Electronic structure of the Y_D tyrosyl radical in photosystem II: A high-frequency electron paramagnetic resonance spectroscopic and density functional theoretical study

We report the high-frequency (139.5 GHz) electron paramagnetic resonance (EPR) spectrum of the Y^._D tyrosyl radical of photosystem II. A rhombic powder pattern with principal g values g₁ = 2.007 82, g₂ = 2.004 50, and g₃ = 2.002 32 is observed. The well-defined turning points and the value of the largest principal g value are indicative of ordered hydrogen bonding to the tyrosyl phenyl oxygen. Hyperfine structure is resolved on all three turning points. Proton hyperfine couplings obtained from the simulation of the 139.5 GHz EPR spectrum are in good agreement with X-band electron spin echo-electron nuclear double resonance studies. The high-frequency EPR spectrum was acquired under conditions of saturation in which the dispersion signal is detected. Proper replication of the high-frequency EPR spectral features is only achieved in simulations which account for the line shapes characteristic of saturated dispersion signals. Comparison of the Y^._D spectrum with spectra of non-hydrogen bonded tyrosyl radicals indicates that the largest principal g value (g₁), oriented along the C-O bond, is sensitive to hydrogen bonding at the phenyl oxygen. Density functional calculations indicate that the decreased downfield shift in g₁ from the free electron g value with increasing hydrogen bond strength arises from both a decreased spin density on the phenyl oxygen-5-30% over a range of reasonable hydrogen bond distances (2.0-1.1 Å) - and an increased splitting between ground state and excited state singly occupied molecular orbitals.