PIGMENT-PIGMENT INTERACTIONS IN THYLAKOIDS AND LHCII OF CHLOROPHYLL AC CONTAINING ALGA PLEUROCHLORIS-MEIRINGENSIS - ANALYSIS OF FLUORESCENCE-EXCITATION AND TRIPLET-MINUS-SINGLET SPECTRA/

Time-resolved triplet-minus-singlet (TmS) difference spectra, Delta A( lambda; t), fluorescence excitation spectra, X(lambda), and absorption spectra, A(lambda), are used for probing pigment-pigment interactions in the thylakoids (Chla/c-Thyl) and isolated light-harvesting complex es associated with photosystem II (Chla/c-LHCII) of the alga Pleurochl oris meiringensis, whose chromophores comprise chlorophyll a (Chla), c hlorophyll c (Chlc), and several carotenoids. The data provide informa tion about interactions between Car-and-Chla(o), Chla dagger-and-Car( o), Car dagger-and-Chla(o) (where the abbreviation Car stands for caro tenoid, 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 (ph i(LH)). determined by comparing A(lambda) and X(lambda), is slightly l ess than unity (ca. 0.85), whereas the efficiency of Chla dagger-->Car dagger transfer of triplet energy (phi(TT)) must be much closer to un ity, since no long-lived Chla dagger could be detected; an interaction between Car dagger and Chla(o), already familiar from investigations concerning the TmS spectra of the trimers and aggregates of Chla/b-LHC II (the light-harvesting complex associated with the photosystem II of higher plants), which manifests itself through a depletion signal (in the Q(y) region of Chla) decaying at the same rate as the Car TmS sig nal, is observed, and explained likewise. In Chla/c-LHCII, both effici encies are found to be much lower; the drastic reduction in the two yi elds is attributed to the perturbation of the native molecular archite cture of the complex by the detergent used in the isolation procedure. The overall TmS signal from Chla/c-LHCII can be decomposed into two c ontributions, Delta A(lambda; t) = Delta(1)A(lambda; t) + Delta(2)A(la mbda; t), where Delta(1)A(lambda; t) with a lifetime of about 8 mu s; Delta(2)A (lambda, t), which persists for several hundred microseconds , is contributed by those Chla dagger molecules which fail to transfer their excitation to a Car neighbour. A comparison of Delta(1)A(lambda ; t) with the TmS signal of thylakoids shows differences which paralle l those previously reported for the TmS spectra of trimers and aggrega tes of Chla/b-LHCII: the carotenoid peak at 510 nm is broader, and the Q(y) depletion signal larger, in the difference spectrum of thylakoid s. The absorption spectrum of Chla/c-LHCII show no signs of Chla-Chla excitonic interactions, since the Chla-contribution to the spectrum ca n be reproduced well by simply red-shifting (by about 200 cm(-1)) the Q bands and the Soret in the absorption spectrum of an ethanolic solut ion of Chla, an observation consistent with the absence, reported in a recent study, of excitonic band in the absorption spectrum of an etha nolic solution of Chla, an observation consistent with the absence, re ported in a recent study. of excitonic bands in the circular dichroism spectrum of Chla/c-LHCII. (C) 1998 Elsevier Science B.V. All rights r eserved.