Nanosecond UV laser photoionization of aqueous tryptophan: temperature dependence of quantum yield, mechanism, and kinetics of hydrated electron decay

The 266 nm photoionization of aqueous tryptophan (Trp) solution at natural pH has been studied by laser flash photolysis using 3-18 mJ, 7 ns, 266 nm pulses. The transient absorption method was employed to detect hydrated electrons. The photoionization mechanism is a combination of mono and biphotonic processes. Results from measurements of the power dependence of photoionization and of triplet (T-1) state production suggest that both processes involve the same one-photon and two-photon singlet intermediate state. The hydrated electron yield increases with temperature, and this can be attributed primarily to the monophotonic photoionization component. Hydrated electrons decay by a non-exponential rate law that is consistent with a decay mechanism involving their bimolecular scavenging by tryptophan radical cations with a rate constant of (7.2+/-0.6) x 10(10) M-1 s(-1). (C) 2000 Elsevier Science S.A. All rights reserved.