Highly electrophilic quinone methide (QM) intermediates often express a surprising selectivity for weak nucleophiles of DNA even when proximity effects do not guide reaction. On the basis of model studies with an unsubstituted ortho-QM, these observations can now be explained by the reversibility of QM alkylation and the time-dependent shift from kinetic to thermodynamic products. The persistent and most commonly identified QM adducts represent thermodynamic products that typically form in low yield by irreversible reaction with weak nucleophiles such as the N1 and N2 of dG and the N6 of dA under neutral conditions. In contrast, strong nucleophiles such as the N1 of dA and the N3 of dC generate relatively high yields of their QM adducts. However, these products dissipate over time as the QM is repeatedly regenerated and repartitioned over the available nucleophiles. The adduct formed by the N7 of dG undergoes a similar release of QM as well as deglycosylation at comparable rates. The kinetic products of QM alkylation serve as a reservoir for QM regeneration and transfer that are likely to prolong the cellular activity of an otherwise highly transient intermediate.
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