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Pigment–pigment interactions in thylakoids and LHCII of chlorophyll a/c containing alga Pleurochloris meiringensis: analysis of fluorescence-excitation and triplet-minus-singlet spectra

1998; Elsevier BV; Volume: 54; Issue: 5 Linguagem: Inglês

10.1016/s1386-1425(98)00006-7

1873-3557

Claudia Büchel, K. Razi Naqvi, T.B. Melø,

Marine and coastal ecosystems

Time-resolved triplet-minus-singlet (TmS) difference spectra, ΔA(λ; t), fluorescence excitation spectra, X(λ), and absorption spectra, A(λ), are used for probing pigment–pigment interactions in the thylakoids (Chla/c-Thyl) and isolated light-harvesting complexes associated with photosystem II (Chla/c-LHCII) of the alga Pleurochloris meiringensis, whose chromophores comprise chlorophyll a (Chla), chlorophyll c (Chlc), and several carotenoids. The data provide information about interactions between Car*-and-Chla0, Chla†-and-Car0, Car†-and-Chla0 (where the abbreviation Car stands for carotenoid, an asterisk and a dagger denote singlet and triplet excitation, respectively, and the superscript 0 denotes a molecule in the ground state). In Chla/c-Thyl, the efficiency of Car*→Chla* transfer (φLH), determined by comparing A(λ) and X(λ), is slightly less than unity (ca. 0.85), whereas the efficiency of Chla†→Car† transfer of triplet energy (φTT) must be much closer to unity, since no long-lived Chla† could be detected; an interaction between Car† and Chla0, already familiar from investigations concerning the TmS spectra of the trimers and aggregates of Chla/b-LHCII (the light-harvesting complex associated with the photosystem II of higher plants), which manifests itself through a depletion signal (in the Qy region of Chla) decaying at the same rate as the Car TmS signal, is observed, and explained likewise. In Chla/c-LHCII, both efficiencies are found to be much lower; the drastic reduction in the two yields is attributed to the perturbation of the native molecular architecture of the complex by the detergent used in the isolation procedure. The overall TmS signal from Chla/c-LHCII can be decomposed into two contributions, ΔA(λ; t)=Δ1A(λ; t)+Δ2A(λ; t), where Δ1A(λ; t) with a lifetime of about 8 μs; Δ2A(λ; t), which persists for several hundred microseconds, is contributed by those Chla† molecules which fail to transfer their excitation to a Car neighbour. A comparison of Δ1A(λ; t) with the TmS signal of thylakoids shows differences which parallel those previously reported for the TmS spectra of trimers and aggregates of Chla/b-LHCII: the carotenoid peak at 510 nm is broader, and the Qy depletion signal larger, in the difference spectrum of thylakoids. The absorption spectrum of Chla/c-LHCII show no signs of Chla–Chla excitonic interactions, since the Chla-contribution to the spectrum can be reproduced well by simply red-shifting (by about 200 cm−1) the Q bands and the Soret band in the absorption spectrum of an ethanolic solution of Chla, an observation consistent with the absence, reported in a recent study, of excitonic bands in the circular dichroism spectrum of Chla/c-LHCII.