Electronic trasitions in molecules in static external fields. I. Indole and Trp-59 in ribonuclease T₁

Electronic transition properties of indole perturbed by its environment were calculated by use of quantum-mechanical semi-empirical numerical methods. The environment was represented by a discrete set of charges placed at different positions around the indole ring. Wavelength shifts and transition intensity changes in indole were evaluated for several, specifically modeled geometries of external charges. This methodology was employed to estimate the extent of spectroscopic changes induced by small nonprotein polar species on the Trp-59 residue in the anisotropic environment of the protein ribonuclease T₁. The geometry of the residue environment was obtained from dynamically equilibrated X-ray crystallographic data of the protein.