Patrik R. Callis
We are currently studying the internal structure and dynamics of proteins through a combination of molecular modeling, quantum mechanics, and ultraviolet spectroscopy. We are primarily exploiting the amino acid tryptophan, whose spectra are sensitive to the internal electric field caused by the surrounding protein scaffolding and solvent. We have already a good quantum mechanical understanding of the tryptophan excited states, one that strongly suggests that the wavelength of the fluorescence emitted from the excited state after excitation with UV light depends almost entirely on the magnitude and direction of the electric field caused by charges and dipoles of the protein and solvent. Current focus is on the more challenging problem of understanding the basis of the 30-fold variation in fluorescence quantum yield found for tryptophans in different protein environments. By combining quantum mechanics and molecular dynamics simulations to generate parameters for electron transfer rate theory, we have found a promising correlation between quantum yield and the computed energy gap between the emitting state and a higher lying charge transfer excited state. The significance of this work lies in the fact that hundreds of experiments are published every year exploiting the changes in tryptophan fluorescence in proteins for a variety of studies, including protein folding pathways and dynamics, enzyme action, and molecular recognition. It is essential that this probe be understood at a fundamental level, because the ultimate detailed pictures of proteins cannot be established from conventional structure methods alone. In addition, if the electrostatic model we have proposed proves correct, tryptophan fluorescence wavelengths will provide excellent tests for the important task of accurately predicting electrostatic fields and potentials in proteins.