EPR AND ESEEM STUDY OF THE PLASTOQUINONE ANION-RADICAL Q(A)(-CENTER-DOT) IN PHOTOSYSTEM-II TREATED AT HIGH PH

The effect of treatment at pH = 11 on the photosystem II was studied b y EPR and electron spin echo envelope modulation (ESEEM). The magnetic interaction between the semiquinone Q(A)(-.) and the non-heme Fe2+(S = 2) was absent. ESEEM showed that the Q(A)(-.) interacts magnetically with two N-14 nuclei. The first interaction has a hyperfine coupling tensor (A(XX), A(YY), A(ZZ)) = (2.0, 1.7, 2.6 MHz) and nuclear quadrup ole interaction parameters e(2)qQ/h = 3.24 MHz and eta = 0.45 while th ose of the second are (A(XX), A(YY), A(ZZ)) = (1.2, 1.5, 2.3 MHz), e(2 )qQ/h = 1.56 MHz and eta = 0.71. These are assigned to an amide nitrog en of the peptide backbone and the amino nitrogen of an imidazole resp ectively. By analogy to the bacterial reaction centre, these nitrogens are attributed to the Ala 261 and His 215 of the D2 protein. It was s hown earlier that the imidazole coupling is absent in cyanide-treated PSII, its presence here is attributed to a difference in the position of the imidazole group itself.