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

Hydrogen bonding to the tyrosyl radical in ribonucleotide reductase (RNR) h as been simulated by a complex between the phenoxyl radical and a water mol ecule. Multiconfigurational self-consistent field linear response theory wa s used to calculate the g-tensor of the isolated phenoxyl radical and of th e phenoxyl-water model. The relevance of the model was motivated by the fac t that spin density distributions and electron paramagnetic resonance (EPR) spectra of the phenoxyl and tyrosyl radicals are very similar. The calcula ted g-tensor anisotropy of the phenoxyl radical was comparable with experim ental 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 th e tyrosyl radical EPR data from Escherichia coli RNR are given in parenthes es. The hydrogen bonding models reproduced a shift toward a lower g(x) valu e that was observed experimentally for mouse and herpes simplex virus RNR w here the H-bond was detected by electron-nuclear double resonance after deu terium exchange. This decrease could be traced to lower angular momentum an d spin-orbit coupling matrix elements between the ground B-2(1) and the fir st excited B-2(2) states (oxygen lone-pair n to pi(SOMO) excitation) upon h ydrogen bonding in a linear configuration. The g(x) value was further decre ased by hydrogen bonding in bent configurations due to a blue shift of this excitation.