Mechanisms of tryptophan fluorescence shifts in proteins

Tryptophan fluorescence wavelength is widely used as a tool to monitor chan ges in proteins and to make inferences regarding local structure and dynami cs. We have predicted the fluorescence wavelengths of 19 tryptophans in 16 proteins, starting with crystal structures and using a hybrid quantum mecha nical-classical molecular dynamics method with the assumption that only ele ctrostatic interactions of the tryptophan ring electron density with the su rrounding protein and solvent affect the transition energy. With only one a djustable parameter, the scaling of the quantum mechanical atomic charges a s seen by the protein/solvent environment, the mean absolute deviation betw een predicted and observed fluorescence maximum wavelength is 6 nm. The mod eling of electrostatic interactions, including hydration, in proteins is vi tal to understanding function and structure, and this study helps to assess the effectiveness of current electrostatic models.