Relaxation behaviour of the tyrosine YD radical in photosystem II: evidence for strong dipolar interaction with paramagnetic centers in the S1 and S2 states

Abstract

Inversion recovery (T1) and microwave power saturation studies have been performed, between 4 and 25 K, on the EPR signal from the stable tyrosyl radical, YD˙, in photosystem II core complex preparations from higher plants. Measurements are reported from the dark stable S1 and first turnover S2 states of the photosystem catalytic Mn cluster and in two cryoprotectant regimes; sucrose and ethylene glycol/glycerol. The inversion recovery kinetics show a dominant, non exponential decay component which is well described by a through space dipolar relaxation model, with a weak exponential decay background (∼an order of magnitude less than the dipolar rate). The dipolar relaxation rate is only modestly temperature dependent and shows no consistent dependence on S state or cryoprotectant. In contrast, the background rate shows a S state dependence, consistent with an interaction between YD˙ and the Mn cluster in the multiline S2 state, over a distance of ∼30 A. The fraction of centers exhibiting the dipolar relaxation component appears to be temperature dependent, but S state independent and consistent with the presence of a fast relaxing species interacting with YD˙. The present results and the possible nature of this interacting species are discussed in comparison with earlier YD˙ relaxation studies on photosystem II membrane samples.