Density Functional and MP2 Calculations of Spin Densities of Oxidized 3-Methylindole:  Models for Tryptophan Radicals

Ab initio calculations have been carried out on 3-methylindole, and the cation and neutral radicals of 3-methylindole, using density functional theory (DFT), the Becke3−Lee−Yang−Parr functional, and the 6-31G*, 6-31+G*, 3-21G*, and TZ2P basis sets. Optimized geometries, vibrational frequencies, and for the radicals, atomic spin densites are calculated. The latter are compared to experimental spin densities recently determined for the tryptophan-191 radical of compound ES of the enzyme cytochrome-c-peroxidase. The DFT spin densities for the cation radical of 3-methylindole are in excellent agreement with the data for the tryptophan-191 radical, which supports the conclusion that the tryptophan radical is a cation radical. The results are compared to calculations using second-order Møller−Plesset theory (MP2) and the 6-31G** basis set. The MP2 spin densities are in significantly worse agreement with the experimental spin densities.