Modeling the fluorescence of protein-embedded tryptophans with ab initio multiconfigurational quantum chemistry: the limiting cases of parvalbumin and monellin.

We show that a quantum-mechanics/molecular-mechanics strategy based on ab initio (i.e., first principle) multiconfigurational perturbation theory can reproduce the spectral properties of a tryptophan residue embedded in the contrasting hydrophobic and hydrophilic environments of parvalbumin and monellin, respectively. We show that the observed absorption and emission energies can be reproduced with a less than 3 kcal mol(-1) error. The analysis of the computed emission energies based on a protein disassembly scheme and protein electrostatic potential mapping allows for a detailed understanding of the factors modulating the tryptophan emission. It is shown that for monellin, where the tryptophan is exposed to the solvent, the fluorescence wavelength is controlled not only by the distribution of the point charges of the protein-solvent environment but also by specific hydrogen bonds and, most important, by the environment-induced change in chromophore structure. In contrast, in parvalbumin, where the chromophore is embedded in the protein core, the structure and emission maxima are the same as those of an isolated 3-methylindole fluorophore. Consistently, we find that in parvalbumin the solvation does not change significantly the computed emission energy.