MICROENVIRONMENTAL EFFECTS ON THE SOLVENT QUENCHING RATE IN CONSTRAINED TRYPTOPHAN DERIVATIVES

Solvent quenching is one of several environmentally sensitive nonradia tive decay pathways available to the indole chromophore. It is charact erized by 2-3-fold deuterium isotope effects and strong temperature de pendence with frequency factors of 10(15)-10(17) S-1 and activation en ergies of 11-13 kcal/mol in aqueous solution. The effects of ionizatio n state, proximity of the amino group to the indole ring, and N-methyl ation of indole nitrogen on the solvent quenching rate were examined i n four constrained tryptophan derivatives: 1,2,3,4-tetrahydro-2-carbol ine, 3-amino-1,2,3,4-tetrahydrocarbazole, 3-amino-1,2,3,4-tetrahydroca rbazole-3-carboxylic acid, and methyl-1,2,3,4-tetrahydro-2-carboline-3 -carboxylic acid. The constrained derivatives had at most two ground-s tate conformations, as determined by X-ray crystallography, molecular mechanics calculations, and H-1 NMR. Fluorescence lifetimes were assig ned to ground-state conformations based on relative populations of con formers and amplitudes of fluorescence decays. Solvent quenching rates were determined from the temperature dependence of the fluorescence L ifetime. The solvent quenching rate is decreased by protonation of the amino group in all compounds. It is slower in the carboline derivativ es, where the amino group is two bonds away from the indole ring, than in the tetrahydrocarbazole derivatives, where the amino group is thre e bonds away. In the tetrahydrocarbazoles, the solvent quenching rate is slower in the conformer with the ammonium in the pseudoaxial positi on closer to the indole ring than in the conformer with the ammonium i n the pseudoequatorial position pointing away from the ring. These res ults suggest that the water quenching rate of tryptophans on protein o r peptide surfaces is modulated by proximal ammonium groups.