Distinguishing Features of Indolyl Radical and Radical Cation: Implications for Tryptophan Radical Studies

Tryptophan radicals or radical cations, currently believed to participate in electron transfer in cytochrome c peroxidase, DNA photolyase, galactose oxidase, and perhaps ribonucleotide reductase, are becoming increasingly conspicuous in proteins. Density-functional quantum chemical calculations for the indolyl radical (Ind • ) and radical cation (IndH •+ ) are reported to aid the distinction between the tryptophan radical (Trp • ) and the tryptophan radical cation (TrpH •+ ). For this evaluation of indole and the indolyl radicals, two density-functional methods were compared: the local spin density exchange functional of Slater combined with the local correlation energy expression of Vosko, Wilk, and Nusair (SVWN) and Becke's gradient-corrected exchange functional combined with Lee, Yang, and Parr's gradient-corrected correlation functional (BLYP). Both were employed with a 6-31G(d) basis set. In addition to providing molecular geometries, atomic spin densities, and approximate isotropic hyperfine coupling constants for modeling EPR and ENDOR spectra, results imply that (a) both Ind • and IndH •+ are π radicals with large spin density on N1 (see 1 ), so the observation of 14 N or 15 N hyperfine interactions alone is not sufficient to distinguish Trp • from TrpH •+, (b) the different spin polarizations at C2 of Ind • and IndH •+ should form the basis for designing experiments to distinguish Trp • from TrpH •+, (c) the large shift of the C2−C3 stretching mode of indole by approximately −200 cm -1 in Ind • may be useful to identify Trp •, and (d) analysis of the angular dependence of hyperfine interactions between the β-methylene protons of Trp • and the large spin density at C3, calculated for both Ind • and IndH •+, may yield the orientations of Trp • side chains in proteins.