Self-assembly and dynamics of complex amphiphilic systems: from block copolymers to phospholipids

The self-assembly of polystyrene-block -poly(L-lysine) copolymers with different block lengths synthesized by anionic polymerization and subsequent ring opening polymerization of N-carboxyanhydrides was studied. The high glass transition temperature of polystyrene leads only to the formation of frozen metastable aggregates in water in the presence of the non-ionic surfactant C12E6. Light scattering measurements and direct imaging techniques show that the copolymers self-assemble into spherical micellar aggregates. For copolymers with large blocks (nPS ≥ 258, nPLL ≥ 107) the solubility is extremely low and large aggregates in solution are detected, even at very low polymer concentrations (0.01wt.%). In this case, depolarized dynamic light scattering improves the particle size determination. The obtained values for the hydrodynamic radius of the micelles are between 20 and 40 nm, depending on the block lenghts. Circular dichroism measurements show the switching of the polypeptide secondary structure upon changes of pH. Poly(L-lysine) block adopts a random coil conformation at low pH and a beta-sheet conformation at pH ≥ 11. The shape of the aggregates does not change with this conformational transition as Cryo-TEM images show, while the average particle size increases at least 10%. The analysis by AFM of the aggregates deposited on graphite surfaces from solutions of the copolymer at different pH reveals changes in the wetting properties of the aggregates with conformation. When polylysine is in the random coil conformation, the aggregates present a smooth flat cross-section profile, while in the apolar beta-sheet conformation, the aggregates wet the graphite surface in some kind of ordered lattice. In toluene, the copolymers are better solubilized. The effect of the addition of pyridine to these solutions was tested by light and neutron scattering techniques. It was shown that pyridine induces a shrinking of the polystyrene chains in the micelle shell region decreasing the aggregates-solvent interface. On the other hand, the addition of benzoic acid to solutions of the copolymers in toluene yields a swelling of the copolymer micelles. This is well correlated with the polypeptide/polystyrene monomer number ratio, suggesting that benzoic acid molecules are able to penetrate into the copolymer micelles. In addition, other amphiphilic systems have been studied. First, the preparation of vesicles by shear of lamellar phases based on PEO-PPO-PEO (P85) copolymer has been tested. The lamellar phase was sheared at different shear rates and the evolution of the viscosity with time monitored. It shows that the system reaches at any shear rate the same final steady state of viscosity. The sheared solutions were measured by dynamic light scattering, presenting an average particle radius around 30 nm, compatible with the theoretical value for P85 vesicles. Secondly, monodisperse unilamellar vesicles composed of phospholipids were prepared by extrusion at different cholesterol contents and studied by scattering techniques, giving coherent values for the bending elastic modulus. It demonstrates that cholesterol increases the flexibility of DPPC bilayers and rigidifies POPC bilayers without any change in the membrane thickness.