Fluorescence of <i>cis</i>-1-Amino-2-(3-indolyl)cyclohexane-1-carboxylic Acid: A Single Tryptophan χ₁ Rotamer Model

A constrained derivative, cis-1-amino-2-(3-indolyl)cyclohexane-1-carboxylic acid, cis-W3, was designed to test the rotamer model of tryptophan photophysics. The conformational constraint enforces a single χ1 conformation, analogous to the χ1 = 60° rotamer of tryptophan. The side-chain torsion angles in the X-ray structure of cis-W3 were χ1 = 58.5° and χ2 = −88.7°. Molecular mechanics calculations suggested two χ2 rotamers for cis-W3 in solution, −100° and 80°, analogous to the χ2 = ±90° rotamers of tryptophan. The fluorescence decay of the cis-W3 zwitterion was biexponential with lifetimes of 3.1 and 0.3 ns at 25 °C. The relative amplitudes of the lifetime components match the χ2 rotamer populations predicted by molecular mechanics. The longer lifetime represents the major χ2 = −100° rotamer. The shorter lifetime represents the minor χ2 = 80° rotamer having the ammonium group closer to C4 of the indole ring (labeled C5 in the cis-W3 X-ray structure). Intramolecular excited-state proton transfer occurs at indole C4 in the tryptophan zwitterion (Saito, I.; Sugiyama, H.; Yamamoto, A.; Muramatsu, S.; Matsuura,T. J. Am. Chem. Soc. 1984, 106, 4286−4287). Photochemical isotope exchange experiments showed that H−D exchange occurs exclusively at C5 in the cis-W3 zwitterion, consistent with the presence of the χ2 = 80° rotamer in solution. The rates of two nonradiative processes, excited-state proton and electron transfer, were measured for individual χ2 rotamers. The excited-state proton-transfer rate was determined from H−D exchange and fluorescence lifetime data. The excited-state electron-transfer rate was determined from the temperature dependence of the fluorescence lifetime. The major quenching process in the −100° rotamer is electron transfer from the excited indole to carboxylate. Electron transfer also occurs in the 80° rotamer, but the major quenching process is intramolecular proton transfer. Both quenching processes are suppressed by deprotonation of the amino group. The results for cis-W3 provide compelling evidence that the complex fluorescence decay of the tryptophan zwitterion originates in ground-state heterogeneity with the different lifetimes primarily reflecting different intramolecular excited-state proton- and electron-transfer rates in various rotamers.