Photophysics of Constrained Tryptophan Derivatives

摘要

The aromatic amino acid tryptophan is widely used as an intrinsic fluorescent probe of the solution conformation and dynamics of peptides and proteins. However, its complex photophysics makes it difficult to interpret the fluorescence results. The biexponential fluorescence decay of the tryptophan zwitterion is presumed to be due to ground-state rotamers. Intramolecular proton and electron transfer reactions involving the excited indole ring and amino acid functional groups have been proposed to account for the lifetime differences among rotamers. Excited-state H-D exchange occurs at the C-4 position of indole. In the proposed mechanism for the photosubstitution reaction, the ammonium group loops back over the aromatic ring and assists the proton exchange. In order to examine the relationship between fluorescence lifetime and ground-state structure, we synthesized two tryptophan derivatives with restricted rotation about the C{sup}α-C{sup}β and C{sup}β-C{sup}γ bonds: W(1) and W(2) W(1) adopts two twist-chair conformations in solution with closely spaced fluorescence lifetimes. As expected from the stereochemical constraint, it shows no excited-state H-D exchange at the indole C-4. The lifetimes are about 20% shorter in the W(2) zwitterion than in the W(1) zwitterion. The only difference between the two derivatives is that the amino group in W(1) is incorporated in the six-membered ring.