Enthalpy of the B-to-Z Conformational Transition of a DNA Oligonucleotide Determined by Isothermal Titration Calorimetry

The influence of high concentrations of Na+ or [Co(NH3)6]3+ on the conformation of two related DNA oligomers was investigated by circular dichroism spectropolarimetry (CD), isothermal titration calorimetry (ITC), and differential scanning calorimetry (DSC). As revealed by CD, DNA oligomers, (dC-dG)4 and (dm5C-dG)4, both form right-handed double helical structures (B-DNA) in standard phosphate buffer with 115 mM Na+ at 25°C. However, at 2.0 M Na+ or 200 μM [Co(NH3)6]3+, (dm5C-dG)4 assumes a left-handed double helical structure (Z-DNA), whereas the unmethylated (dC-dG)4 analog remains right-handed under those conditions. ITC was then used to determine the enthalpy change upon increasing the concentration of either Na+ or [Co(NH3)6]3+ for both DNA oligomers at 25°C. The titration with Na+ resulted in endothermic isotherms with (dm5C-dG)4 being more endothermic than (dC-dG)4 by 700 cal/mol basepair. In contrast, titration with [Co(NH3)6]3+ resulted in exothermic isotherms with (dC-dG)4 being more exothermic than (dm5C-dG)4 by 720 cal/mol basepair. We attribute the enthalpy difference to the conformational transition from B-form DNA to Z-form DNA for (dm5C-dG)4, a transition which does not occur for the unmethylated (dC-dG)4. The value of ∼700 cal/mol basepair for the enthalpy of the B-Z transition compares favorably with previously published results obtained by different techniques. DSC was used to monitor the duplex to single strand transitions for both oligomers under the different concentrations. These results indicated that methylation of the cytidine destabilizes (dm5C-dG)4 relative to (dC-dG)4. Coupling the DSC data with the ITC data allowed construction of a thermodynamic cycle which gives insight into the influence of both temperature and ionic strength on the heat content of the two DNA systems studied. Further, this study reveals the utility of using ITC for determinations of transition enthalpies with the appropriate choice of control.