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

Abstract

Wavelength-selective photo-excitation of samples containing a detergent and LHCII (the main light-harvesting complex pertaining to photosystem II of green plants) is used for recording time-resolved triplet-minus-singlet (TmS) difference spectra, with a view to probing interactions between chlorophyll a (Chl a) and chlorophyll b (Chl b), and between Chl a and lutein (Lut). Once the detergent concentration ( C D) exceeds a threshold, C ©, the TmS spectrum becomes sensitive to λ ⊗, 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 †→ x † transfer (triplet–triplet transfer from Chl a to Lut) and a rise in both the triplet formation yield and the fluorescence yield of Chl a. What is more, b*→ a* transfer (singlet–singlet transfer from Chl b to Chl a) slackens to such an extent that Chl b*→Chl b † intersystem crossing, negligible when C D is below C ©, begins to vie with transfer, for the deactivation of Chl b* (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 Kühlbrandt–Wang–Fujiyoshi model of LHCII, which posits each Chl b in contact with a Chl a and each Chl a in contact with a Lut, and (ii) assuming that the detergent severs contact between adjacent chromophores. That a growth in the triplet yield of Chl a* accompanies the detergent-induced decrease in the efficiency of a †→ x † transfer becomes intelligible if one assumes, further, that internal conversion in Chla * is faster than that in Chla * , where under or over lining betokens the presence or absence of a carotenoid neighbour. When C D is close to C ©, most Chl a molecules are adjacent to a Lut, internal conversion dominates, and the overall triplet yield is low. As C D is gradually raised the Chla → Chla transformation sets in, causing concomitant drops in the efficiencies of a †→ x † transfer and internal conversion, and a consequent rise in the overall yields of Chl a fluorescence and formation of Chl a triplets.