The kinetics of the rearrangement of some isopyrimidines to pyrimidines studied by pulse radiolysis

Abstract

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.

The reactions of thymine and thymidine with ozone

“…refs. 42,43). In the present system, addition of water to the N(1)-C(6) double bond will result in the formation of thymine glycol 20 [reaction (33)].…”

Singlet dioxygen formation in ozone reactions in aqueous solution

“…Table 1). We suggest that when the neutral molecule predominates, the ozone reaction follows the Criegee mechanism [overall reactions (18) and ( 19)], but upon deprotonation [reaction (17)] an isopyrimidine 54, 55 is formed [reaction (20)]. Preliminary experiments indicate that 5chlorouracil indeed yields HCl, parabanic acid (imidazolidinetrione), and formic acid [reaction (19)], whereas in the reaction of the 5-chlorouracilate ion with ozone the yields of parabanic acid and formic acid are reduced and isodialuric acid (dihydro-6-hydroxypyrimidine-2,4,5(3H )-trione) is formed instead [reactions ( 20)-( 23)].…”

OH Radical Reaction of 5-Substituted Uracils: Pulse Radiolysis and Product Studies of a Common Redox-Ambivalent Radical Produced by Elimination of the 5-Substituents

“…12,13 While the pyrimidine C(5) radical 2b has an oxidizing property analogous to β-oxoalkyl radicals, 15 the pyrimidine C(6) radical 3b is of reducing property and thereby is an exclusive precursor of thymine glycol 16 under oxygen-free conditions. The distributions of OH radical attacking sites for uracil (1a) and its derivatives have been determined by pulse radiolysis in conjunction with a redox titration method, [17][18][19][20][21][22][23] using the difference in redox properties of the OH adduct carbon-centered radicals. More recently, theoretical calculations were also performed to predict the distribution of OH radical attack.…”

“…In attacking thymine ( 1b ) base constituents, OH radicals add preferentially to the C(5)−C(6) double bond to produce 6-hydroxy-5,6-dihydrothymin-5-yl radical ( 2b ) and 5-hydroxy-5,6-dihydrothymin-6-yl radical ( 3b ) (reactions 1 and 2 in Scheme ). − These OH radical additions are also accompanied by relatively minor hydrogen atom abstraction at the methyl group. , While the pyrimidine C(5) radical 2b has an oxidizing property analogous to β-oxoalkyl radicals, the pyrimidine C(6) radical 3b is of reducing property and thereby is an exclusive precursor of thymine glycol under oxygen-free conditions. The distributions of OH radical attacking sites for uracil ( 1a ) and its derivatives have been determined by pulse radiolysis in conjunction with a redox titration method, − using the difference in redox properties of the OH adduct carbon-centered radicals. More recently, theoretical calculations were also performed to predict the distribution of OH radical attack .…”