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)6 as 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 pKa values of 9.8 and 10.6, respectively. In the absence of an oxidant the radicals decay by second order (2k = 7.0×108 dm3 mol-1 s-1 and 2k = 4.4×108 dm3 mol-1 s-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-1 and 0.56 µmol J-1 (in monomer units), resp.). A small portion however disproportionates via abstraction of a C-6-bound Η atom followed by isomerization to 2,5-dihydroxypyrazines (pKa values 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 hydrogen atom is prefered to the transfer of a nitrogen-bound hydrogen atom. No disproportionation 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 dm3 mol-1 s-1 was obtained for this reaction by measuring and computing the dehydrodimer yields as a function of dose rate and solute concentration. Thus the observed transient spectrum 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)6 3- reflects the pKa of the transient radical. The rate constant for oxidation of the (protonated) radical derived from glycine anhydride is: k = 1.0x 108 dm3 mol-1, the corresponding radical anion is oxidized with k = 3.1 × 108 dm3 mol-1 s-1. No change with pH was observed in the case of the alanine anhydridederived radicals (k = 7.9x 108 dm3 mol-1 s-1). In contrast to the disproportionation, oxidation by Fe(CN)6 3- leads to the removal of a proton from the heteroatom, a carbocation being the intermediate. The resulting dehydropiperazines rapidly add water to yield the corresponding 2,5-diketo-3-hydroxypiperazines (G = 0.61 μmol J-1 after oxidation of the glycine anhydride-derived radicals, G = 0.58 µmol J-1 after oxidation of the alanine anhydride-derived radicals). The radicals derived from sarcosine anhydride are readily oxidized with k = 4.0×108 dm3 mol-1 s-1, independent of pH. 1H and 13C{1H} NMR-spectroscopic and mass-spectroscopic data of the products are given.