Determination of the distance between Y ox⋅Z and Q −⋅A in photosystem II by pulsed EPR spectroscopy on light-induced radical pairs

Abstract

Out-of-phase electron spin echo envelope modulation (ESEEM) spectroscopy was used to determine the distances within two consecutive radical pair states initiated by a laser flash in photosystem II membrane fragments at pH 11. The distance between the spin density centers of the primary electron donor cation radical, P +⋅ 680, and the reduced plastoquinone acceptor, Q −⋅ A, has been found to be 27.7±0.7 Å in agreement with previous results. Near room temperature and at high pH, P +⋅ 680 is reduced by Y Z, a redox active tyrosine residue, on a sub-microsecond timescale. As a consequence, the subsequent radical pair state, Y ox⋅ ZQ −⋅ A, could be investigated after almost complete reduction of P +⋅ 680 by Y Z. The determined dipolar electronic spin-spin coupling within the radical pair Y ox⋅ ZQ −⋅ A corresponds to a distance of 34±1 Å between the two molecules.