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

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, Y_D^., 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 Y_D^. and the Mn cluster in the multiline S₂ state, over a distance of ∼30 Å. 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 Y_D ^.. The present results and the possible nature of this interacting species are discussed in comparison with earlier Y_D^. relaxation studies on photosystem II membrane samples.