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 Ce (4+) 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.
Connor
Henry D
HD
Laboratory of Pharmacology and Chemistry, National Institute of Environmental Health Sciences, National Institutes of Health, P.O. Box 12233, Research Triangle Park, North Carolina 27709, USA.
Sturgeon
Bradley E
BE
Mottley
Carolyn
C
Sipe
Herbert J
HJ
Mason
Ronald P
RP
D002412:Cations / Q000737:chemistry; D002563:Cerium / Q000737:chemistry; D003198:Computer Simulation; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods*; D005609:Free Radicals / Q000737:chemistry; D010084:Oxidation-Reduction; D012996:Solutions; D014364:Tryptophan / Q000737:chemistry*
D002412:Cations; D005609:Free Radicals; D012996:Solutions; D002563:Cerium; D014364:Tryptophan