Laser flash photolysis of aqueous tryptophan at 265 nm generates the neutral radical product of indole ring N-H bond photolysis (Xmax 510 nm, 1800 M~l cm-1; 330 nm, 3100 M-1 cm-1), the triplet state (Xmax 460 nm), and the hydrated electron as initial products. The photoionization quantum yields for tryptophan, 1methyltryptophan, tyrosine, and glycine-tryptophan peptides have been determined using ferrocyanide as the reference system. The electron and aromatic radical decay by a previously unreported first-order back reaction of 0.80-µ8ß half-time, leading to residual yields of the separated products after several microseconds significantly smaller than the photoionization yield. The kinetics results are consistent with the formation of a loose complex between the hydrated electron and the radical in which recombination competes with separation. The implications of this process for tryptophyl residue destruction in proteins are considered. A comparable first-order electron decay stage was observed with tryptophan derivatives, tyrosine, I-, and Fe(CN)64-.

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