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(680). Tyr(Z), part of the water-oxidizing complex, is a preferential fast electron donor while Tyr(D) is a slow auxiliary donor to P(680) (+). 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(D) occurs via equilibrium with Tyr(Z) (•) at pH 4.7 and 6.3. At pH 8.5 direct Tyr(D) oxidation by P(680) (+) 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(680) (+)) on the Chl(D1) in Mn-depleted PS II after the far-red light-induced charge separation at room temperature is suggested. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s11120-017-0442-3) contains supplementary material, which is available to authorized users.