Tryptophan photoionization from prefluorescent and fluorescent states

Contributions into tryptophan ionization from non-relaxed prefluorescent S∗ and relaxed fluorescent S1 states are separated by measurement of the ionization quantum yield in neutral and extremely acidic solutions. The rate constant of the ionization from the prefluorescent S∗ state is temperature independent, the rate constant of ionization from S1 state has frequency factor Aion=(2.3±0.9)×1015 s−1 and activation energy Eion=50±5 kJ/mol. For l-tryptophan at room temperature the ionization from S1 state is completely suppressed by other processes of S1 state decay, in particular – by the intramolecular proton transfer (IPT) from the protonated amino group to the indole ring (Aipt=(6.1±1.9)×1010 s−1, Eipt=14.0±1.8 kJ/mol).