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

Journal Article · Wed May 25 04:00:00 EDT 1977 · J. Am. Chem. Soc.; (United States)

DOI:https://doi.org/10.1021/ja00453a019· OSTI ID:7211120

Taniguchi, H; Kirino, Y

Free radicals formed by the reaction of OH or O/sup -/ 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 ..pi..-electron density on the nitrogen atom decreases in cyclic peptide radicals with a more delocalized ..pi..-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.

• Research Organization: Carnegie-Mellon Univ., Pittsburgh
• OSTI ID: 7211120
• Journal Information: J. Am. Chem. Soc.; (United States), Journal Name: J. Am. Chem. Soc.; (United States) Vol. 99:11; ISSN JACSA
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEAR CHEMISTRY
400301 -- Organic Chemistry-- Chemical & Physicochemical Properties-- (-1987)
400600* -- Radiation Chemistry
AMIDES
AZOLES
CHEMICAL RADIATION EFFECTS
CHEMICAL REACTIONS
CHEMISTRY
DECOMPOSITION
DISSOCIATION
ELECTRON SPIN RESONANCE
HETEROCYCLIC COMPOUNDS
LACTAMS
MAGNETIC RESONANCE
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
PEPTIDES
PH VALUE
PROTEINS
PYRROLES
PYRROLIDONES
RADIATION CHEMISTRY
RADIATION EFFECTS
RADICALS
RADIOLYSIS
RESONANCE