Kinetics and Mechanisms of Initial Electron-Transfer Reactions in Rhodopseudomonas sphaeroides Reaction Centers

Abstract

The reaction center (RC) of the photosynthetic bacterium Rhodopseudomonas sphaeroides consists of three small polypeptides that hold a cluster of four molecules of bacteriochlorophyll-a (BChl), two molecules of bacteriopheophytin-a (BPh), one nonheme iron atom, and two molecules of ubiquinone [1, 2]. The RCs can be purified readily, and they recently have been crystallized [3], Although the crystal structure has not yet been solved, RCs from Rps. sphaeroides are functionally very similar to those from Rps. viridis whose structure has been solved to 3 Å resolution [4]. In both species, two of the four BChls form a closely interacting pair (P) that undergoes oxidation to a radical cation (P⁺) when the RC is excited with light. The electron is transferred to a BPh, and from there to a quinone (Q) that is close to, but probably not bound to the Fe. (In Rps. vi ridis, Q is menaquinone instead of ubiquinone.) In Rps. sphaeroides RCs, the photochemical electron-transfer reaction from P to Q has a quantum yield of essentially 100% [5], The kinetics of electron-transfer from the intermediate BPh anion radical (BPh⁻) to Q have been measured under a variety of conditions, and are similar in the two species, in spite of the difference between the qui nones [6–10], It has been suggested that one of the two BChls that are not part of P mediates electron-transfer from the excited singlet state of P (P*) to the BPh [10–12]. However, the evidence for the formation of a P⁺ BChl⁻ radical pair prior to the formation of P⁺ BPh⁻ is inconclusive [13].