L-tryptophan radical cation electron spin resonance studies: Connecting solution-derived hyperfine coupling constants with protein spectral interpretations

Fast-flow electron spin resonance (ESR) spectroscopy has been used to detect a free radical formed from the reaction of L-tryptophan with Ce4+ in an acidic aqueous environment. Computer simulations of the ESR spectra from L-tryptophan and several isotopically modified forms strongly support the conclusion that the L-tryptophan radical cation has been detected by ESR for the first time. The hyperfine coupling constants (HFCs) determined,from the well-resolved isotropic ESR spectra support experimental and computational efforts to understand L-tryptophan's role in protein catalysis of oxidation-reduction processes. L-Tryptophan HFCs facilitated the simulation of fast-flow ESR spectra of free radicals from two related compounds, tryptamine and 3-methylindole, Analysis of these three compounds' beta-methylene hydrogen HFC data along with equivalent L-tyrosine data has led to a new computational method that can distinguish between these two amino acid free radicals in proteins without dependence on isotope labeling, electron-nuclear double resonance, or high-field ESR. This approach also produces geometric parameters (dihedral angles for the beta-methylene hydrogens) that should facilitate protein site assignment of observed L-tryptophan radicals as has been done for L-tyrosine radicals.