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

• Published in: Journal of physical chemistry (1952) 1996-02, Vol.100 (5), p.1530-1535
• Main Authors: Walden, Susan E, Wheeler, Ralph A
• Format: Article
• Language: English
• ISSN: 0022-3654; 1541-5740
• DOI: 10.1021/jp951838p

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 14N or 15N 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.