Electrochemical oxidation of bacteriochlorophyll a in reaction centers and antenna complexes of photosynthetic bacteria

The oxidation of BChl in several pigment-protein complexes involved in bacterial photosynthesis was investigated by optical spectroelectrochemistry, with the aim to obtain information on the tuning of the BChl oxidative midpoint potential (E-m) through BChl-BChl and BChl-protein interactions. The two accessory BChls in reaction centers (RCs) of Rhodobacter sphaeroides (R-26 and wild-type) have E-m of 0.73V and 0.83V (+/-0.01V), appreciably higher than the E,, fur pentacoordinated monomeric BChl in nonprotic solvents (similar to0.66V). The self-aggregation of BChl in vivo leads to a lowering of the oxidation potential: E-m = 0.50 V (the primary donor P865 in Rb. sphaeroides RCs), E-m = 0.57 V (the light-harvesting complex LH1 of Rhonopseudomonas acidophila), E-m = 0.60V (LH1 complex of Rb. sphaeroides). Only 20-35% of the total absorbance at 875 (880) nm in LH1 complexes corresponds to reversible BChl oxidation, whereas further bleaching is irreversible. Complete irreversibility of BChl oxidation was observed in LH2, complexes of Rps. acinophila for both monomeric (B800) and aggregated (B850) pigments, with the B850 component bleached somewhat more easily than the B800 component. One of the products of degradative oxidation is 2-desvinyl-2-acetyl Chi. The following order of susceptibility to oxidation is observed: P865 (RCs) > B880 (LH1) similar or equal to B875 (LH1) > B850 similar or equal to B800 (LH2) > B800 (RCs). It is suggested that irreversible oxidation of BChl in antenna complexes results from dismutation of BChl and carotenoid (Car) cation pairs (BChl (+)- BChl(+) and BChl(+)- Car(+)), resulting in the formation of the highly reactive BChl dication. The remarkable difference in LH1 and LH2 oxidation could be due to the presence of oxidized Car in LH2 at early oxidation stages of BChl, The results are discussed in terms of the X-ray diffraction structures of the RC and antenna complexes.