HISTIDINE OXIDATION IN THE S-2 TO S-3 TRANSITION PROBED BY FTIR DIFFERENCE SPECTROSCOPY IN THE CA2-DEPLETED PHOTOSYSTEM-II - COMPARISON WITH HISTIDINE RADICALS GENERATED BY UV IRRADIATION()

FTIR difference and EPR spectroscopies were used to identify the organ ic radical species formed during the S-2 to S-3 transition in Ca2+-dep leted, EGTA-treated, and polypeptide-reconstituted photosystem II memb ranes (denoted S-2' and S-3', respectively). Ferricyanide was added to the samples to act as an exogenous electron acceptor. Using EPR spect roscopy, it was shown that, under the experimental conditions used, on ly the species oxidized in the S-3' State was detected during the time required for the acquisition of the FTIR difference spectra. No contr ibutions from the electron acceptor side were observed. The correspond ing S-3'/S-2' FTIR difference spectra were recorded at 10 degrees C in H2O, D2O, and with N-15-labeled photosystem II membranes. Spectra wer e compared with radical-minus-neutral FTIR difference spectra of amino acid model compounds generated by UV irradiation at low temperature. Under our experimental conditions, we did not observe FTIR difference Signals consistent with tyrosine oxidation in the S-2' to S-3' transit ion. The infrared signals characteristic of radical formation with 4-m ethylimidazole and histidine obtained by UV irradiation of 4-methylimi dazolium at pH 6 and of a His-Tyr dipeptide at pH 7 are presented. The analogy found between these spectra and the S-3'/S-2' spectrum obtain ed in situ supports the oxidation of a histidinium in the S-2' to S-3' transition.