[Formation of 55-kDa Fragments under Impaired Coordination Bonds and Hydrophobic Interactions in Peripheral Light-Harvesting Complexes Isolated from Photosynthetic Purple Bacteria].

Size exclusion chromatography was used to assess the relative size of intact and diphenylamine-treated (DPA, with suppressed carotenoid synthesis) peripheral light-harvesting complexes (LH2 complexes) of the sulfurbacterium Allochromatium minutissimum. Both LH2 complexes were nonamers and had the same elution volume V(e), coinciding with that for the LH2 complex of Rhodoblastus acidophilus (strain 10050). Their molecular mass was 150 kDa. Bot pheophytinization of bacteriochlorophyll (BChl) at low pH and treatment with the detergent LDAO, affecting the hydrophobic interactions between the neighboring protomers, result in the fragmentation of the ring of the isolated LH2 complexes and formation of 55-kDa fragments with molecular masses corresponding to one-third of the initial value. Fragmentation caused by both pheophytinization and detergent treatment was much more rapid in DPA-treated LH2 complexes than in the intact ones. The 55-kDa fragments formed at low pH values contained monomeric bacteriopheophytin, while the fragments of a similar molecular mass formed at pH 8.0 in the presence of the detergent contained monomeric BChl. The observed fragmentation was hypothesized to reflect the inherent C3 symmetry of the LH2 complexes, with the preliminarily assembled trimers used as building blocks.

Orientation and linear dichroism of the reaction centers from Rhodopseudomonas sphaeroides R-26.

Linear dicroism of chromatophores and isolated reaction centers from the photosynthetic bacterium Rhodopseudomonas sphaeroides strain R-26 was studied using a novel technique of orientation. The results are discussed in view of the reaction center structure and its position in the membrane. The advantages of the new orientation technique are also outlined.