Charge recombination from the P+QA- state in reaction centers from Rhodopseudomonas viridis

Abstract

The rate of decay of the flash-oxidized primary electron donor, P+, from the state P+QA- was studied in reaction centers from Rhodopseudomonas viridis, containing only the primary menaquinone electron acceptor (QA). At 295 K, in 100 mM NaCl and in the presence of o-phenanthroline, the rate of recombination was 470 +/- 15 s-1 at pH 7 and 570 +/- 20 s-1 at pH 9. The rate at ambient temperatures varied somewhat with viscosity, pH and ionic strength. Between 310 K and 275 K, the temperature dependences of the rate, at pH 7 and pH 9, were linear in an Arrhenius plot, with apparent activation energies of 0.20 eV and 0.16 eV, respectively. At lower temperatures, however, the dependences deviated from this behavior. In 60% glycerol (pH 7) the recombination rate was 370 +/- 10 s-1 at 295 K. As the temperature was lowered, the rate decreased but leveled off to a value of 105 +/- 5 s-1 at 170 K and was independent of temperature from 170 K to 100 K. In 60% ethylene glycol, the temperature dependence was similar, but the rate fell to a minimum of 75 s-1 at 170 K and then increased slightly at lower temperatures; it finally became temperature independent, with a value of about 100 s-1, at 110 K. The overall temperature dependence is consistent with charge recombination by two competing pathways: a direct electron-tunneling process which dominates at low temperature (less than 250 K), and a thermally activated process via a higher energy state, M, which decays rapidly to the ground state. The indirect route dominates at high temperature (above 250 K). Taking into account the contribution from the low-temperature pathway, the activation energy (enthalpy) for the activated process, in aqueous buffer, was determined to be 0.25 eV (at pH 7) and 0.19 eV (at pH 9). A likely candidate for M is P+I- (PF), where I is the intermediate bacteriopheophytin electron acceptor, and energetic arguments are presented in favor of this assignment. If a rate of decay of P+I- to the ground state, derived from the experimental value, was used in the description of the thermally activated P+QA- recombination process, the free-energy gap separating M and P+QA- could be estimated to be 0.27-0.28 eV, placing it about 0.95 eV above the ground state and 0.30 eV below the excited singlet state, P*.(ABSTRACT TRUNCATED AT 400 WORDS)