaceae , molecule , halobacterium salinarum , membrane , organic chemistry , ion , biochemistry , physics , quantum mechanics
Bacteriorhodopsin, bR, the other photosynthetic system in nature (besides chlorophyll) absorbs light and undergoes a cycle involving intermediates appearing in different time domains (from fractions of picoseconds to several milliseconds). As a result, the protonated Schiff base (PSB) deprotonates, and protons are pumped from inside to the outside of the cell membrane, creating proton gradients that are energetically responsible for making ATP. Using time‐resolved Raman and optical spectroscopic experiments, the following results and possible conclusions are obtained. (1) Unlike chlorophyll, bR does not use an antenna system, i.e., each absorbing molecule is a reaction center. (2) Isomerization of its retinal chromophore, which leads to the first step in energy storage by charge separation, occurs on the subpicosecond time scale. (3) The deprotonation of the PSB and a tyrosine, which occur on the 40 μs time scale, is found to have activation energies comparable to H‐bond energies. This, together with the fact that the temporal quenching of the tryptophan fluorescence follows the time profile of the deprotonation strongly suggests that the latter process is controlled by protein conformation changes. (4) Cations are found to be required for the deprotonation process and are believed to control the protein conformation required for this process. Possible mechanisms responsible for the decrease in the pK a of the PSB from 13.3 to <2.6 during the photocycle, and are thus responsible for the deprotonation process, are discussed.