Spectral and Kinetic Study of 3-Methylquinoxalin-2-ones Photoreduced by Amino Acids: N-Phenylglycine Radical Chain Reactions and N-Acetyltryptophan Decarboxylation.

Transient intermediates were identified in the photoreduction of 3-methylquinoxalin-2-one derivatives by N-phenylglycine, NPG, and N-acetyltryptophan, NAT. For both reductants it can be postulated a sequence of reaction comprising first a photoinduced single electron transfer followed by a proton transfer from the radical cation of the electron donor to the radical anion of the 3-methylquinoxalin-2-one giving rise to the reported products. The effect of the concentrations of NPG and the quinoxalin-2-one on the rate of photoconsumption of this last were quantified, and the lifetimes of the possible intermediates estimated. In the photoreduction by NAT, processes leading to the decarboxylation of NAT and radical adduct product compete with the expected SET from the indoyl N to the excited triplet of quinoxalin-2-ones as revealed by the detection of the deprotonated N-acetyltryptophan radical [NAT-H](•). This radical is formed almost instantly after the laser pulse and has a secondary delayed growth via a delayed proton transfer from the indoyl radical cation NAT(•+) to the quinoxalin-2-one radical anions. The decarboxylation of NAT that mimics C-terminus tryptophan in proteins is biologically relevant because might cause damages at cellular and the whole organism level. As far as we know this is the first report of a radical decarboxylation of N-acetyltryptophan leading to photoproducts.