The structural basis for the spectral red shift in the near-IR absorption band of the B875 light-harvesting complex was examined by treatment of membranes from Rhodobacter sphaeroides M21 with acid. This mutant strain lacks the overlapping spectral bands of the B800-850 light-harvesting antenna and gives rise to membrane fragments with both surfaces accessible to protons. At pH 2.2, about half the absorption at 876 nm was converted within 10 min to a 'free' pigment band; the remaining absorption appeared at 880 nm and shifted to ~ 845 nm over the next three hours. These spectral shifts could not be reversed by alkali. Approximately one-third of the characteristic near-IR CD signal of B875 was also lost initially and replaced by a broad trough centered near 854nm. Thereafter, the CD spectrum was dominated by the strong conservative signal of the 845 nm absorbing component which was attributed to an oligomeric bacteriopheophytin a species, probably a dimer. A kinetic analysis of the acid-induced absorption changes suggested a multi-step model with rate constants of 0.37 min-1 for the initial rapid change and 0.05 and 0.11 min-~ for the respective subsequent steps. The non-conservative nature of the near-IR CD spectrum of the intact complex, together with the spectral changes observed after the initial loss of near-IR absorption and CD, suggest that pigment-pigment interactions are not solely responsible for the red shift in this complex.