Hydrogen bonding to tyrosyl radical analyzed by ab initio g-tensor calculations

Hydrogen bonding to the tyrosyl radical in ribonucleotide reductase (RNR) has been simulated by a complex between the phenoxyl radical and a water molecule. Multiconfigurational self-consistent field linear response theory was used to calculate the g-tensor of the isolated phenoxyl radical and of the phenoxyl-water model. The relevance of the model was motivated by the fact that spin density distributions and electron paramagnetic resonance (EPR) spectra of the phenoxyl and tyrosyl radicals are very similar. The calculated g-tensor anisotropy of the phenoxyl radical was comparable with experimental findings for tyrosyl in those RNRs where the H-bond is absent: g(x) = 2.0087(2.0087), g(y) = 2.0050(2.0042), and g(z) = 2.0025(2.0020), where the tyrosyl radical EPR data from Escherichia coli RNR are given in parentheses. The hydrogen bonding models reproduced a shift toward a lower g(x) value that was observed experimentally for mouse and herpes simplex virus RNR where the H-bond was detected by electron-nuclear double resonance after deuterium exchange. This decrease could be traced to lower angular momentum and spin-orbit coupling matrix elements between the ground B-2(1) and the first excited B-2(2) states (oxygen lone-pair n to pi(SOMO) excitation) upon hydrogen bonding in a linear configuration. The g(x) value was further decreased by hydrogen bonding in bent configurations due to a blue shift of this excitation.