Determination of Permeability Coefficients of Polymersomal Membranes for Hydrophilic Molecules.

Polymer vesicles, so-called polymersomes, can be applied as carrier-systems and universal reaction compartments, due to the possibility to encapsulate guest molecules. Compared to common lipid vesicles, polymersomes show an increased stability and decreased membrane permeability. Control of the mass transport across the membrane is necessary for any application, requiring the precise knowledge of the permeability. So far, data on permeability coefficients of polymersomal membranes are scarce because commonly applied release assays are confronted with the challenge of high detection limits and alternative methods developed so far are either restricted to the use of a certain permeating molecule or rely on the use of nuclear magnetic resonance measurements. In contrast, an influx assay that is broadly applicable to hydrophilic molecules and does not involve specialized equipment was developed in this work, which is based on the passive diffusion of compounds into initially empty vesicles. The method is valid for hydrophilic molecules that show no membrane retention and, thus, do not accumulate within the membrane. Using this method, the permeability of polymersomes made of poly(2-methyloxazoline)15-poly(dimethylsiloxane)68-poly(2-methyloxazoline)15 for seven model compounds was investigated under varying conditions. Permeability coefficients as low as 1.9 × 10-14 cm s-1 could be measured.

Simple surface functionalization of polymersomes using non-antibacterial peptide anchors.

BACKGROUND: Hollow vesicles formed from block copolymers, so-called polymersomes, have been extensively studied in the last decade for their various applications in drug delivery, in diagnostics and as nanoreactors. The immobilization of proteins on the polymersomes' surface can aid in cell targeting, lead to functional biosensors or add an additional reaction space for multistep syntheses. In almost all surface functionalization strategies to date, a chemical pre-conjugation of the polymer with a reactive group or ligand and the functionalization of the protein are required. To avoid chemical pre-conjugation, we investigated the simple and quick functionalization of preformed poly(2-methyloxazoline)-poly(dimethylsiloxane)-poly(2-methyloxazoline) (PMOXA-PDMS-PMOXA) polymersomes through the spontaneous insertion of four hydrophobic, non-antibacterial peptide anchors into the membrane to display enhanced green fluorescent protein (eGFP) on the polymersomes' surface. RESULTS: Three of the four hydrophobic peptides, the transmembrane domains of a eukaryotic cytochrome b 5 , of the viral lysis protein L and of the yeast syntaxin VAM3 could be recombinantly expressed as soluble eGFP-fusion proteins and spontaneously inserted into the polymeric membrane. Characterization of the surface functionalization revealed that peptide insertion was linearly dependent on the protein concentration and possible at a broad temperature range of 4-42 °C. Up to 2320 ± 280 eGFP molecules were immobilized on a single polymersome, which is in agreement with the calculated maximum loading capacity. The peptide insertion was stable without disrupting membrane integrity as shown in calcein leakage experiments and the functionalized polymersomes remained stable for at least 6 weeks. CONCLUSION: The surface functionalization of polymersomes with hydrophilic proteins can be mediated by several peptide anchors in a spontaneous process at extremely mild insertion conditions and without the need of pre-conjugating polymers.