Theoretical determination of the vibrational absorption and Raman spectra of 3-methylindole and 3-methylindole radicals

Abstract

We report theoretical calculations of vibrational absorption and Raman spectra for the tryptophan analog 3-methylindole using density functional theory, the Becke3LYP hybrid functional, and the TZ/2P basis set. These results are compared to experimentally measured vibrational absorption and Raman spectra for 3-methylindole. The theoretical calculations represent accurate predictions of the observed vibrational frequencies and intensities, and of the Raman intensities. Currently, tryptophan radicals, either neutral or cationic, are believed to participate in electron transfer in enzymes such as cytochrome c peroxidase, ribonucleotide reductase, and DNA photolyase. In this paper we also report theoretical vibrational absorption and Raman spectra for 3-methylindole cation radical and 3-methylindole neutral radical. These predictions should provide specific spectroscopic markers for the detection of neutral or cationic tryptophan radicals in biological systems, providing a complement to the data available from electron paramagnetic resonance experiments. Raman spectroscopy of tryptophan is already in use in the study of protein conformations; theoretical predictions for the radical species provides a new tool for the detection of neutral or cation radicals of tryptophan in natural systems predisposed to the appropriate experiment.