DNA binding studies of abietane diterpenes natural products using isothermal titration calorimetry, circular dichroism, fluorescence and gel assays

A group of four structurally related abietane diterpenes (including royleanone and 7-acetoxy-horminone) were investigated for their DNA binding capabilities using isothermal titration calorimetry (ITC), circular dichroism (CD) and fluorescence displacement spectroscopy, and a topoisomerase DNA-unwinding assay. Both ITC and CD spectroscopy data indicate that the abietane diterpenes of this study exhibit strong binding to DNA, likely preferring a DNA-groove binding mode. Binding constants (K) were found to be in the order of 10^6–8 M⁻¹ and were strongly enthalpically driven. The binding of all compounds to DNA was accompanied by large negative enthalpy changes (more than −20 kcal/mol) and negative entropy changes. The substituent at the 7th position of the abietane ring system was important in determining both the magnitude of binding and the propensity to stack within their DNA binding sites, with derivatives 7-one/ene > 7-OAc/H. This is significant given the reports that the identity of the substituent at position-7 is a strong determinant for bioactivity. Although the specific CD data observed was compound-dependent, most showed strong signal perturbations around one or both of the DNA signature wavelengths (245 and 280 nm). However, we do not attribute these perturbations to DNA intercalation. Compounds that were capable of self-stacking (i.e., 7-ene and 7-one) were also able to elicit very strong positively induced CD signal (ICD) around 330 nm, as well as perturbations at higher wavelengths. Additionally, topoisomerase DNA-unwinding and ethidium fluorescence displacement assays were used to corroborate the DNA binding mode, which was found to be consistent with the abietane diterpene compounds adopting a non-intercalative DNA binding mode.