UV-induced phosphorescence from the lowest triplet state T1 of tryptophan (Trp) residues is widely used to monitor the structural dynamics of host proteins on long time scales. Probing the intrinsic properties of this optically "dark" state requires studies of Trp isolated in the gas phase, which are challenging due to low sample concentrations and the need to monitor the triplet over extended time scales. We discovered that excitation of cold, protonated noncovalent TrpH+-(H2O)n complexes (n ≥ 6) by UV light of specific wavelengths induces evaporation of two water molecules and promotes tryptophan to the triplet state. Subsequent photofragmentation dynamic, monitored by mass spectrometry and ion spectroscopy, yields rate constants for intrinsic triplet-state quenching via phosphorescence and reverse intersystem crossing. The T1 lifetime is approximately 1 s at 7 K and is dominated by phosphorescence; it decreases to tens of milliseconds at ∼40 K and is estimated to be ∼10 ms at room temperature.
Boyarkin et al. (Thu,) studied this question.