Effect of Cofactor Structure on Control of Electron Transfer Rates at the QA Site of the Reaction Center Protein

Abstract

The bacterial photosynthetic reaction center protein (RC) catalyzes the conversion of light to electrochemical energy through a sequence of photon- initiated electron transfer reactions between redox cofactors held at fixed distances in the protein [1–4]. The primary processes are elicited by absorption of a photon by the primary donor, a dimer of bacteriochlorophyll ([BChl]2). The first excited singlet state of the dimer, [BChl]2*, transfers an electron over 10 A in 3 ps to a bacteriopheophytin (BPh). The BPh− in turn reduces a quinone bound at the QA site to form an anionic semiquinone in 0.2 ns (for a review, see: [5]). Numerous experimental efforts have aimed to identify the factors which control the remarkable near unit quantum yield and temperature independence of the RC processes [6–9], often with an eye toward emulation in artificial photosynthetic devices [10]. Here, we examine the role of structural components of the quinone cofactor in determining electron transfer rates at the RC QA site.