Similarities of the Primary Change Separation Process in the Photosynthesis of Rhodobacter sphaeroides and Rodopseudomanas viridis

Abstract

Photosynthetic conversion of light into chemical energy starts via a series of electron transfer reactions in pigment-protein complexes called reaction centers (RC’s). The most direct access to the primary reaction dynamics offers time resolved optical spectroscopy. During the past few years, this technique has been continuously improved permitting advanced experiments with high temporal and amplitude resolution. In this paper, we show, that RC’s from Rhodobacter (Rb.) sphaeroides and Rhodopseudomonas (Rps.) viridis exhibit common features in the absorption transients. This points to a substantial similarity of the elementary molecular processes. This fact is not self-evident, since different polypeptides and different types of bacteriochlorophylls (BChl) and bacteriopheophytins (BPh) are present in various reaction centers, e.g. BChl a and BPh a are essential pigments in the reaction centers of Rb. sphaeroides while BChl b and BPh b are active in the RC’s of Rps. viridis. For both reaction centers x-ray structures are now available /1–3/. It was shown that the prosthetic groups and neighbouring amino acids are in a very similar arrangement: most importantly are two BChl molecules in close contact which act as the primary electron donor P. The other pigments are arranged in two branches, A and B. Starting from the primary donor, the special pair P, one finds a monomeric bacteriochlorophyll (B), a bacteriopheophytin (H), and a quinone (Q) on each branch. It was shown that the electron transfer occurs via the A-branch and that after about 3 – 4 ps a radical pair P⁺H_A ⁻ is formed. Approximately 200 ps later the electron reaches the quinone Q_A building the intermediate P⁺Q_A ⁻.