An ESR study of the acid dissociation of NH protons. 2. Cyclic peptide radicals and related radicals

Taniguchi, H.; Kirino, Y.

Journal of the American Chemical Society 99(11): 3625-3631


ISSN/ISBN: 0002-7863
PMID: 192774
DOI: 10.1021/ja00453a019
Accession: 068523494

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Free radicals formed by the reaction of OH or O- radicals with alicyclic compounds containing a peptide group (.sbd.CONH.sbd.) and structurally related compounds were studied by the in situ radiolysis-steady state ESR method. Eight cyclic peptide radicals resulting from H abstraction from the C.sbd.H bond adjacent to the peptide group were observed. They are divided into 2 groups on the basis of the skeletal structure of the radicals: > .ovrhdot.C.sbd.NH.sbd.CO.sbd. and > .ovrhdot.C.sbd.CO.sbd.NH.sbd. 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 pKa values for the NH proton dissociation were determined to be in the range of 7.6-13.6 for 8 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 pKa values are considerably lower than those for corresponding linear peptide radicals, partly because .pi.-electron density on the N atom decreases in cyclic peptide radicals with a more delocalized .pi.-electron system. Within each group of cyclic peptide radicals with > .ovrhdot.C.sbd.NH.sbd.CO.sbd. or > .ovrhdot.C.sbd.CO.sbd.NH.sbd. the same trend of changes in ESR parameters was observed upon the dissociation of peptide proton. In the first group with structure similar to linear peptide radicals the g value, and the .gamma.- or .delta.-proton and N coupling constants, increase while .alpha.- and .beta.-proton coupling constants decrease upon the dissociation. Apparently 0.07-0.09 of spin density flows from the .alpha.-C atom to the peptide C atom after NH proton dissociation in the 3 pyrrolidinone radicals. In the 2nd group the g values and N coupling constants decrease upon dissociation and no remarkable changes in .alpha.-proton coupling constant are observed, suggesting that local rearrangement of .pi.-electron distribution occurs in the dissociating peptide group upon the dissociation of peptide proton.