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

Out-of-phase electron spin echo envelope modulation (ESEEM) spectroscopy wa s used to determine the distances within two consecutive radical pair state s initiated by a laser flash in photosystem II membrane fragments at pH 11. The distance between the spin density centers of the primary electron dono r cation radical, P-680(+.) and the reduced plastoquinone acceptor, Q(A)(-. ), has been found to be 27.7 +/- 0.7 Angstrom in agreement with previous re sults. Near room temperature and at high pH, P-680(+.) is reduced by Y-z, a redox active tyrosine residue, on a submicrosecond timescale, As a consequ ence, the subsequent radical pair state, Y(Z)(ox.)Q(A)(-.), could be invest igated after almost complete reduction of P-680(+.) by Y-Z. The determined dipolar electronic spin-spin coupling within the radical pair Y(Z)(ox.)Q(A) (-.) corresponds to a distance of 34 +/- 1 Angstrom between the two molecul es. (C) 1999 Federation of European Biochemical Societies.