CONFORMER INTERCONVERSION IN THE EXCITED-STATE OF CONSTRAINED TRYPTOPHAN DERIVATIVES

The conformer model of tryptophan photophysics ascribes the multiple f luorescence lifetimes to ground-state heterogeneity. It is usually ass umed that the different conformers do not interconvert in the excited state. Previous studies of two constrained tryptophan derivatives supp orted this assumption (Colucci, W. J.; Tilstra, L.; Sattler, M. C.; Fr onczek, F. R.; Barkley, M. D. J. Am. Chem. Sec. 1990, 112, 9182-9190; Yu, H.-T.; Vela, M. A.; Fronczek, F. R.; McLaughlin, M. L.; Barkley, M . D. J. Am. Chem. Sec. 1995, 117, 348-357). Five constrained derivativ es have been synthesized and shown to undergo conformer inversion duri ng the lifetime of the excited state. All derivatives have two ground- state conformations as determined by X-ray crystallography, molecular mechanics calculations, and H-1-NMR. Fluorescence lifetime data were f it to single and double-exponential models and to a reversible two-sta te excited-state reaction model. -Amino-1,2-dihydrocyclopenta[b]indole -2-carboxylic acid has a single-exponential decay consistent with conf ormer inversion much faster than fluorescence decay. 1,2,3,4-Tetrahydr ocarbazole-3-carboxylic acid, ethyl 1,2,3,4-tetrahydrocarbazole-3-carb oxylate, and their 9-methyl derivatives have double-exponential decays with a minor second component of small positive or negative amplitude . Conformer inversion rates of similar to 10(7) s(-1) were determined by analyzing the fluorescence decay data using the excited-state react ion model. Temperature dependence of the fluorescence lifetimes was me asured in H2O and D2O, and solvent quenching rates were calculated fro m the Arrhenius parameters. The carboxylate and carbonyl functional gr oups appear to have little effect on solvent quenching of indole fluor escence. Model calculations examining the effect of conformer inversio n rate on the decay parameters of a biexponential model indicate that the presence of a small amplitude, short lifetime component may be a g ood predictor of excited-state conformer interconversion of tryptophan s in peptides and proteins.