More on the catalysis of internal conversion in chlorophyll a by an adjacent carotenoid in light-harvesting complex (Ch1a/b LHCII) of higher plants: time-resolved triplet-minus-singlet spectra of detergent-perturbed complexes

Wavelength-selective photo-excitation of samples containing a detergent and LHCII (the main light-harvesting complex pertaining to photosystem II of g reen plants) is used for recording time-resolved triplet-minus-singlet (TmS ) difference spectra, with a view to probing interactions between chlorophy ll n (Chla) and chlorophyll b (Chlb), and between Chla and lutein (Lut). On ce the detergent concentration (C-D) exceeds a threshold, Cc, the TmS spect rum becomes sensitive to lambda x, the wavelength of excitation, and to t, the delay between excitation and observation. Each increment in C-D brings about a diminution in the efficiency of a(dagger -->) x(dagger) transfer (t riplet-triplet transfer from Chla to Lut) and a rise in both the triplet fo rmation yield and the fluorescence yield of Chla. What is more, b* --> a* t ransfer (singlet-singlet transfer from Chlb to Chla) slackens to such an ex tent that Chlb* --> Chlb(dagger) intersystem crossing, negligible when C-D is below Cc, begins to vie with transfer, for the deactivation of Chlb* (in the foregoing an asterisk/dagger denotes singlet/triplet excitation). The reduction in the efficiencies of the two transfers is easily understood by: (i) invoking the Kuhlbrandt-Wang-Fujiyoshi model of LHCII, which posits ea ch Chlb in contact with a Chla and each Chin in contact with a Lut, and (ii ) assuming that the detergent severs contact between adjacent chromophores. That a growth in the triplet yield of Chla* accompanies the detergent-indu ced decrease in the efficiency of a(dagger) --> x(dagger) transfer becomes intelligible if one assumes, further, that internal conversion in <(Chla)un der bar>* is faster than that in <(Chla)over bar>*, where under or over lin ing betokens the presence or absence of a carotenoid neighbour. When C-D is close to Cc, most Chla molecules are adjacent to a Lut, internal conversio n dominates, and the overall triplet yield is low. As C-D is gradually rais ed the <(Chla)under bar> --> <(Chla)over bar>Chla transformation sets in, c ausing concomitant drops in the efficiencies of a(dagger) --> x(dagger) tra nsfer and internal conversion, and a consequent rise in the overall yields of Chla fluorescence and formation of Chla triplets. (C) 1999 Elsevier Scie nce B.V. All rights reserved.