Chlorophyll Triplet States Associated with Photosystem II of Thylakoids

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Abstract

The analysis of FDMR thylakoid spectra, determined at multiple emission wavelengths, by a global decomposition technique, has revealed the presence of three previously undescribed triplet populations at emission wavelengths characteristic of Photosystem II chlorophyll/protein complexes. Their zero-field splitting parameters have been determined in order to compare them with the well-studied PSII recombination triplet state. None of these triplets have the zero-field splitting parameters characteristic of the recombination triplet and are therefore probably not generated directly in the reaction center. On the basis of their microwave-induced emission spectra, it is suggested that two are probably generated in the core complex(es) while the third may be generated in the external antenna. These triplets are formed under nonreducing redox conditions, when the recombination triplet is undetectable. It is suggested that they may be involved in the photoinhibitory damage of Photosystem II. The triplet-minus-singlet spectrum associated with the recombination triplet state has been determined for thylakoids after reduction of the secondary acceptors. Its main peak is at 685 nm, slightly red shifted with respect to earlier reports, with a weak signal, of opposite sign at approximately 675 nm. The 685 nm peak indicates that at cryogenic temperatures, the triplet is located on the long-wavelength chlorophyll state present in the reaction center complex of Photosystem II (D1.D2.Cytb(559) complex). From the absence of a clear structure in the 680 nm absorption region, this long-wavelength absorbing state does not appear to be strongly coupled to P(680), though it must be associated with one of the "inner core" pigments recently identified in the photosystem II crystallographic structure [Zouni et al. (2001) Nature 408, 739-743].

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