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l-Tryptophan Radical Cation Electron Spin Resonance Studies: Connecting Solution-Derived Hyperfine Coupling Constants with Protein Spectral Interpretations

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journal contribution
posted on 21.05.2008, 00:00 by Henry D. Connor, Bradley E. Sturgeon, Carolyn Mottley, Herbert J. Sipe, Ronald P. Mason
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’ β-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 β-methylene hydrogens) that should facilitate protein site assignment of observed l-tryptophan radicals as has been done for l-tyrosine radicals.