Flash-induced enhancements in the proton NMR relaxation rate of Photosystem II particles

Abstract

Flash-induced enhancements in the NMR spin-lattice relaxation rate of solvent protons have been detected in suspensions of Photosystem II particles. The relaxation enhancements are small (less than 1% of background) and have been detected using signal-averaging techniques. The enhancements correlate with the known properties of the S states with respect to (1) decay kinetics, (2) extractants of manganese, (3) sensitivity to atrazine-type inhibition of electron transport, (4) reagents which accelerate S-state decay (ADRY reagents), and (5) the two-flash retardation in the S-state advancement that is produced by low concentrations of NH2OH. The transient proton relaxation enhancement observed after a single flash arises from a strongly relaxing paramagnetic species that is produced by the S1 --> S2 transition of the water-oxidizing center. The appearance of such a species on an oxidative transition is suggestive of an Mn(III) --> Mn(IV) oxidation.