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 st radicals with large spin density on N1 (see 1), so the observation of N-14 Or N-15 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 beta-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.