Study of Quinizarin Interaction with SDS Micelles as a Model System for Biological Membranes

Investigation of the interaction of quinizarin (Q), an analogue of the core unit of different anticancer drugs, with anionic SDS micelles has been performed by absorption and conductance measurements in 0.1 M phosphate buffer, pH 7.4 and over the temperature range of 298.15-323.15 K. The values of binding constant (Kb), partition coefficient (Kx) and the corresponding thermodynamic parameters (Gibbs free energy, enthalpy, entropy) for the binding and distribution of quinizarin between the bulk aqueous solution and surfactant micelles have been determined and discussed in terms of possible intermolecular interactions. Values of critical micelle concentration (CMC) and degree of ionization (?) for SDS in the absence and the presence of quinizarin have been evaluated from conductometric study. Comparing the absorption spectra of quinizarin in SDS micelles with the spectra in different solvents revealed that quinizarin molecules are located in the hydrophilic region of SDS micelles. The trend of changes in Gibbs free energy, enthalpy and entropy with temperature shows that both binding and partition processes are spontaneous and entropy driven. In addition, the hydrophobic interactions are the main forces involved in binding and partition processes.