Flavin Sensitized Photooxidation of (Poly)amino Acids: Fate of the Photosubstrate

With respect to literature data, which seemed to be contradictory, we examined the fate of ethylenediaminetetraacetate (EDTA) and nitrilotriacetate (NTA) in flavin‐sensitized photooxidation reactions by quantitation of all possible products, i.e. CO 2 by acidimetric titration, CH 2 O by colometry and glyoxylate enzymatically; N‐methyl groups were quantified by 1 H‐NMR. Nitrilotriacetate yields 100% oxidative photodecarboxylation with CH 2 O formation, while EDTA behaves quite differently, yielding quantitative amounts of CO 2 and glyoxylate, but no CH 2 O. The electron balance in this reaction is found to be maintained by co‐formation of one equivalent >N‐CH 3 . From these data it is concluded that the photooxidation of the structural element >NCH 2 C0 2 containing a tertiary nitrogen atom, occurs by electron transfer, followed by decarboxylation. The neutral radical that is formed is reductant and will give off an electron equivalent to yield the easily hydrolysed structural element >N + = CH 2 . With EDTA, containing the fragment –O 2 CCH 2 >NCH 2 CH 2 N< CH 2 CO 2 –, the electron transfer towards flavin is strongly favoured by stabilisation of a cisoid mesomeric radical cation ( 02 cch 2 > + NCH 2 CH 2 N CH2CO2 o 2 cch 2 >NCH 2 CH 2 N + CH2CO2 ), which, after loss of CO 2 undergoes internal 1,6‐hydrogen shift to yield the more stable radical: CH2 >CH 2 CH 2 N CH 2 CO 2 CH 2 lt NCH 2 CH 2 N HCO2 . The latter radical will easily be oxidized to the iminium ion >N * = CH‐C0 2 ‐ . which in turn hydrolyses to glyoxylate. Chelation of EDTA by metal cations prevents the 1,6‐hydrogen shift, and leads to the product pattern obtained in the NTA photoreaction.