Enthalpy - entropy contribution to carcinogen-induced DNA conformational heterogeneity

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DNA damage by adduct formation is a critical step for the initiation of carcinogenesis. Aromatic amines are strong inducers of environmental carcinogenesis. Their DNA adducts are known to exist in an equilibrium between the major groove (B) and base-displaced stacked (S) conformations. However, the factors governing such heterogeneity remain unclear. Here we conducted extensive calorimetry/NMR/CD studies on the model DNA lesions caused by fluorinated 2-aminfluorene (FAF) and 4-aminobiphenyl (FABP) in order to gain thermodynamic and kinetic insights into the S/B conformational equilibrium.We demonstrate that there are large differences in enthalpy-entropy compensations for FABP and FAF. The small and flexible FABP exclusively adopts the less perturbed B conformer with small enthalpy (ΔΔH-2.7 kcal/mol)/entropy (ΔΔS-0.7 eu) change. In contrast, FAF stacks better and exists as a mixture of B and S conformers with large enthalpy (ΔΔH-13.4 kcal/mol)/entropy (ΔΔS-34.2 eu) compensation. van't Hoff analysis of dynamic 19F NMR data indicated ΔHB↔S=4.1 kcal/mol in favor of the B conformer and ΔSB↔S=15.6 cal mol -1 K-1 in favor of the intercalated S conformer. These findings demonstrate that the favorable entropy of the S conformer over B conformer determines the S/B population ratios at physiological temperatures. ©2009 American Chemical Society.

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