FLUORESCENCE OF A ROTATIONALLY CONSTRAINED TRYPTOPHAN DERIVATIVE, 3-CARBOXY-1,2,3,4-TETRAHYDRO-2-CARBOLINE

Despite a plethora of work on tryptophan photophysics, the origin of the complex fluorescence decay kinetics is still unknown. A question remains whether the two fluorescence lifetimes of the tryptophan zwitterion are due to ground-state and therefore excited-state heterogeneity resulting from rotameric forms. To test the conformer model, we synthesized a tryptophan derivative, 3-carboxy-1,2,3,4-tetrahydro-2-carboline, which restricts rotation of the alanyl side chain. The constraint limits the number of conformations available to the molecule without greatly affecting the electronic properties. The absorption and emission spectra and the pK of the amino group are similar to those of tryptophan. The fluorescence decay of the constrained derivative is an apparent monocxponential. The zwitterion has a lifetime of 6.2 ± 0.2 ns and the anion has a lifetime of 4.7 ± 0.2 ns at 25 °C. Global analysis of time-resolved emission spectral data for the zwitterion reveals a double-exponential decay, which may reflect two stable conformations of the partially unsaturated six-membered ring. This is discussed in detail in the following paper. © 1990, American Chemical Society. All rights reserved.