Reaction of HOCl with amino acids and peptides: EPR evidence for rapid rearrangement and fragmentation reactions of nitrogen-centred radicals

Abstract

EPR spin trapping together with UV–VIS spectroscopy has been employed to examine the reaction of HOCl with amino acids and some small peptides. Evidence has been obtained for the formation and subsequent decomposition of short-lived chloramine derivatives from free amine groups present on both amino acid side-chains and at the N-terminus. Radical formation, detected by EPR spin trapping, occurs concurrently with chloramine decomposition. This process is enhanced by, but does not require, the presence of added Fe²⁺. With some substrates nitrogen-centred (aminyl, RNH˙ or RNH₂^˙+) radicals, formed from cleavage of the N–Cl bond of the chloramine, can be detected. These initial aminyl radicals undergo a variety of hydrogen atom abstraction, rearrangement and fragmentation reactions to give carbon-centred species. Evidence has been obtained for both inter- and intra-molecular (1,2- and 1,5-) hydrogen atom abstraction reactions, decarboxylation and β-scission processes. The last of these only occurs to a significant extent where the resulting radical is highly stabilised, for example, by aromatic substituents, and results in loss of the amino acid side-chain. Studies with N-acetyl derivatives and peptides are consistent with reaction of HOCl at amide (peptide) bonds to give transient chloramides which rapidly decompose to give (undetected) amidyl [˙N(R)C(O)R′] radicals. These species undergo rapid 1,2-hydrogen atom shift reactions to give (stabilised) α-carbon radicals with most peptides. Evidence has also been obtained for the occurrence of hydrogen atom abstraction and decarboxylation reactions with these substrates.