EPR and ESEEM study of the plastoquinone anion radical QA−. in photosystem II treated at high pH

The effect of treatment at pH = 11 on the photosystem II was studied by EPR and electron spin echo envelope modulation (ESEEM). The magnetic interaction between the semiquinone QA−. and the non-heme Fe2+ (S = 2) was absent. ESEEM showed that the QA−. interacts magnetically with two 14N nuclei. The first interaction has a hyperfine coupling tensor (AXX, AYY, AZZ)=(2.0, 1.7, 2.3 MHz) and nuclear quadrupole interaction parameters e2qQ/h=3.24 MHz and η = 0.45 while those of the second are (AXX, AYY, AZZ)=(1.2, 1.5, 2.3 MHz), e2qQ/h = 1.56 MHz and η = 0.71. These are assigned to an amide nitrogen of the peptide backbone and the amino nitrogen of an imidazole respectively. By analogy to the bacterial reaction centre, these nitrogens are attributed to the Ala 261 and His 215 of the D2 protein. It was shown 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.