The antenna system of Rhodospirillum rubrum: Radical formation upon dark oxidation of bulk bacteriochlorophyll

Abstract

11. Such near infrared changes are very similar to those observed upon light-induced or dark oxidation of photoreaction center bact- eriochlorophyll (the primary donor of bacterial photosynthesis), i.e., a bleaching at 865 nm and increase in optical density at 1245 nm [2]. Besides, the oxidized primary donor exhibits an ESR signal with a Gaussian lineshape, a g-value of 2.0025 and a peak-to-peak derivative linewidth of 9.5 G 131. In view of the similarities existing between the near infrared changes that follow oxidation of antenna and photoreaction center bacteriochlorophylls, it seemed interesting to investigate whether the oxi- dized antenna pigment is a paramagnetic species which can also be detected by ESR spectroscopy. As reported here, a nearly Gaussian ESR signal with a g-value of 2.0025 and a linewidth of 3.8 G appears to be due to the oxidized bacteriochlorophyll con- stituent which exhibits the 1230-nm band. Such a constituent seems to account only for - 1/3rd of total antenna bacteriochlorophyll and has an ap- parent midpoint redox potential of -555 mV (pH 8.0). A preliminary report of this work has been presented in [4].