The Genuine Article Number: YG526
Weinkauf R., Schiedt J .
ENERGETICS OF PHOTOINDUCED ELECTRON TRANSFER IN THE INDOLE-
CENTER-DOT-O-2 CLUSTER IN GAS PHASE - POSSIBLE CONSEQUENCES FOR
PHOTOEXCITED TRYPTOPHAN IN SOLUTION
Photochemistry & Photobiology. 66(5):569-575, 1997 Nov.
Room-temperature. Aqueous-solution. Amino-acids. Fluorescence. Proteins.
State. Phosphorescence. Photoionization. Indole. Spectroscopy.
A direct process for an activationless electron transfer from photoexcited
tryptophan to molecular oxygen is proposed. By photodetachment of
mass-selected indole . O-2(-) clusters in gas phase a neutral indole(+). O-2(-)
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 L-1(b) and L-1(a)
and the lower even below the first triplet state T-1 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 . O-2 in aqueous environment and find it without activation barrier
and presumably extremely fast. In the collisional tryptophan*O-2 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
O-2 and propose electron transfer to be the relevant process. [References:
Biochemistry & biophysics.
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