Hydrogen Bonding to Tyrosyl Radical Analyzed by Ab Initio g-Tensor Calculations

Abstract

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 2 B 1 and the first excited 2 B 2 states (oxygen lone-pair n to πSOMO 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.

A Tryptophan Neutral Radical in the Oxidized State of Versatile Peroxidase from Pleurotus eryngii

“…7), for two different side chain orientations. Considering a reported general ϳ10% overestimation by DFT methods (63,64) of the g-tensor shifts (⌬g jj ) from the free electron value g e , (⌬g jj ϭ g jj Ϫ g e (jj ϭ xx, yy, zz), g e ϭ 2.002319), the calculated g-tensor values are in good agreement with the experimental values. Similar g-tensor values were, however, also calculated for the tryptophan cation radical (Table 2).…”

Displacement of the tyrosyl radical cofactor in ribonucleotide reductase obtained by single-crystal high-field EPR and 1.4-Å x-ray data

“…This result shows that H 2 O 2 treatment of the met R2 crystals did not change the chemical environment of the radical, e.g., by hydrogen bonding to a possible reaction product. It is well known that the g x -value of a tyrosyl radical is shifted from Ϸ2.0091 [hydrophobic environment, E. coli (37,38)] to values of Ϸ2.0076 for the case of a more polar environment (refs. 12, 14, and 37; see Table 2).…”

Photosystem II single crystals studied by EPR spectroscopy at 94 GHz: The tyrosine radical Y\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy}\usepackage{mathrsfs}\setlength{\oddsidemargin}{-69pt}\begin{document}\begin{equation*}{\mathrm{_{D}^{{\bullet}}}}\end{equation*}\end{document}

“…The g tensor, particularly g x , is a sensitive probe for hydrogen bond strength and orientation (34,46,47). This is of particular interest because the hydrogen bond is believed to have a significant effect on redox potential and radical reactivity (48).…”