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

`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 beta-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) a-carbon radicals with most peptides. Evidence has also been obtained for the occurrence of hydrogen atom abstraction and decarboxylation reactions with these substrates.