Theg-values and hyperfine coupling of amino acid radicals in proteins: comparison of experimental measurements withab initio calculations

Abstract

Electron paramagnetic resonance (EPR) spectroscopy has been extensively used to identify and characterize protein-based redox active amino acid radicals based on their g-values and hyperfine couplings. To better understand how these parameters depend on the electronic structure and environment of the radical, the theoretical g-values and proton hyperfine tensors of three models corresponding to the tyrosyl, tryptophanyl and glycyl radicals were calculated using Gaussian 03. The g-values were determined using the B3LYP/6-31+G(D,P) combination of density functional and basis set, while the hyperfine tensors were determined using the B3LYP/EPR-III and PBE0/EPR-III combinations. Comparisons are made to measured values. It was found that by appropriately accounting for hydrogen bonds and the dielectric constant of the environment, good agreement could be achieved between the calculated and measured g-values. In all three cases, the g-anisotropy arose from significant spin density on a nitrogen or oxygen atom. The calculated hyperfine tensors for the three radicals did not differ significantly from previous calculations. In the case of the tyrosyl radical, it is shown for the first time that the para-position substituent that is opposite of the C-O group can break the symmetry of the phenyl ring, leading to different hyperfine tensors for the two large ortho proton couplings. For the tyrosyl and tryptophanyl models, the calculated hyperfine couplings to hydrogen-bonding protons were in very good agreement with measured values for both the tyrosyl and tryptophanyl models.