The mechanism of photo-oxidation of tryptophan (Trp) sensitized by riboflavin (RF) was examined employing high concentrations of Trp and RF, with a high intensity 365 nm light emitting diode (LED) source under N-2, 20% and 100% O-2 atmospheres. Dimerization of Trp was a major pathway under the N2 atmosphere, though this occurred with a low yield ((D)phi(Trp) = 5.9x10(-3)), probably as a result of extensive back electron transfer reactions between RF center dot- and Trp(H)(center dot+). The presence of O-2 decreased the extent of this back electron transfer reaction, and the extent of Trp dimerization. This difference is attributed to the formation of O-2(center dot-) (generated via electron transfer from RF center dot- to O-2) which reacts rapidly with Trp(center dot) leading to extensive consumption of the parent amino acid and formation of peroxides and multiple other oxygenated products (N-formylkynurenine, alcohols, diols) of Trp, as detected by LC-MS. Thus, it appears that the first step of the Type 1 mechanism of Trp photooxidation, induced by this high intensity 365 nm light source, is an electron transfer reaction between the amino acid and (RF)-R-3, with the presence of O-2 modulating the subsequent reactions and the products formed, as a result of O-2(center dot-) formation. These data have potential biological significance as LED systems and RF-based treatments have been proposed for the treatment of pathological myopia and keratitis.