1D- and 2D-ESEEM Study of the Semiquinone Radical QA- of Photosystem II

Abstract

The semiquinone radical QA- has been studied by Electron Spin−Echo Envelope Modulation (ESEEM) spectroscopy in Photosystem II membranes at various pH values. The observed nuclear modulations have been assigned by the use of two-dimensional Hyperfine Sublevel Correlation Spectroscopy (HYSCORE) and numerical simulations. Two protein 14N nuclei (NI and NII) were found to be magnetically coupled with the QA- spin, and on the basis of 14N-NQR and 14N-ESEEM data from the literature, NI is assigned to an amide nitrogen from the protein backbone while NII is assigned to the amino nitrogen, Nδ, of an imidazole. A physical explanation for such couplings is suggested where the coupling occurs through H-bonds from the protein to the carbonyls of the semiquinone. In PSII membranes treated with CN-, only the NI coupling is present above pH 8.5 while both NI and NII couplings are present at lower pH values. In samples treated at high pH to remove the iron, both NI and NII couplings are present throughout the pH range st...