Multichannel flash spectroscopy (with microsecond time resolution) has been applied to carotenoid (Car)-containing and Car-less reaction centers (RC) of Rhodobacter sphaeroides with a view to investigate the interaction between the Car and its neighboring pigments at room temperature. Under neutral redox potential conditions, where the primary quinone acceptor (QA) is oxidized, the light-induced spectral changes in the 350–1000 nm region are attributed to the photochemical oxidation of the special pair (denoted here as P870), the generation of P 870Q−A, and the attendant electrochromism of adjacent chromophores. A bathochromic shift of <1 nm in the visible absorption region of Car reveals the sensitivity of Car to the P870 photooxidation. Under low redox potential conditions, where QA is reduced, P870 triplets (P†870) are formed. The time-resolved triplet-minus-singlet (TmS) spectrum of Car-less RC shows a deep bleaching at 870 nm, which belongs to P†870, and additional (but smaller) bleaching at 800 nm; the entire spectrum decays at the same rate (with a lifetime of about 50 μs). The bleaching at 800 nm arises from the pigment interaction between P†870 and the accessory bacteriochlorophylls on A and B branches (BA,B). In Car-containing RC, the TmS spectra of Car are accompanied by two smaller, negative signals—a sharp peak at 809 ± 2 nm and a broad band at 870 nm—which decay at the same rate as the TmS spectrum of Car (ca 10 μs). The former is ascribed to the perturbation, by Car†, of the absorption spectrum of BB; the latter, to the TmS spectrum of P†870, a species that appears to be in approximate thermal equilibrium with Car†. These assignments are consistent with the absorption-detected magnetic resonance spectra obtained by other workers at low temperatures.
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Vol. 79 • No. 1