engleski

Reactions of N-centered Radicals Derived from alfa-Amino Acids; A Time Resolved Study

Time resolved methods (pulse radiolysis and laser flash photolysis) were used to investigate reactions of aliphatic alfa-amino acids in anionic form with hydroxyl radicals(^.^OH) and 4-carboxybenzophenone triplet (CB^*^) in aqueous solutions. Besides H-atom abstraction from alfa-C and/or side-chain C-H bonds, ^.^OH radicals also readily undergo a one-electron transfer reaction from the free electron pair on nitrogen. The resulting aminium radicals, H"2"N^.+^-CR"2"-CO"2"^–^ (R = H, alkyl), decaboxylate and transfer a proton to OH^-^, which is still located within the solvent cage, at a ratio of ca 1:2. As products, strongly reducing alfa-aminoalkyl and oxidizing aminyl radicals are respectively formed. The aminyl radicals further degrade via b-scission, yielding CO"2"^.^ as another reducing species. In the presence of proton donors, they can also be re-protonated into the fast decarboxylating aminium radical, or may abstract H-atoms from, e.g., C"alfa"-H bonds in the amino acid anions. The quenching of CB^*^ by amino acid anions proceeds via the one-electron transfer mechanism, resulting in the formation of aminium radicals as the main initial product. Within the solvent cage, with CB^.^ as the counter ion, this radical either decarboxylates or deprotonates at a ratio of ca 9:1. Deprotonation into aminyl radicals by the OH^-^ ions in the bulk starts to be competitive at [OH^-^ > 1 . This reaction enabled the determination of a rate constant of (1.0  0.3) x 10^11^ s^-1^ for the actual aminium radical decarboxylation process. Specific scavenger methods were used to experimentally determine yields and rate constants of individual radical products from glycine, alanine and 2-aminoisobutyric acid.

aminium radical; amino acids; aminyl radical; decarboxylation; laser flash photolysis; rate constants; reaction mechanism

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