Rate constants of ozone reactions with DNA, its constituents and related compounds

Abstract

The rate constants of the reaction of ozone with DNA, its constituents and related compounds have been determined as a function of pH by competition with nitrite and/or buten-3-ol and, when the rate constant was ≤10³ dm³ mol⁻¹ s⁻¹, by the indigo method. Depending on the degree of protonation, the rate constant (in units of dm³ mol⁻¹ s⁻¹) varies substantially, e.g. in the case of cytosine, k = 18 (protonated), k = 1.4 × 10³ (neutral) and k = 1.5 × 10⁶ (deprotonated). A similar variation has been found with the other nucleobases. Upon deprotonation the mechanism of the ozone reaction may also change; e.g. no singlet dioxygen (O₂¹Δ_g) is formed in its reaction with 5-chlorouracil, but when the 5-chlorouracilate ion predominates it becomes a major product (∼42%). Rate constants for the neutral compounds are: thymine (4.2 × 10⁴), thymidine (3.0 × 10⁴), 1,3-dimethyluracil (2.8 × 10³), uracil (650), 6-methyluracil (140), 5-chlorouracil (4.3 × 10³), orotic acid (5.9 × 10³), isoorotic acid (3.7 × 10³), 2′-deoxycytidine (3.5 × 10³), cytidine (3.5 × 10³), adenine (12), 2′-deoxyadenosine (14), adenosine (16), guanosine (1.6 × 10⁴), 2′-deoxyguanosine (1.9 × 10⁴) and DNA (410). In the case of adenine and its derivatives, and thus also in the case of DNA, ˙OH is produced (via O₂˙⁻ as an intermediate). For the determination of their intrinsic ozone rate constants, tert-butyl alcohol was added as the ˙OH scavenger.