On the molecular mechanisms of solar energy storage during the photocycle of the other photosynthetic system in nature, bacteriorhodopsin
- Univ. of California, Los Angeles (United States)
Bacteriorhodopsin (bR) is the only protein found in the purple membrane of Halobacterium halobium, a light-harvesting bacterium. bR 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 {mu}s 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{sub a} of the PSB from 13.3 to <2.6 during the photocycle, and are thus responsible for the deprotonation process, are discussed.
- OSTI ID:
- 5681216
- Journal Information:
- International Journal of Quantum Chemistry, Quantum Chemistry Symposium; (United States), Vol. 22; ISSN 0161-3642
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
PHOTOSYNTHESIS
MOLECULAR BIOLOGY
PHOTOSYNTHETIC BACTERIA
PHYSIOLOGY
ACTIVATION ENERGY
ATP
CATIONS
PHOTOCHEMICAL REACTIONS
PROTEIN STRUCTURE
PROTONS
BARYONS
CHARGED PARTICLES
CHEMICAL REACTIONS
ELEMENTARY PARTICLES
ENERGY
FERMIONS
HADRONS
IONS
NUCLEONS
NUCLEOTIDES
ORGANIC COMPOUNDS
SYNTHESIS
551000* - Physiological Systems