Properties of excited states of aqueous tryptophan

The short-lived intermediate (τ≈26 ns) with absorption maximum at 400 nm, observed in 308 nm photolysis of aqueous tryptophan, is identified as a protonated triplet state. It is shown that the main channel of photoexcited singlet state S1 decay is intramolecular (in neutral solutions) or through-solvent (in acidic solutions) protonation followed by a rapid intersystem crossing into the protonated triplet state. The dissociation constant of triplet tryptophan pKa=3.2 and the lower limit of the dissociation constant of singlet-excited tryptophan pKa≥2.2 are determined from the pH dependence of the triplet lifetime and quantum yield. Ground-state tryptophan and acetone quench triplet tryptophan with the rate constants kT-TrpH=1.2×107 M−1 s−1 and kT-Ac=1.9×106 M−1 s−1, respectively. The major precursor of monophotonic tryptophan photoionization at room temperature is a non-relaxed prefluorescent state.