Abstract

Electron paramagnetic resonance (EPR) and pulsed electron-nuclear double resonance (ENDOR) spectra of oxidized tyrosines Z and D (Y Z ⋅ and Y D ⋅, respectively) in Mn-depleted photosystem II have been studied in the pH range from 5.5 to 8.0. Above pH 7.0 the hyperfine couplings of 3,5-ring protons of both tyrosines were similar. The hyperfine constants of the strongly-coupled β-methylene protons were slightly different, 28.4 MHz in Y D ⋅ vs. 33.6 MHz in Y Z ⋅. This difference was explained by different orientations of the β-methylene group with respect to the aromatic ring plane. From the similarity of the hyperfine couplings it was concluded that at high pH Y Z ⋅, same as Y D ⋅, is a neutral radical. The EPR and ENDOR spectra of Y D ⋅ did not change in the studied pH range. However, the light-minus-dark spectra below pH 6.0 were considerably different from those at higher pH. To explain these spectra, two models were discussed, with Y Z supposed to be a cation-radical or a neutral radical with spin densities close to those known in literature. Assuming Y Z to be a cation radical with a spin density distribution close to that known for phenolic cation radicals, we were able to explain the shape of the ENDOR, S-(2GHz) and X-band EPR spectra observed at pH 5.5. The deuteron ENDOR spectra of Y Z ⋅ in D 2O-washed sample were similar both at low and high pH, suggesting the change in hydrogen bondings may be orientational without change in their distance. The change of the Y Z ⋅ structure at low pH was suggested to be caused by the protonation of some amino acid residue (possibly, histidine 190 of the D1 protein) in the vicinity of Y Z.

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