E.s.r. of spin-trapped radicals in aqueous solutions of peptides. Reactions of the hydroxyl radical.

The reactions of hydroxyl radicals with 30 dipeptides and several larger peptides were studied in aqueous solutions. The OH radicals were generated by U.V. photolysis of H2O2. The short-lived peptide radicals were spin-trapped using t-nitrosobutane and identified by e.s.r. For dipeptides containing the amino terminal residues glycine, alanine and phenylalanine, abstraction of the hydrogen from the carbon adjacent to the peptide nitrogen was the major process leading to the spin-adducts. Such radicals will be referred to as backbone radicals. Dipeptides with a carbonyl terminal serine residue and also glycylglutamic acid form both backbone and side-chain radicals, with the latter being formed in larger quantities. For dipeptides, side-chain radicals were detected on either the carboxyl or amino terminal residues of both. The effect of pD on the e.s.r. sectrum of the spin-adducts of glycylglycine was studied and the pK of the carboxyl group of this radical was determined to be 2.5. For (Ala)3 and (Ala)n, with an average value of n = 1800, backbone and minor side-chain radicals were observed. For ribonucleases-S-peptide, containing 20 amino acid residues, both backbone and side-chain radicals were detected.