The differences in microenvironments and functions of tyrosine radicals Y-Z and Y-D in photosystem II studied by EPR

Electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) were performed to investigate the difference in microenvironments and functions between tyrosine Z (Y-Z) and tyrosine D (Y-D). Mn-depletion or Ca2+-depletion causes extension of the lifetime of tyrosine radical Y-Z(center dot), which can be trapped by rapid freezing after illumination at about 250 K. Above pH 6.5, Y-Z(center dot) radical in Mn-depleted PS II shows similar EPR and ENDOR spectra similar to that of Y-D(center dot) radical, which are ascribed to a typical neutral tyrosine radical. Below pH 6.5, Y-Z(center dot) radical shows quite different EPR and ENDOR spectra. ENDOR spectra show the spin density distribution of the low-pH form of Y-Z(center dot) that has been quite different from the high-pH form of Y-Z(center dot). The spin density distribution of the low-pH Y-Z(center dot) can be explained by a cation radical or the neutral radical induced by strong electrostatic interaction. The pH dependence of the activation energy of the recombination rate between Y-Z(center dot) and Q(A)-shows a gap of 4.4 kJ/mol at pH 6.0-6.5. In the Ca2+-depleted PS II, Y-Z(center dot) signal was the mixture of the cation-like and normal neutral radicals, and the pH dependence of Y-Z(center dot) spectrum in Ca2+-depleted PS II is considerably different from the neutral radical found in Mn-depleted PS II. Based on the recent structure data of cyanobacterial PS II, the pH dependence of Y-Z(center dot) could be ascribed to the modification of the local structure and hydrogen-bonding network induced by the dissociation of ASP170 near Y-Z.