Nucleobases, Nucleosides and Nucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 272

Table 10. 25. Rate constants of halide elimination from the radical anions of 5-halouracils

Rate constant/s −1

5-Halouracil Reference

4 × 10 8 5IUra Rivera and Schuler (1983)

1 × 10 8

5BrUra Rivera and Schuler (1983)

1.4 × 10 5
9 × 10 4

5ClUra Rivera and Schuler (1983)

Wagner and Schulte-Frohlinde (1975)

5FUra Slow Bansal et al. (1972)

Table 10. 26. Compilation of rate constants of ET from electron adducts and their proton-
ated forms of nucleobases and nucleosides. (Nese et al. 1992)

k/dm 3 mol − 1 s − 1

Substrate Reaction Reference

Thy Thy • − + 5BrUra

Thy • − + Cyt
Thy • − + Guo

1.1 × 10 9
1.7 × 10 9
5.0 × 10 8
7. 0 × 10 7

Adams and Willson (1972)
Nese et al. (1992)
Nese et al. (1992)
Nese et al. (1992)

2.3 × 10 7
7. 2 × 10 8

Thd ThdH • + 5BrUra

Thd • − + 5BrUra

Nese et al. (1992)
Nese et al. (1992)

AdoH • + 5BrUra

3.5 × 10 8
2.0 × 10 8

Ado Adams and Willson (1972)

Nese et al. (1992)

Cy tH • + 5rBrUra
Cy tH • + Thy

10 7
2.0 × 10 7
2.0 × 10 6

Cy t < 5 × Adams and Willson (1972)

Nese et al. (1992)
Nese et al. (1992)

GuoH • + 5BrUra Guo No transfer Nese et al. (1992)

1994). It is discussed above that the nucleobase radical anions can be proton-
ated at a heteroatom and/or carbon. Only the heteroatom-protonated species re-
tains reducing properties, and thus the rate of protonation at carbon determines
whether or not an ET to 5BrUra is observed under the given condition. Proton-
ation at carbon is especially fast in the case of Guo, and for this reason an ET to
5BrUra was not observed (Nese et al. 1992). A compilation of the rate constants
for such ET reactions is found in Table 10.26. As can be seen from this table, the
radical anions transfer an electron to 5BrUra at practically diffusion-controlled
rates, while the heteroatom-protonated species react two orders of magnitude
more slowly.