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

Abstract

Abstract— In the reaction center of photosynthetic bacteria, with the primary ubiquinone reduced, the triplet state PR 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 PR and 3Car in the reaction center of several strains of R. sphaeroides. The rate of triplet energy transfer, measured as the decay of PR 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 PR (about 100 μs). In several cases it is clear that PR 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 PR 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.