IDENTIFICATION AND CHARACTERIZATION OF EPR SIGNALS INVOLVING QB SEMIQUINONE IN PLANT PHOTOSYSTEM-II

We have investigated the EPR characteristics of native Q(B) and Q(B) analogues in higher plant PS II. We show that, as in cyanobacteria, an interaction between Q(A) and Q(B) iron-semiquinones (Q(A)--Fe2+-Q(B)-) is observed which gives an EPR signal near g = 1.6. Bicarbonate binding close to the non-haem iron is required to observe this interaction. The EPR signal of Q(B) semiquinone is weak and difficult to distinguish from that of Q(A). The appearance of the g = 1.6 signal from Q(A)--Fe2+-Q(B)- after 77 K illumination is a better marker for the presence of Q(B) semiquinone. The yield of Q(B) semiquinone in plant PS II is lower than found in cyanobacteria. The brominated quinones DBMIB, TBTQ and bromanil were used as Q(B) analogues to increase the yield of Q(A)--Fe2+-Q(B)-. These analogues act by forming a stable semiquinone at the Q(B) site and not by covalent binding.