CONFORMATIONAL STUDIES OF A CONSTRAINED TRYPTOPHAN DERIVATIVE - IMPLICATIONS FOR THE FLUORESCENCE QUENCHING MECHANISM

The ground-state conformation of a rotationally constrained tryptophan derivative, 3-carboxy-1,2,3,4-tetrahydro-2-carboline, W(1), is determined from single-crystal X-ray diffraction, MM2 calculations, and *H NMR coupling constants. The solid-state structure represents the predominant solution conformation. W(1) populates only two minimum-energy conformations in solution, which correspond to the half-chair forms of cyclohexene. The conformers are distinguished mainly by distance of the carboxylate from the indole ring. The MM2-computed barrier for ring inversion in W(1) is 5.91 kcal/mol. The constrained tryptophan derivative and its ethyl ester, W(1)E, are used to investigate nonradiative decay pathways in tryptophan photophysics. The conformational restriction eliminates the excited-state intramolecular proton-transfer reaction observed with tryptophan (Saito, I.; Sugiyama, H.; Yamamoto, A.; Muramatsu, S.; Matsuura, T. J. Am. Chem. Soc. 1984, 106, 4286-4287). Global analysis of time-resolved fluorescence data reveals biexponential decays with lifetimes of 3.6 and 6.3 ns for the W(l) zwitterion and 2.9 and 4.8 ns for the W(l) anion. The relative amplitudes (α1 = 0.12-0.20 and α2 = 0.88-0.80) match the relative populations of the two conformers (0.3 and 0.7). Consequently, one lifetime is assigned to each conformer. The shorter lifetime component is associated with the conformer having the carboxylate closest to the indole ring. Esterification replaces the carboxylate of W(1) with a better electron acceptor and shortens both lifetimes, suggesting that intramolecular electron transfer may be an important mode of quenching. Arrhenius parameters concur that the temperature-dependent nonradiative process occurring in W(1) and W(1)E probably involves electron transfer. © 1990, American Chemical Society. All rights reserved.