Beta-scission of side-chain alkoxyl radicals on peptides and proteins results in the loss of side-chains as aldehydes and ketones

Exposure of proteins to radicals in the presence of O2 results in side-chain oxidation and backbone fragmentation; the interrelationship between these processes is not fully understood. Recently, initial attack on Ala side-chains was shown to give α-carbon radicals (and hence backbone cleavage) and formaldehyde, via the formation and subsequent β-scission, of C-3 alkoxyl radicals. We now show that this side-chain to backbone damage transfer, is a general mechanism for aliphatic side-chains. Oxidation of Val, Leu, and Asp residues by HO•/O2 results in the release of a family of carbonyls (including formaldehyde, acetone, isobutyraldehyde, and glyoxylic acid) via the formation, and subsequent β-scission of alkoxyl radicals. The concentration of these products increases with the HO• flux. The release of multiple carbonyls confirms the occurrence of oxidation at C-3 and C-4 for Val, and these sites, plus C-5, for Leu. The detection of glyoxylic acid and CO2−• from Asp demonstrates the occurrence of competing β-scission processes for the Asp C-3 alkoxyl radical. The yield of hydroperoxides and released carbonyls account for 10–145% of the initial HO•. The greater than 100% yields confirm the occurrence of chain reactions in peptide/protein oxidation, with more than one residue being damaged per initiating radical.