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

K. Razi Naqvi, Tamás Jávorfi, T. B. Melø, Gyözö Garab

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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 (Chla) and chlorophyll b (Chlb), and between Chla and lutein (Lut). Once the detergent concentration (CD) 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 CD brings about a diminution in the efficiency of a† → cursive chi† transfer (triplet-triplet transfer from Chla to Lut) and a rise in both the triplet formation yield and the fluorescence yield of Chla. What is more, b* → a* transfer (singlet-singlet transfer from Chlb to Chla) slackens to such an extent that Chlb* → Chlb† intersystem crossing, negligible when CD is below C©, 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 Kühlbrandt-Wang-Fujiyoshi model of LHCII, which posits each Chlb in contact with a Chla and each Chla 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-induced decrease in the efficiency of a† → cursive chi† 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 CD is close to C©, most Chla molecules are adjacent to a Lut, internal conversion dominates, and the overall triplet yield is low. As CD is gradually raised the Chla → Chla transformation sets in, causing concomitant drops in the efficiencies of a† → cursive chi† transfer and internal conversion, and a consequent rise in the overall yields of Chla fluorescence and formation of Chla triplets.

Original languageEnglish
Pages (from-to)193-204
Number of pages12
JournalSpectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy
Volume55
Issue number1
Publication statusPublished - Dec 1 1998

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Keywords

  • Carotenoids
  • Chlorophyll fluorescence quenching
  • Light-harvesting
  • Photosynthesis
  • Triplet-triplet transfer

ASJC Scopus subject areas

  • Analytical Chemistry
  • Atomic and Molecular Physics, and Optics
  • Instrumentation
  • Spectroscopy

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