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 sulfur bacterium Allochromatium minutissimum. Both LH2 complexes were nonamers and had the same elution volume V, coinciding with that for the LH2 complex of Rhodoblastus acidophilus (strain 10050). Their molecular weight was 150 kDa. Both pheophytinization of bacteriochlorophyll (BChl) at low pH and treatment with the detergent LDAO, which affects 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 weights corresponding to one-third of the initial value. Fragmentation caused by both pheophytinization and detergent treatment was much more rapid in DPA 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 weight formed at pH 8.0 in the presence of the detergent contained monomeric BChl. The observed fragmentation was hypothesized to reflect the inherent C symmetry of the LH2 complexes, with the preliminarily assembled trimers used as building blocks. [ABSTRACT FROM AUTHOR]