Composition and Properties of Complexes between Spherical Polycationic Brushes and Anionic Liposomes

A spherical polycationic brush (SPB) is made by graft-polymerizing a cationic monomer onto the surface of a 100 nm polystyrene bead. It is possible to adsorb anionic liposomes (40-60 nm diameter) onto the SPBs while maintaining the liposome integrity. The liposomes were constructed with phosphatidyl choline (PC) admixed with 0.05-0.4 mol fraction of an dianionic lipid, cardiolipin (CL2-). As shown by electrophoretic mobility measurements, SPB-to-liposome complexation leads to a conversion from the initial positive charge of the copolymer to a negative charge. The higher the CL2- content of the liposomes, the lower the concentration needed for charge neutralization. Dynamic light scattering (DLS) revealed that multicomplex aggregates are formed with a maximum size at the SPB/liposome charge-equivalence point. Experiments with fluorescent-labeled liposomes show that at low CL2- content about 80 liposomes are adsorbed per SPB. As the mole fraction of CL2- increases from 0.05 to 0.4, fewer liposomes adsorb owing to electrostatic repulsion among neighboring liposomes. The effect of added NaCl also depends upon the CL2- content. With 0.05 mol fraction CL2-, the SPB/liposome complex dissociates into its components at 0.15 M NaCl. With a mole fraction of >0.1, complexes fail to dissociate even at 1.2 M NaCl. Additional information about the SPB/liposome morphology was obtained from cryo-TEM. For example, cryo-TEM data confirm liposome integrity upon complexation, a behavior that contrasts with the liposome destruction as found with adsorption to many other types of surfaces.