ESR study of the acid dissociation of NH protons. II. Cyclic peptide radicals and related radicals

Free radicals formed by the reaction of OH or O- radicals with alicyclic compounds containing a peptide group (-CONH-) and structurally related compounds have been studied by the in situ radiolysis-steady state ESR method. Eight cyclic peptide radicals resulting from hydrogen abstraction from the C-H bond adjacent to the peptide group have been observed. They are divided into two groups on the basis of the skeletal structure of the radicals: greater than C-NH-CO- and greater than C-CO-NH-. In basic solutions significant changes occur in the ESR parameters of these radicals which can be interpreted in terms of the dissociation of the NH proton in a peptide group. The pK/sub a/ values for the NH proton dissociation have been determined to be in the range of 7.6 to 13.6 for eight cyclic peptide radicals from 5-methyl-2-pyrrolidinone, 2-pyrrolidinone, 2-pyrrolidone-5-carboxylic acid, hydantoin (for both first and second NH proton dissociations), 1-methylhydantoin, 2-thiohydantoin (second dissociation only), succinimide, and 2,5-piperazinedione (first dissociation only) and 10.9 for a related cyclic radical from 2-oxazolidone. These pK/sub a/ values are considerably lower than those for corresponding linear peptide radicals, partly because π-electron density on the nitrogen atom decreases in cyclic peptide radicals with a more delocalized π-electron system. Within each group of cyclicpeptide radicals with greater than C-NH-CO- or greater than C-CO-NH- the same trend of changes in ESR parameters was observed upon the dissociation of peptide proton