Photosystem II single crystals studied by transient EPR: the light-induced triplet state

Transient electron paramagnetic resonance (TR EPR) at 9.8 GHz has been used to study the light-induced triplet state in single crystals of Photosystem 11 (PS 11). The crystals were grown from a solution of PS 11 core complexes from the thermophilic cyanobacterium Synechococcus elongatus. The core complexes contain at least 17 subunits, including the water-oxidizing complex, and 32 chlorophyll a molecules per PS 11 complex. The PS 11 complexes are active in light-induced electron transfer and water oxidation. The crystals belong to the orthorhombic space group P2(1)2(1)2(1), with four dimers of PS 11 complexes per unit cell. Laser excitation was used to generate the recombination triplet state in PS 11 which was then studied by EPR at low temperatures (10 K). The crystal spectra show the same magnitude of the zero-field splitting (ZFS) values D, E as spectra obtained earlier for the triplet state of PS 11 in frozen solution. The orientation of the ZFS tensor D of the triplet state with respect to the crystallographic axes has been deduced from the analysis of angular-dependent EPR spectra. Knowledge of the orientation of the D tensor component perpendicular to the plane of the chlorophyll (D-Z) allows an assignment on which chlorophyll of the reaction centre the triplet state is localized at low temperatures. Furthermore, the orientation of the D-X and D-Y components of the D tensor yielded the in-plane orientation of the respective chlorophyll in the reaction centre providing first experimental evidence for the orientation of this molecule in the PS II. (C) 2003 Elsevier B.V. All rights reserved.