REACTION-CENTER PHOTOCHEMISTRY IN CYANIDE-TREATED PHOTOSYSTEM-II

EPR was used to study the triplet state of chlorophyll generated by ra dical pair recombination in the photosystem II (PSII) reaction centre. The spin state of the non-haem Fe2+ was varied using the CN--binding method (Y. Sanakis, V. Petrouleas, B.A. Diner, Biochemistry 33 (1994) 9922-9928) and the redox state of the quinone acceptor (Q(A)) was chan ged from semi-reduced to fully reduced (F.J.E. van Mieghem, W. Nitschk e, P. Mathis, A.W. Rutherford, Biochim. Biophys. Acta 977 (1989) 207-2 14). It was found that the triplet was not detectable using continuous wave EPR when Q(A)(-) was present irrespective of the spin-state of t he Fe2+. It was also found that the triplet state became detectable by EPR when the semiquinone was removed (by reduction to the quinol) and that the triplet observed was not influenced by the spin state of the Fe2+. Since it is known from earlier work that the EPR detection of t he triplet reflects a change in the triplet lifetime, it is concluded that the redox state of the quinone determines the triplet lifetime (a t least in terms of its detectability by continuous wave EPR) and that the magnetic state of the iron, (through the weakly exchange-coupled Q(A)(-) Fe2+ complex) is not a determining factor. In addition, we loo ked for polarisation transfer from the radical pair to Q(A)(-) in PSII where the Fe2+ was low spin. Such polarisation is seen in bacterial r eaction centres under comparable conditions. In PSII, however, we were unable to find evidence for such polarisation of the semiquinone. It is suggested that both the short triplet lifetime in the presence of Q (A)(-) and the lack of polarised Q(A)(-) might be explained in terms o f the electron transfer mechanism for triplet quenching involving the semiquinone which was proposed previously (F.J.E. van Mieghem, K. Bret tel, B. Hillmann, A. Kamlowski, A.W. Rutherford, E. Schlodder, Biochem istry 34 (1995) 4798-4813). It is suggested that this mechanism may oc cur in PSII (but not in purple bacterial reaction centres) due the tri plet-bearing chlorophyll being adjacent to the pheophytin at low tempe rature as suggested from structural studies (F.J.E. van Mieghem, K. Sa toh, A.W. Rutherford, Biochim. Biophys. Acta 1058 (1992) 379-385). (C) 1998 Elsevier Science B.V. All rights reserved.