Electron spin polarization (CIDEP) of a primary electron acceptor in Photosystem II

Abstract Photosystem II particles prepared according to Berthold et al. (Berthold, D.A., Babcock, G.T. and Yocum. C.F. (1981) FEBS Lett. 134, 231–234) and to Ganago and Klimov (Ganago, I.B. and Klimov, V.V. (1985) Biofizika, in the press) were subjected to an iron extraction procedure and cooled in the light under reducing conditions. The samples showed a 0.9 mT wide EPR line at g = 2.0044 attributed to the reduced primary acceptor Q − A . Further prolonged illumination at 15 K generated a wide, somewhat asymmetric EPR signal at g = 2.0034−2.0038 that showed strong, reversible polarization upon continuous illumination at 15 K and below. The signal is ascribed to an acceptor that becomes spin-polarized through exchange-mediated transfer of polarization as described previously for photosynthetic bacteria (Gast, P. and Hoff, A.J. (1979) Biochim. Biophys. Acta 548, 502–535). Arguments are given that the aceptor may be intermediate between the pheophytin transient acceptor and Q A .