Unusual triskelion patterns and dye-labelled GUVs: consequences of the interaction of cholesterol-containing linear-hyperbranched block copolymers with phospholipids

Dense Monolayer Network Structures of Double Hydrophilic Hyperbranched Copolymers at the Air/Water Interface

Influences of subphase pH and temperature on the interfacial aggregation behavior of two double hydrophilic hyperbranched copolymers of poly[oligo(ethylene glycol) methacrylate-co-(2-diisopropylamino)ethyl methacrylate] (P(OEGMA-co-DIPAEMA)) at the air/water interface are studied by the Langmuir film balance technique. Morphologies of their Langmuir-Blodgett (LB) films are characterized by atomic force microscopy (AFM). At the interface, P(OEGMA-co-DIPAEMA) copolymers tend to form a dense network structure of circular micelles composed of branching agent-connected carbon backbone cores and mixed shells of OEGMA and DIPAEMA segments (pendant groups). This network structure containing many honeycomb-like holes with diameters of 6-8 nm is identified for the first time and clearly observed in the enlarged AFM images of their LB films. Under acidic conditions, surface pressure versus molecular area isotherms of the two copolymers in the low-pressure region show larger mean molecular area than those under neutral and alkaline conditions due to the lack of impediment from DIPAEMA segments. Upon further compression, each isotherm exhibits a wide pseudo-plateau, which corresponds to OEGMA segments being pressed into the subphase. Furthermore, the isotherms under neutral and alkaline conditions exhibit the lower critical solution temperature behavior of OEGMA segments, and the critical temperature is lower when the hyperbranched copolymer contains higher OEGMA content.

Cospreading of Anionic Phospholipids with Peptides of the Structure (KX)4K at the Air-Water Interface: Influence of Lipid Headgroup Structure and Hydrophobicity of the Peptide on Monolayer Behavior

Mixtures of anionic phospholipids (PG, PA, PS, and CL) with cationic peptides were cospread from a common organic solvent at the air-water interface. The compression of the mixed film was combined with epifluorescence microscopy or infrared reflection adsorption spectroscopy (IRRAS) to gain information on the interactions of the peptide with the different lipids. To evaluate the influence of the amino acid X of peptides with the sequence (KX)₄K on the binding, 1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol (DPPG) was mixed with different peptides with increasing hydrophobicity of the uncharged amino acid X. The monolayer isotherms of DPPG/(KX)₄K mixtures show an increased area for the lift-off due to incorporation of the peptide into the liquid-expanded (LE) state of the lipid. The surface pressure for the transition from LE to the liquid-condensed (LC) state is slightly increased for peptides with amino acids X with moderate hydrophobicity. For the most hydrophobic peptide (KL)₄K two plateaus are seen at a charge ratio PG to K of 5:1, and a strongly increased transition pressure is observed for a charge ratio of 1:1. Epifluorescence microscopy images and infrared spectroscopy show that the lower plateau corresponds to the LE-LC phase transition of the lipid. The upper plateau is connected with a squeeze-out of the peptide into the subphase. To test the influence of the lipid headgroup structure on peptide binding (KL)₄K was cospread with different anionic phospholipids. The shift of the isotherm to larger areas for lift-off and to higher surface pressure for the LE-LC phase transition was observed for all tested anionic lipids. Epifluorescence microscopy reveals the formation of LC domains with extended filaments indicating a decrease in line tension due to accumulation of the peptides at the LC-domain boundaries. This effect depends on the size of the headgroup of the anionic phospholipid.