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

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 3 Car 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 3 Car 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 3 Car 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 3 Car, in the context of the equilibrium 3 P ← 3 D where 3 P is the localized triplet state of P‐870 and 3 D is another triplet state. A reasonable kinetic scheme leads us to estimate that 3 D is 0.0025 ± 0.005 eV above 3 P. 3 D 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.