Microcalorimetric study on the interaction of dissymmetric gemini surfactants with DNA

The interaction of a series of dissymmetric gemini surfactants, [CmH2m+1(CH3)2N(CH2)6N(CH3)2CnH2n+1]Br2 (designated as CmC6CnBr2, with constant m+n=24, and m=12, 14, 16, and 18) with DNA in 10 mM NaCl solution has been investigated by isothermal titration microcalorimetry (ITC). The curves for titration of the surfactants into DNA solution show noticeable differences from those into 10 mM NaCl solution without DNA. It is attributed to the interaction between DNA and surfactants. The critical aggregation concentration (CAC), the saturation concentration (C2), and the thermodynamic parameters for the aggregation and interaction processes were obtained from the calorimetric titration curves. The results show that the dissymmetry degree (m/n) has a marked effect on the interaction of the CmC6CnBr2 surfactants with DNA. The CAC and C2 tend to become smaller with increased m/n. The enthalpy change (ΔHagg) and the Gibbs free energy change (ΔGagg) for aggregation become more negative down the series, indicating that the hydrophobic interaction between the hydrophobic chains of the surfactant molecules increases and the aggregation process is more spontaneous with increased m/n. The entropy changes of aggregation (ΔSagg) are all positive and TΔSagg is much larger than |ΔHagg|, revealing that the aggregation process is mainly entropy-driven. However, the calculated Gibbs free energy (ΔGDS) for the interaction between the gemini surfactants and DNA becomes less negative with increased m/n, which reveals that the interaction between the gemini surfactants and DNA tends to be weaker with increased m/n. This is induced by the disruption of the chain-chain hydrophobic interaction between the surfactant molecules at higher m/n, where the entropy change ΔSDS for the interaction process tends to be an unfavorable factor. In addition, the DNA concentration also has a remarkable influence on the interaction.