The γ-radiolysis of N2O-saturated aqueous solutions generates OH radicals and a small amount of H atoms. In 1,3-dimethyluracil solutions (10-3 M) these radicals add mainly to the 5-position of the C(5) -C(6) double bond of the solute. The resulting products have been identified by GC-MS after trimethylsilylation. The major products (G values in brackets) are dimers of the C(5) OH-adduct radicals (3.4), 5,6-dihydro-5,6-dihydroxy-1,3-dimethyl- uracil (0.85), 5,6-dihydro-5-hydroxy-1,3-dimethyluracil (0.75), dimer of the C(5)-OH- adduct with the C(5)-H-adduct radical (0.2) and 5,6-dihydro-6-hydroxy-1,3-dimethyl- uracil (0.2). G( 1,3-dimethyluracil consumption) has been found to be 5.7.The major reaction in this system is the dimerisation of the primary radicals. To a small extent the primary radicals also disproportionate by electron transfer to give the glycol 5,6-dihydro-5,6-dihydroxy-1,3-dimethyluracil as one of the products. Radiolytically formed H2O2 which could oxidise the C(5)-OH-adduct radical does not contribute to the formation of the glycol because the H2O2 yield is not reduced with respect to its expected “molecular yield” (G = 0.75). In contrast to the uracil system where it has been reported that drastic changes in radiolytic yields occur on going from neutral to alkaline irradiation conditions no such changes have been observed with 1,3-dimethyluracil. However, an acid-catalysed rearrangement from the C(5)-OH-adduct to the C(6)-OH-adduct radical is observed in the present system

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The nature and yields of the products of radiolysis of aqueous solutions of dihydrouracil containing both N 2 0 and O2 strongly depend on pH and dose rate. At a dose rate of 0.3 Gy s-' and at pH 3, the major products are barbituric acid (G 2.4), labile material (G 2.8), and uracil ( G 0.1). The labile material is converted into uracil upon treatment with acid and is largely composed of 5,6-dihydro-6-hydroxyuracil. At pH 7 barbituric acid is absent and uracil becomes the major product (G 4.5) ; some labile material is also formed (G 0.7). At pH 5 the pattern of the product distribution changes as a function of dose rate. With increasing dose rate G( uracil) decreases whereas G( barbituric acid) increases. The predominant radical formed in the dihydrouracil system is the 6-peroxyl radical, and it has been found, using pulse radiolysis with both optical and conductometric detection, that this peroxyl radical can eliminate 02-' to give uracil via an unstable isomeric form of the pyrimidine (isouracil). The elimination is base-catalysed so that, under alkaline conditions, uracil is the major radiolysis product. At lower pH, and also at higher dose rates, bimolecular decay of the peroxyl radical competes with the 02-' elimination process, barbituric acid being a specific product of the bimolecular decay route. The mechanisms of these processes are discussed.Irradiation of dihydrothymine-N20-02 solutions gave similar results.The radiolysis of pyrimidines and dihydropyrimidines (as models for DNA) in aqueous solutions has received considerable attention (for reviews see refs. 1 and 2), and some details are fairly well understood.In the radiolysis of N20-saturated aqueous solutions, OH radicals are the predominant primary species available for reaction with solutes: H20 radiolysis -+ OH', edaq, H', H3O+, Hz, H D z e-aq + NzO-*OH + N2 + OH-The OH radicals react with pyridimines by addition to the 5,6-double bond; with uracil the preferential site of attack is C(5).3 In the case of dihydropyrimidines the OH radicals abstract H atoms at the C(5)-C(6) bond; with dihydrouracil this occurs mostly at C(6) [> 90%; reaction ( l ) ] and much less at C(5) [ca. 5%; reaction (2)].4 Thus in both cases 6-yl radicals are formed predominantly. The 6-yl radicals are readily oxidized by transition metal ions [reaction ( 3 ) ] . s * 6 The intermediate cation is not stable and rapidly loses the proton at N( 1) [reaction (4)]. The product is an unstable isomeric form of the pyrimidine, isouracil (IV).

lsopyrimidines are formed as a result of the oxidation of the 6-yl radicals derived by 'OH attack on pyrimidines and dihydropyrimidines. The kinetics of the rearrangement of the isopyrimidines into the corresponding pyrimidines has been followed by pulse radiolysis. The rearrangement of isouracil into uracil is proton-catalysed (k 1.8 x 1 O7 I mol s-' ). Around pH 7 a spontaneous reaction, k 3 000 s ', is observed. On increasing the pH the isouracil deprotonates at N(3) (pK, ca. 9.4). The spontaneous rearrangement of the isouracil anion is considerably slower (k I 50 s '). At pH > 10.5 an OH-catalysed reaction sets in (k 4.9 x 1 Ob I mol ' s ' ) which involves a second deprotonation, at C( 5).Similar results have been obtained for the rearrangement of 5hydroxyisouracil into isobarbituric acid. On blocking the N(3) position as in 3-methylisouracil, the OH--induced rearrangement sets in at a much lower pH (pH I 9.5), i.e. the rearrangement is faster (k 2.7 x 1 O7 I molV s-') than that observed in the other two systems.

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