Time-resolved EPR study of the primary donor triplet in D1-D2-cyt b559 complexes of photosystem II: temperature dependence of spin−lattice relaxation

Abstract

The temperature dependence of the EPR spectrum and kinetics of the primary donor triplet state (3)P680 are measured with direct-detection CW EPR and electron spin echo (ESE) spectroscopy, respectively. The EPR spectrum was recorded up to 230 K; kinetics could be traced up to 70 K. The observed anisotropy of the temperature dependence of the EPR spectrum recorded within 1 mu s after a laser flash is explained by anisotropic spin relaxation in the precursor primary radical pair. The ESE-detected decay kinetics of the Z-canonical peak of (3)P680 is close to its optical lifetime for T < 20 K but deviates strongly from the lifetime for T > 30 K, due to a rapid acceleration of spin relaxation with temperature. This relaxation is not caused by the presence of oxygen or the paramagnetic heme iron in the b559 cytochrome; it is attributed to slow tripler hopping in the P680 dimer itself. Comparison of the low-temperature kinetics with that of chlorophyll a in solution confirmed that the central Mg ion in the triplet-bearing Chi of (3)P680 is pentacoordinated.