Hyperfine structure of the photoexcited triplet state 3P680 in plant PSII reaction centres as detennined by pulse ENDOR spectroscopy

The triplet states in plant photosystem 11 (PS 11), (3)P680, and from chlorophyll a, (3)Chl a, in organic solution have been investigated using pulse ENDOR combined with repetitive laser excitation at cryogenic temperature with the aim to obtain their hyperfine (hf) structure. The large zero field splitting (ZFS) tensor of (3)P680 enabled orientation selection via the electron spin resonance (EPR) field setting along the ZFS tensor axes. ENDOR spectra have been obtained for the first time also for the in-plane X- and Y-orientations of the ZFS tensor. This allowed a full determination of the hf-tensors of the three methine protons and one methyl group of (3)P680. Based on the orientations of the axes of these hf-tensors, a unique orientation of the axes of the ZFS tensor of (3)P680 in the Chl a molecular frame was obtained. These data serve as a structural basis for determining the orientation of (3)P680 in the PS 11 protein complex by EPR on single crystals (see M. Kammel et al. in this issue). The data obtained represent the first complete set of the larger hf-tensors of the triplet state (3)P680. They reflect the spin density distribution both in the highest occupied (HOMO) and lowest unoccupied (LUMO) orbitals. The data clearly confirm that (3)P680 is a monomeric Chl a species at low temperature (T=10 K) used, as has been proposed earlier based on D- and E-values obtained from EPR and optically detected magnetic resonance (ODMR) studies. Comparison with the hf data for the cation and anion radicals of Chl a indicates a redistribution of spin densities in particular for the LUMO orbital of the triplet states. The electron spin distribution in the LUMO orbital is of special interest since it harbours the excited electron in the excited P680 singlet state, from which light-induced electron transfer proceeds. Observed shifts of hf couplings from individual nuclei of (3)P680 as compared with (3)Chl a in organic solution are of special interest, since they indicate specific protein interactions, e.g. hydrogen bonding, which might be used in future studies for assigning (3)P680 to a particular chlorophyll molecule in PS II. (C) 2003 Elsevier B.V All rights reserved.