Fluorescence of Tryptophan Residues

Fluorescence occurs when a photon is absorbed by a molecule and re-emitted at a longer wavelength. When photons excite electrons, the change of electronic distribution can cause atomic re-bonding or defect formation. Such rearrangements can significantly alter the fluorescence spectrum, as is the case of tryptophan (Trp) residues in proteins. Because of the strong sensitivity of its optical gap to the local environment, a single Trp molecule can be used to make an ideal intrinsic biological fluorescent probe.

Most molecular dynamics simulations are performed on a single ground state potential energy surface. On the other hand, following the absorption of a photon, the molecular motion involves crossings of different electronic states, rendering the Born-Oppenheimer approximation invalid. To date, only a few attempts have been tried to do dynamical simulations of small molecules involving excited states.

We have finished the analysis of ground-state and excited-state properties of tryptophan in vacuum, and are implementing a practical algorithm to perform MD of an excited molecule in solution.

Being able to model photo-excited processes not only allows us to understand or even design light-activated materials such as bio-molecules or nanotubes, but also allows us to study photochemical processes in condensed phases. We plan to work intensively in this rich field and look forward to collaborating with experimentalists studying these problems.