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

Abstract

Abstract— 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. CO2 by acidimetric titration, CH2O by colometry and glyoxylate enzymatically; N‐methyl groups were quantified by 1H‐NMR. Nitrilotriacetate yields 100% oxidative photodecarboxylation with CH2O formation, while EDTA behaves quite differently, yielding quantitative amounts of CO2 and glyoxylate, but no CH2O. The electron balance in this reaction is found to be maintained by co‐formation of one equivalent >N‐CH3. From these data it is concluded that the photooxidation of the structural element >NCH2C02 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+= CH2. With EDTA, containing the fragment –O2CCH2 >NCH2CH2N< CH2CO2–, the electron transfer towards flavin is strongly favoured by stabilisation of a cisoid mesomeric radical cation (02cch2 >+ NCH2CH2NCH2CO2 o2cch2 >NCH2CH2N+CH2CO2), which, after loss of CO2 undergoes internal 1,6‐hydrogen shift to yield the more stable radical: CH2>CH2CH2N CH2CO2CH2lt NCH2CH2NHCO2. The latter radical will easily be oxidized to the iminium ion >N*= CH‐C02‐. 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.

Photo-induced reduction of flavin mononucleotide in aqueous solutions

“…The photo-reduction of FMN ox in pH 8 buffer with 1 mM EDTA occurs according to a formal overall reaction [30] …”

“…It lowers the pH to below pK a = 6.7 for dominant fully reduced neutral FMN formation. The details of the photo-reaction of FMN ox with EDTA are rather complex, they are described in [27,[30][31][32]. EDTA is irreversibly modified (indicated by Photoproducts) with CO 2 release and hydrolysis to glyoxylate.…”

“…Under anaerobic conditions stable photo-reduction to the fully reduced form is achieved [14,16,[20][21][22][23][24][25][26][27][28][29] even in the absence of exogenous reducing agents [22,24,29]. The involved mechanisms in the photo-reduction of flavins are discussed in [20][21][22][23][24][25][26][27][28][29][30][31][32]. It is generally accepted that the photo-excitation leads to triplet state population by intersystem crossing, and that the reduction occurs from the triplet state [21,22,[24][25][26][27][28][29][30][31][32].…”

“…The involved mechanisms in the photo-reduction of flavins are discussed in [20][21][22][23][24][25][26][27][28][29][30][31][32]. It is generally accepted that the photo-excitation leads to triplet state population by intersystem crossing, and that the reduction occurs from the triplet state [21,22,[24][25][26][27][28][29][30][31][32]. Some investigated reducing agents for photo-induced reduction of flavins are ethylene-diamine-tetraacetic acid (EDTA) [20,[22][23][24][27][28][29][30][31][32][33], nitrilotriacetate [20], phenylacetic acid [26], indole-acetic acid [26,28], ascorbic acid (vitamin C) [28], and tryptophan [28].…”

Reactions of lumiflavin with amine radicals

“…For EDTA k, is apparently much larger (6.2 X 10' for 3-methyllumiflavin (14)). The products derived from EDTA for radical reductions of FI to FIH-have been discussed by Armstrong, Hemmerich, and Traber (29). In the absence of metal ions 1 mol of glyoxalate and of CO, were formed per mol of flavin reduced.…”