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Multichannel Flash Spectroscopy of the Reaction Centers of Wild-type and Mutant Rhodobacter Sphaeroides: BacteriochlorophyllB-mediated Interaction Between the Carotenoid Triplet and the Special Pair

AuthorsArellano, Juan B. CSIC ORCID ; Melo, Thor Bernt; Fyfe, Paul K.; Cogdell, Richard J.; Naqvi, K. Razi
KeywordsReaction center
Rhodobacter sphaeroides
Triplet energy transfer
Issue DateJan-2004
PublisherAmerican Society for Photobiology
CitationPhotochemistry and Photobiology
(2004) 79(1):68-75
AbstractMultichannel 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 P870(+)QA(-), 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 (P870(+)) are formed. The time-resolved triplet-minus-singlet (TmS) spectrum of Car-less RC shows a deep bleaching at 870 nm, which belongs to P870(+), and additional (but smaller) bleaching at 800 nm; the entire spectrum decays at the same rate (with a lifetime of about 50 micros). The bleaching at 800 nm arises from the pigment interaction between P870(+) 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 micros). The former is ascribed to the perturbation, by Car(+), of the absorption spectrum of BB; the latter, to the TmS spectrum of P870(+), 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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