Dynamics of the primary events in bacterial photosynthesis
This paper considers the primary light-induced electron-transfer (ET) processes in the reaction center of photosynthetic bacteria which involved ET from the electronically excited state of the bacteriochlorophyll a dimer (BChL)/sub 2/ to bacteriopheophytin (BPh) ((I.1)) and ET from BPh/sup -/ to ubiquinone (Q) ((I.2)). Ultrafast reactions I.1 and I.2, which are practically temperature independent over the range 4 to 300 K, cannot be accounted for in terms of low-temperature nuclear tunneling through a nuclear barrier. Two mechanisms for ultrashort, temperature-independent processes I.1 and I.2 were examined. The rate of the ET reaction (I.2) is considerably longer than characteristic medium-induced vibrational relaxation rates, so that process I.2 has to occur from a thermally equilibrated nuclear configuration of BPh/sup -/Q. Reaction I.2 is assigned to an activationless nonadiabatic ET process, the short lifetimes for this reaction stemming from a large value of the electronic coupling V approximately equal to 4 cm/sup -1/. We propose that the ultrafast reaction (I.1) occurs from a nonequilibrium nuclear configuration of the (BChl)/sub 2/*BPh initially excited state which is located above the crossing point of the nuclear potential surfaces for (BChl)/sub 2/*BPh and for (BChl)/sub 2//sup +/BPh/sup -/. Such a novel ET mechanism involves competition between ET and vibrational relaxation. A theory has been developed to handle this problem and applied to reaction I.1. A microscopic molecular scheme for the primary events of charge separation in bacterial photosynthesis is proposed, which rests on the optimization of the intramolecular distortions of the equilibrium nuclear configurations and the intermolecular spatial organization of the donor and the acceptor. The molecular scheme is successful in accounting for the qualitative and the quantitative features of the primary ET rates.
- Research Organization:
- Tel-Aviv Univ., Israel
- OSTI ID:
- 6786694
- Journal Information:
- J. Am. Chem. Soc.; (United States), Vol. 102:22
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
CHLOROPHYLL
BIOCHEMICAL REACTION KINETICS
PHOTOSYNTHETIC BACTERIA
PIGMENTS
QUINONES
COUPLING
ELECTRON TRANSFER
EXCITED STATES
LOW TEMPERATURE
RELAXATION
AROMATICS
CARBOXYLIC ACIDS
ENERGY LEVELS
HETEROCYCLIC ACIDS
HETEROCYCLIC COMPOUNDS
KINETICS
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
ORGANIC OXYGEN COMPOUNDS
PHYTOCHROMES
PORPHYRINS
REACTION KINETICS
550200* - Biochemistry