Electron transport and triplet formation in membrane fragments of the green sulfur bacterium Prosthecochloris aestuarii

Abstract

Under conditions where normal electron transport is blocked, at high pH in the presence of dithionite, three triplets could be observed upon flash illumination of membrane fragments of the green sulfur bacterium Prosthecochloris aestuarii. The first triplet decayed in 7.5 μs and is assigned to carotenoid. The second triplet decays in 67 μs and is assigned to bacteriochlorophyll (BChl) a of the Fenna-Matthews-Olson (FMO) complex, since a triplet with the same spectrum and lifetime was also formed in the isolated FMO complex. The third triplet decayed in 165 μs and is assigned to BChl a of the reaction center core complex, based on its main bleaching at 837 nm. There is insufficient evidence to decide whether this triplet is located on the primary electron donor P840 or on a long-wavelength antenna BChl a of the core. A multiple-flash experiment indicated the presence of two photo-oxidizable hemes per reaction center (RC), both having a difference spectrum centered around 553 nm. The oxidation time was 25 μs for both cytochromes. However, a 75- μs delay was observed for the oxidation of the second heme, indicating that another process must take place before this reaction can occur. This result, together with the observed low efficiency of oxidation of the second heme, suggests the presence of a four-heme cytochrome (as observed in some other species of green sulfur bacteria), with only one heme in direct contact with the RC, rather than a model with two cytochromes symmetrically attached to the RC, as proposed by others. The observed delay can then be explained by a relatively slow heme-to-heme electron transfer. The cytochrome oxidation time of both hemes increased with viscosity, suggesting that some molecular motion is involved in the oxidation process.