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

We report theoretical calculations of vibrational absorption and Raman spec tra for the tryptophan analog 3-methylindole using density functional theor y, the Becke3LYP hybrid functional, and the TZ/2P basis set. These results are compared to experimentally measured vibrational absorption and Raman sp ectra for 3-methylindole. The theoretical calculations represent accurate p redictions of the observed vibrational frequencies and intensities, and of the Raman intensities, Currently, tryptophan radicals, either neutral or ca tionic, 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 spec tra for 3-methylindole cation radical and 3-methylindole neutral radical. T hese predictions should provide specific spectroscopic markers for the dete ction of neutral or cationic tryptophan radicals in biological systems, pro viding a complement to the data available from electron paramagnetic resona nce experiments. Raman spectroscopy of tryptophan is already in use in the study of protein conformations; theoretical predictions for the radical spe cies provides a new tool for the detection of neutral or cation radicals of tryptophan in natural systems predisposed to the appropriate experiment. ( C) 2001 Published by Elsevier Science B.V.