Effect of Zwitterionic Phospholipid on the Interaction of Cationic Membranes with Monovalent Sodium Salts

Cationic lipids have attracted much attention because of their potential for biomedical applications, such as gene delivery. The gene transfection efficiency of cationic lipids is greatly influenced by the counterions as well as salt ions. We have systematically investigated the interaction of different monovalent sodium salts with positively charged membrane, composed of 1,2-dioleoyl- sn-glycero-3-phosphocholine (DOPC)/1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) and DOTAP, using dynamic light scattering, zeta potential, isothermal titration calorimetry (ITC), and fluorescence spectroscopy techniques. Our results reveal that the affinity of anions with cationic membranes follows the sequence I^- ≫ Br^- > Cl^- according to descending order of their sizes and is consistent with the Hofmeister series. Interestingly, the electrostatic behavior of the DOTAP membrane in the presence of monovalent anions differs significantly from the DOPC/DOTAP membrane. This difference is due to the strong interplay between phosphocholine and trimethylammonium-propane (TAP) headgroups leading to the reorientation of the TAP group in the membrane. The binding constant of anions, derived from zeta potential and ITC is in agreement with the affinity of anions mentioned above. Among all anions, I^- shows strongest affinity, as evidenced from the rapid increase in hydrodynamic radius which eventually leads to the formation of large aggregates. The fluorescence spectroscopy of a lypophilic probe Nile red in the presence of cationic vesicles containing ions complements the I^- adsorption onto the membrane. Nonlinear Stern-Volmer plot, consisting of accessible and inaccessible Nile red to I^- is consistent with the zeta potential as well as ITC results.