Indoles undergo two isotopically sensitive temperature-dependent fluorescence quenching processes: solvent quenching and excited-state proton transfer. Fluorescence quantum yields of simple indoles in protium and deuterium solvents were measured in the absence and presence of glycine. Photochemical H-D exchange was monitored by NMR and mass spectrometry.Although the fluorescence quantum yield and lifetime of 2-methylindole show large deuterium isotope effects in aqueous solutions, photochemical H-D exchange was not detected after extensive irradiation, whereas, H-D exchange is readily observed for 2-and 3-methylindole in solutions containing glycine. Stern-Volmer plots of glycine quenching data give bimolecular rate constants kq from (0.5-3) X 108 M™1 11s™1 for indoles in water. The kq values of 2-and 3-methylindole are faster in protium than in deuterium solvents. The isotope effect on kq implicates excited-state proton transfer in the collisional quenching mechanism.This contrasts with iodide quenching which has no isotope effect on kq. A glycine derivative lacking the ammonium protons, iV,iV,7V-trimethylglycine, does not quench indole fluorescence. The intermolecular excited-state reaction of 2-and 3-methylindole with 0.3 M glycine-d5 in 50% D20/CD30D induces H-D exchange at three ring carbons. In 2-methylindole the exchange is fastest at C3 and occurs with similar rates at C4 and C7 on the indole ring. The temperature dependence of 3-methylindole fluorescence in 0.5 M glycine was also determined. The large difference in temperature dependence for solvent quenching and glycine quenching causes curvature in the Arrhenius plot. The frequency factor A2 = 7.2 X 1010 s™1 and activation energy E2* = 3.6 kcal/mol for glycine quenching are similar to the values for intramolecular excited-state proton transfer in tryptamine.Possible mechanisms for the excited-state proton transfer reaction and the implications of this reaction for tryptophan fluorescence in proteins are discussed.

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53) Pernot, C.; Lindqvist, L. J . Photochem. 1976/1977, 6 , 215-220. (54) Strickland, E. H.; Billups, C.; Kay, E. Biochemistry 1972, 11, 3657-3662.over the range pH 3-1 l.I7 The radiative rates calculated from quantum yield and lifetime data are (4.8-5.2) X lo7 s-I for indole,'4Js, (3.1-5.0) X IO7 s-' for 3-methylind0le,6~'~~'~~~~ and 3.5 X 1 O7 s-' for 2,3-dimethylind0le.'~~~~ The estimated intersystem crossing rate is equal to the radiative rate in aqueous indoleI2 and -3.3 X IO7 s-I in 3-methylind01e.l~ W(1) is a 2,3-dialkylindole. Although the zwitterion has a somewhat longer lifetime than aqueous 2,3-dimethylindole, the 4.94s lifetime of the anion approaches the lifetime of 2,3-dimethylindole. This is opposite to the trend in tryptophan, where the zwitterion lifetimes are much shorter than the lifetime of 3-methylindole and the anion lifetime is about the same as 3-methylind0le.'~ The complex fluorescence decays observed in many 3-substituted indoles are due to the functional groups on the alkyl side hai in,'^*^**^^ not the alkane moiety. Abstract:The ground-state conformation of a rotationally constrained tryptophan derivative, 3-carboxy-l,2,3,4-tetrahydro-2-carboline, W( I), 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(1) zwitterion and 2.9 and 4.8 ns for the W(I) anion. The relative amplitudes (al = 0.12-0.20 and a2 = 0.88-0.80) match the rclativc populations of the two conformers (0.3 and 0.7). Consequently, one lifetime is assigned to each conformer. The shorter lifctimc component is associated with the conformer having the carboxylate closest to the indole ring. Esterification replaces thc 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 proccss occurring in W( I ) and W(I)E probably involves electron transfer.Although the fluorescence of tryptophan is widely studied, its photophysics is not fully understood.'J Numerous explanations have been proposed for the complex fluorescence decays of the tryptophan zwit...

The equation is a cosine series in 8, the torsion angle between vicinal hydrogens. The feature which distinguishes this equation from similar equations is the inclusion of ASi terms, which describe the magnitude of each substituent's effect. These substituent constants have been defined from experimental data. An orientation effect which is dependent on the torsion angle(s) between substituent(s) and a vicinal hydrogen is included. Substituent constants have been defined for 39 groups, of which 15 have been experimentally determined herein. The parameters for the equation have been defined from 49 torsion angles in 19 conformationally rigid compounds. The torsion angles have been determined from x-ray crystal structure data and molecular mechanics calculations. The multiplicity of structures used to determine the substituent constants should allow for the application of this equation to a wide variety of structures.

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