Experimental Evaluation of the B‐DNA → Z‐DNA Transition: Energetics of a Reversible, Thermally Induced Helix/Helix Transition of Poly dG‐m5dC in Physiological Mg2± Concentrations

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Abstract
UV spectroscopic studies show that poly dG‐m5dC above 2 mM Mg2+ undergoes a reversible, thermally induced helix/helix transition from the B‐ to the Z‐DNA conformation. The observed cooperativity of the transition suggested to monitor the enthalpy change of the interconversion by means of an adiabatic scanning microcalorimeter. The transition is endothermic, the transition temperature at half conversion is 59°C (10 mM Mg2+), the transition enthalpy is 4.77 kJ per mole base pairs, the transition entropy amounts to 14.35 J · mol−1 K−1. From the slope of the absorbance/temperature scan at 280 nm as well as from the plot of the degree of transition/temperature ΔHvH is calculated (1078 kJ/mol). The ratio of the ΔHvH and the true enthalpy obtained by calorimetry gives the number of base pairs (226) which undergo the transition simultaneously. The Gibbs enthalpy of the B→Z transition yields 0.5 kJ per mole base pairs.