Redox state dependence of rotamer distributions in tyrosine and neutral tyrosyl radical

摘要

Redox state-dependent changes in the relative orientation of the phenol side chain and the peptide group in model tyrosine have been characterized using specific ~2H isotopic labelling and X-band electron paramagnetic resonance (EPR) spectroscopy. Tyrosyl radicals were generated by UV photolysis of tyrosine trapped in rigid polycrystalline basic-aqueous medium at T ≤ 170 K. Ring-~2H_4 and β-~2H_2 substitutions on tyrosine were used to enhance the lineshape contributions from β-hydrogen or ring-hydrogen hyperfine interactions, respectively. The EPR lineshape at 120 K of the trapped ring-~2H_4-tyrosyl radical is altered dramatically after annealing at 235 K. In contrast, the lineshape of the β-~2H_2-tyrosyl radical is impervious to annealing. The effect of annealing on the lineshape therefore arises from a change in the isotropic hyperfine coupling between unpaired π-electron spin density at the ring carbon atom C_1 and the β-hydrogen nuclei, which is caused by rotational relaxation of the ring and peptide group about the C_1-C_β bond. EPR simulations indicate angular distributions of the peptide group (R-) of 0°≤ θ_R ≤30°and 0°≤ θ_R ≤ 18°in the rigid and relaxed radical states, respectively. Redox-induced changes in the C_1-C_β rotamer distribution must be accounted for in assessments of stable amino acid side chain equilibrium structures, and may influence catalytic tyrosyl radical/tyrosine function in enzymes.