Protein's role in triplet energy transfer in bacterial reaction centers

摘要

When photosynthetic organisms are subjected to high-light conditions in nature,211u001eelectron transfer becomes blocked as the rate of conversion of light into charge-211u001eseparated states in the reaction center (RC) exceeds the capacity of the soluble 211u001ecarriers involved in cyclic electron transfer. In that event, a well-211u001echaracterized T(sub 0)-polarized triplet state (sup T)P, is formed on the primary 211u001edonor, P, from the P(sup +)H(sub A)(sup (minus)) state (reviewed in (1)). In an 211u001eaerobic or semi-aerobic environment, the major role of the carotenoid (C), also 211u001ebound by the RC, is to quench (sup T)P prior to its sensitization of the (sup 211u001e1)(Delta)(sub g) singlet state of oxygen--a potentially damaging biological 211u001eoxidant. The carotenoid performs this function efficiently in most bacterial RCs 211u001eby rapidly accepting the triplet state from P and dissipating this excited-state 211u001eenergy into heat through internal conversion. The lowest-lying triplet states of 211u001eP and the carotenoid are sufficiently different that (sup T)P can promote oxygen 211u001eto its excited singlet state whereas (sup T)C can quench the (sup T)P state 211u001e(reviewed in (2)).