Reaction center bacteriochlorophyll triplet states: Redox potential dependence and kinetics

Abstract

Triplet states of reaction center (bacterio-) chlorophylls have been observed by low temperature EPR in a wide variety of photosynthetic preparations ranging from whole cells to purified reaction centers. The reaction center triplet state is seen under conditions where electron transfer from the reaction center chlorophyll is prevented by reduction of the primary electron acceptor. The redox potential dependence shows one-electron titrations with midpoint potentials of −50 mV in Rhodopseudomonas spheroides and approx. −120±20 mV in R. rubrum and Chromatium D. Kinetic measurements with bacterial reaction centers show an approx. 6-μs triplet decay time. Observed zero-field splitting parameters D and E are appreciably lower in the reaction center triplets than in isolated chlorophyll, suggesting that the reaction center photo-active (bacterio-) chlorophyll is not monomeric. EPR lineshapes of the triplet signals (equal mixtures of microwave absorption and emission) show almost complete spin polarization demonstrating that inter-system crossing is a highly selective process.