THE primary light-induced electron transfer events of both green plant and bacterial photosynthesis result in very rapid charge separation which subsequently generates a chemical potential within the organism. There is much information concerning the primary events of bacterial photosynthesis but for green plants the details of these processes are just beginning to become available. Strong evidence based mainly on electron spin resonance and electron-nuclear double resonance studies of oxidised bacterial reaction centres favours a dimeric bacteriochlorophyll a (BChl a) structure for the primary photochemical electron donor1. The dimeric BChl a species absorbs light at 865nm in reaction centres from Rhodopseudomonas sphaeroides yielding an excited state which rapidly transfers an electron to a molecule of bacteriopheophytin a (BPh a) in <10 ps (refs 2-4). Within about 150 ps the electron is transferred to a quinone molecule which in turn transfers an electron to a secondary quinone within a few microseconds5,6. In reaction centres for which the quinones have been either extracted or chemically reduced before light excitation, BPh- back transfers an electron to (BChl a)2+ in 10-20 ns (ref. 7). Thus, the in vivo geometry of the reaction centre is such that the reverse electron transfer is 5-10 × 103 times slower than the forward reaction. We now report the first in vitro model that duplicates both the rapid light-induced charge transfer and the slow back reaction of the reaction centre.
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