Oxido‐reduction of B800–850 and B880 holochromes isolated from three species of photosynthetic bacteria as studied by electron‐paramagnetic resonance and optical spectroscopy

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Abstract
Certain redox properties of bacteriochlorophyll a were used to probe the structure of several light-harvesting pigment-protein complexes or holochromes. To attribute redox properties unequivocally to a given holochrome, purified holochromes were used. Purification procedures were developed for the B880 holochromes from Rhodospirillum rubrum, Rhodopseudomonas sphaeroides and Ectothiorhodospira sp. and for the B800-850 holochromes from the latter 2 spp. In all these holochromes, bacteriochlorophyll a could be oxidized by ferricyanide as witnessed by the bleacking of their near-IR absorption bands. However, only in B880 holochromes was this oxidation reversible. Another important difference between the B800-850 and the B880 holochromes is that oxidation of the latter gives rise to a g = 2.0025 EPR signal with linewidth varying, according to species, from 0.37 mT to 0.48 mT. Both the reversible EPR signal and absorption changes titrate with a midpoint redox potential (pH 8.0) of .apprx. 570 mV. Linewidth narrowing can be interpreted by delocalization of the free electron spin over .apprx. 12 bacteriochlorophyll molecules. While the B880 holochromes from the 3 spp. considered had indistinguishable redox properties, the B800-850 holochromes differed from one another by their circular dichroic spectra and by the relative ease of oxidation of their 800-nm and 850-nm bands. This indicates that, contrary to the B880 holochromes, the B800-850 holochromes may not form a homogeneous class.