Energetics of photoinduced electron transfer in the indole.O2 cluster in gas phase: possible consequences for photoexcited tryptophan in solution.

Abstract

A direct process for an activationless electron transfer from photoexcited tryptophan to molecular oxygen is proposed. By photodetachment of mass-selected indole+.O2- clusters in gas phase a neutral indole+.O2- charge-separated exciplex state is found at 2.25 +/- 0.2 eV above the neutral ground state. By theory also, the existence of an excited charge separated state at 3.05 +/- 0.2 eV is postulated. In gas phase both charge-separated cluster states are energetically below the first singlet states 1Lb and 1La and the lower even below the first triplet state T1 of indole. In gas-phase clusters these energetics imply a very efficient quenching of photoexcited indole by fast electron transfer to oxygen. We discuss a similar mechanism for tryptophan.O2 in aqueous environment and find it without activation barrier and presumably extremely fast. In the collisional tryptophan*-O2 complex the efficiency and the time scale of the charge transfer process should be mostly solvent independent. In polar solvent a complete charge separation and free superoxide formation are expected. We correlate this model with previous fluorescence and phosphorescence quenching data of excited tryptophan by O2 and propose electron transfer to be the relevant process.