DNA-bound Fe(II) generates hydroxyl radical that causes single-strand backbone nicks. DNA-localized antioxidants could shield DNA from hydroxyl radical damage but this requires targeting antioxidants to the DNA. Imidazole thiones and selones are iron-binding sulfur- and selenium-containing antioxidants that can prevent iron-mediated DNA damage, so we are exploring whether their metal-binding abilities can target them to DNA. Circular dichroism (CD) titration data with Fe(II) and Zn(II) and DNA show similar decreases in the intensity of the CD band at 275 nm, consistent with alteration of DNA secondary structure as a result of metal binding. Due to the similar CD results for Fe(II)- and Zn(II)-DNA interactions, Zn(II) titrations into DNA using isothermal titration calorimetry (ITC) were used to mimic Fe(II) to avoid hydrolysis and precipitation over the longer times required for these studies. Initial negative enthalpy peaks were observed, indicating enthalpy only dominates the reaction as Zn(II)-DNA binding approaches saturation. Zn(II) binding to DNA is a spontaneous reaction (ΔG = -6.9 kcal/mol). The number of binding sites (0.59 ± 0.02) agrees with reported Raman and IR studies of Zn(II)-DNA interactions and indicates one Zn(II) binds for every two nucleotides. The calculated K_dvalue of the Zn(II)-DNA interactions (9 ± 2 µM) is comparable to the K_dvalue reported by Ouameur et al. for Fe(II)-DNA interactions as determined by IR spectroscopy. To examine DNA interactions of Zn(dmit)₂Cl₂ (dmit = N,N’-dimethylimidazole thione), this complex was titrated into DNA and observed by CD spectroscopy, resulting in an increase in the helicity band (245 nm) and a decrease in the base-stacking (275 nm) band. ITC data are similar for Zn(II) and Zn(dmit)₂Cl₂titrated into DNA; however, the enthalpic contributions differ by 3.5-fold, indicating that Zn(dmit)₂Cl₂ interacts significantly more strongly with DNA. These differences in Zn(II) and Zn(dmit)₂Cl₂complex interactions with DNA suggest that metal ions potentially mediate antioxidant-DNA interactions to protect against oxidative DNA damage.

Craig Goodman

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