Redox state dependence of rotamer distributions in tyrosine and neutral tyrosyl radical.
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 < or = 170 K. Ring-2H(4) and beta-2H(2) substitutions on tyrosine were used to enhance the lineshape contributions from beta-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 beta-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 pi-electron spin density at the ring carbon atom C(1) and the beta-hydrogen nuclei, which is caused by rotational relaxation of the ring and peptide group about the C(1)-C(beta) bond. EPR simulations indicate angular distributions of the peptide group (R-) of 0 degrees < or = theta(R) < or = 30 degrees and 0 degrees < or = theta(R)< or = 18 degrees in the rigid and relaxed radical states, respectively. Redox-induced changes in the C(1)-C(beta) 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.
Warncke
K
K
Department of Physics, 1001 Rollins Research Center, Emory University, 1510 Clifton Road, Atlanta, GA 30322, USA. kwarncke@physics.emory.edu
Perry
M S
MS
D002384:Catalysis; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals; D008956:Models, Chemical; D015394:Molecular Structure; D010084:Oxidation-Reduction; D010777:Photochemistry; D014443:Tyrosine / Q000737:chemistry* / Q000528:radiation effects; D014466:Ultraviolet Rays
D005609:Free Radicals; D014443:Tyrosine