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 Q_A^−. and the non-heme Fe²⁺ (S = 2) was absent. ESEEM showed that the Q_A^−. interacts magnetically with two ¹⁴N nuclei. The first interaction has a hyperfine coupling tensor (A_XX, A_YY, A_ZZ)=(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 (A_XX, A_YY, A_ZZ)=(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.