Peptide Free-Radicals: The Reactions of OH Radicals with Glycine Anhydride and its Methyl Derivatives Sarcosine and Alanine Anhydride.A Pulse Radiolysis and Product Study

Abstract

The reactions of radiolytically generated OH radicals and H atoms with the cyclic dipeptides of glycine, alanine and sarcosine in deoxygenated aqueous solutions and the subsequent reactions of the transient peptide radicals were studied in the absence and presence of K3Fe(CN)6as oxidant by pulse radiolysis and product analysis.Hydroxyl radicals and H atoms react with glycine anhydride and alanine anhydride by abstracting an H atom bound at C-3; there is no evidence for any other site of attack at these two peptides.The resulting radicals have pK, values of 9.8 and 10.6, respectively.In the absence of an oxidant the radicals decay by second order (2k = 7.0x108dm3mol−1s−1and 2k = 4.4x 108dm3mol−1s−1 resp.), the main fraction (94% of the glycine anhydride-derived radicals, 90% of the alanine anhydride-derived radicals) yielding dehydrodimers (G = 0.58 μmol J−1and 0.56 μmol J−1(in monomer units), resp.).A small portion however disuproportionates via abstraction of a C-6-bound H atom followed by isomerization to 2,5-dihydroxypyrazines (pKavalues of the parent 2,5-dihydroxypyrazine at about 7.9 and 10.1) and subsequent addition of water to 2,5-diketo-3-hydroxypiperazines.thus indicating that the transfer of a carbon-bound hvdrogen atom is prefered to the transfer of a nitrogen-bound hvdrogen atom.No disuproportionation products but three different dehydrodimers (G = 0.36, 0.18 and 0.04 μmol J−1(in monomer units)) were found after irradiation of sarcosine anhydride.In this case a dose rate and solute concentration dependence of dehydrodimer formation indicates a radical-solute reaction converting part of the N-methyl radicals (21% of 'initial' attack) into the C-3-yl radicals.A rate constant of k = 600 ± 50 dm3mol−1s−1was obtained for this reaction by measuring and computing the dehydrodimer yields as a function of dose rate and solute concentration.Thus the observed transient supectrum accounts only for about 79% of the radicals from the 'initial' attack at C-3.The rate of oxidation of the glycine anhydride-derived radicals by Fe(CN)63reflects the pKaof the transient radical.The rate constant for oxidation of the (protonated) radical derived from glycine anhydride is: K = 1.0x108dm3mol−1s−1, the corresponding radical anion is oxidized with k = 3.1 x 108dm3mol−1s−1No change with pH was observed in the case of the alanine anhydride-derived radicals (k = 7.9x 10sdm3mol−1s−1).In contrast to the disuproportionation, oxidation by Fe(CN)63-leads to the removal of a proton from the heteroatom, a carbocation being the inter-mediate.The resulting dehydropiperazines rapidly add water to yield the corresponding 2,5-diketo-3-hydroxypiperazines (G = 0.61 μmol J−1after oxidation of the glycine anhydride-derived radicals, G = 0.58 μmol J−1after oxidation of the alanine anhydride-derived radicals).The radicals derived from sarcosine anhydride are readily oxidized with k = 4.0x108dm3mol−1s−l, independent of pH.1H and3C{1H} NMR-supectroscopic and mass-supectroscopic data of the products are given. © 1989, Verlag der Zeitschrift. All rights reserved.