Abstract
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 854 nm. 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-1 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.
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Abbreviations
- BChl:
-
bacteriochlorophyll a
- BPheo:
-
bacteriopheophytin a
References
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Sturgis, J.N., Niederman, R.A. Acid denaturation of the B875 light-harvesting complex in membranes of Rhodobacter sphaeroides . Photosynth Res 23, 241–248 (1990). https://doi.org/10.1007/BF00034854
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DOI: https://doi.org/10.1007/BF00034854