Formation of tyrosine radicals in photosystem II under far-red illumination.

Photosystem II (PS II) contains two redox-active tyrosine residues on the donor side at symmetrical positions to the primary donor, P. Tyr, part of the water-oxidizing complex, is a preferential fast electron donor while Tyr is a slow auxiliary donor to P. We used PS II membranes from spinach which were depleted of the water oxidation complex (Mn-depleted PS II) to study electron donation from both tyrosines by time-resolved EPR spectroscopy under visible and far-red continuous light and laser flash illumination. Our results show that under both illumination regimes, oxidation of Tyr occurs via equilibrium with Tyr at pH 4.7 and 6.3. At pH 8.5 direct Tyr oxidation by P occurs in the majority of the PS II centers. Under continuous far-red light illumination these reactions were less effective but still possible. Different photochemical steps were considered to explain the far-red light-induced electron donation from tyrosines and localization of the primary electron hole (P) on the Chl in Mn-depleted PS II after the far-red light-induced charge separation at room temperature is suggested.