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

Abstract

Certain redox properties of bacteriochlorophyll alpha were used to probe the structure of several light-harvesting pigment-protein complexes or holochromes. To attribute redox properties unequivocally to a given holochrome, we worked with purified holochromes. We developed purification procedures for the B880 holochromes from Rhodospirillum rubrum, Rhodopseudomonas sphaeroides and Ectothiorhodospira sp. and for the B800-850 holochromes from the latter two species. In all these holochromes, bacteriochlorophyll alpha could be oxidized by ferricyanide as witnessed by the bleaching of their near-infrared 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 electron paramagnetic resonance (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 approximately 570 mV. Linewidth narrowing can be interpreted by delocalization of the free electron spin over approximately 12 bacteriochlorophyll molecules. While the B880 holochromes from the three species 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.