Nucleobases, Nucleosides and Nucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 244

Table 10.17. Rate constants of isopyrimidine → pyrimidine rearrangements at 20 °C.

(Schuchmann et al. 1984a)

Neutral H + -
catal. dm 3
mol − 1 s − 1

Neutral OH − -
catal. dm 3
mol − 1 s − 1

Anion OH − -
catal. dm 3
mol − 1 s − 1

Isopyrimidine Neutral

spont. s −1

Anion
spont.
s −1

Isouracil 3000

1.8 × 10 7

Absent 50

4.9 × 10 5

3-Methyl-
isouracil

2500

1.1 × 10 7 2.7 × 10 7

Absent Absent

2.6 × 10 7 × 10 5 5-Hydroxy-

isouracil

2000 Absent < 50

Deprotonation of the carbocation at N (1) [reaction (137)] is kinetically favored
over the deprotonation at C (5) [reaction (136)]. Thus, isouracil is an important
intermediate in these reactions. It is sufficiently long-lived to follow its reactions
by pulse radiolysis. Isouracil is in equilibrium with its anion [equilibrium (138)].
This allows reactions (139) and (140) to proceed as well. However, when N (3) is
substituted such as in 3MeUra, this additional pathway is no longer possible. For
a compilation of rate constants see Table 10.17.