Intramolecular Proton Transfer Quenching in Tryptophan and Tryptamine: Temperature Dependence

摘要

There are two dominant intramolecular quenching reactions for the deactivation of the excited singlet state of tryptophan and its related chromophores. These mechanisms are charge transfer to an electrophilic side chain group and proton transfer from a side chain to the indole ring. When occurring in D{sub}2O, the photoinduced proton transfer reaction results in an exchange of the hydrogen isotope at position 4 of the benzenoid ring of indole (Saito et al. J. Amer. Chem. Sot. 106, 4286), which can be quantitated by 1{sup left}H-NMR studies. We have determined the rate constant for this photoinduced H-D exchange for tryptophan and some of its analogs as a function of temperature. We show that the proton transfer quenching reaction makes a large contribution to the quenching of tryptophan, whereas for other chromophores, charge transfer quenching must dominate.