TRIPLET FORMATION AND TRIPLET DECAY IN REACTION CENTERS FROM THE PHOTOSYNTHETIC BACTERIUM Rhodopseudomonas sphaeroides

Abstract— In the reaction center of photosynthetic bacteria, with the primary ubiquinone reduced, the triplet state P^R of the primary electron donor (a pair of bacteriochlorophylls named P) is PO ulated with a takes place in a few ns. We measured by flash absorption spectroscopy the influence of temperature on formation and decay kinetics of P^R and ³Car in the reaction center of several strains of R. sphaeroides . The rate of triplet energy transfer, measured as the decay of P^R after a flash, decreases when the temperature is lowered. Between 60 and 30 K the half-time of energy transfer becomes longer than the ³Car half-time decay (about 6 μs) and below 20 K the transfer is slower than the internal decay of P^R (about 100 μs). In several cases it is clear that P^R and ³Car decay independently and are not in thermal equilibrium. The singlet energy transfer from carotenoid to P occurs with a high efficiency at all temperatures.
The data can be accounted for on the basis of estimated energy levels of P^R and ³Car, in the context of the equilibrium ³P ←³D where ³P is the localized triplet state of P-870 and ³D is another triplet state. A reasonable kinetic scheme leads us to estimate that ³D is 0.0025 ± 0.005 eV above ³P. ³D may thus be the state observed by Shuvalov and Parson (1981). We propose that both triplet and singlet energy transfer between P and the carotenoid occur via a bacteriochlorophyll, to which the carotenoid should be tightly coupled via exchange interaction.