Fluorescence Studies with Conformationally Constrained Tryptophan Analogs: Implications on the Mechanisms of Intramolecular Quenching

摘要

The fluorescence properties of a set of tryptophan analogs, including several that are conformationally restricted, is reported. The conformationally restricted analogs include tetrahydro-carboline-3-carboxylic acid, 3-amino-3-carboxytetrahydrocarbazole, 3-aminotetrahydrocarbazole, and 3-carboxytetrahydrocarbazole. Steady-state and time-resolved measurements were made. The fluorescence decay of most of the conformationally restricted analogs is a bi-exponential. The fluorescence quantum yield and average lifetime of the restricted analogs are higher than the values for tryptophan and corresponding flexible tryptophan analogs. This indicates that the effective rate constant for intramolecular non-radiative (quenching) processes, k{sub}(nr) is smaller for the restricted tryptophan analogs than for the flexible analogs. For a series of flexible analogs, having a bifunctional side chain, there is synergism between the two functional groups (i.e., α-NH{sub}3{sup}+ and α-CO{sub}2{sup}- groups) in determining the effective k{sub}(nr). Potential mechanisms for intramolecular quenching mechanisms, which contribute to k{sub}(nr) include proton transfer from a side chain ammonium group to position 4 of the excited indole ring and charge transfer from the excited indole ring to a side chain acceptor group. By measuring the rate of photo-induced isotope exchange into position 4 of an analog's indole ring, we have estimated the contribution of proton transfer quenching to the total intramolecular quenching rate constant for several flexible tryptophan derivatives.