Nucleobases, Nucleosides and Nucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 250

Table 10. 20. Rate constants for protonation of the pyrimidine radical anion at C (6) by
H 2 PO 4 − using various approaches of evaluating the data (Deeble et al. 1985)

/10 7 dm 3 mol −1 s −1

Pyrimidine p K a k

1,3Me 2 Thy 7.2 1.6/0.74 /0.95

Thy 7.2 0.32/0.33

Thd 7.2 0.19/0.21

1MeUra 7.0 0.14/0.25

Ura 7.3

∼ 0.1

Urd 7.2

∼ 0.05/ ∼ 0.05

6MeUra 7.3 <0.01

The other functional groups, especially the second carbonyl function, with-
draws considerable electron density, and hence the p K a values of these O -pro-
tonated radical anions are much lower than those of the simple

-hydroxyalkyl
radicals (Thy: p K a = 6.9 (Hayon 1969); Ura: p K a = 7.2 (Shragge and Hunt 1974);
the value of 4.3 reported for the p K a for the protonated 5,6Me 2 Ura radical anion
(Aravindakumar et al. 1998) is surprising low and difficult to explain on the ba-
sis of the effects of methyl substituents on the p K a values of

α

-hydroxyalkyl radi-

α