The reaction order of the dissociation reaction of the B820 subunit of Rhodospirillum rubrum light-harvesting I complex.

We have studied the equilibrium between the dissociated B777 form (absorbing at 777 nm) of the light-harvesting complex of Rhodospirillum rubrum and the oligomeric B820 form. Analysis of the reaction order for the B820 dissociation reaction to form B777 shows that this reaction depends on the concentration of octylglucoside detergent (n-octyl-beta-D-glucopyranoside (betaOG)) present in the sample. At low betaOG concentrations (less than 1.2%) this reaction requires two components, presumably one alpha-B777 and one beta-B777, implying that the B820 subunit is a dimer. At higher betaOG concentrations this reaction requires four components, implying that B820 is a tetramer. These results partly explain the discrepancies in the literature about the stoichiometry of B820 and open an original way for studying protein-detergent interactions.