Planar Bilayer Measurements of Alamethicin and Gramicidin Reconstituted in Biomimetic Block Copolymers.

This work explores methods for forming and characterizing biomimetic planar membranes composed of amphiphilic block copolymers. The membranes are diblocks and triblocks with hydrophilic blocks of poly(2-methyl-2-oxazoline) (PMOXA) and hydrophobic blocks of poly(dimethylsiloxane) (PDMS). Experiments with the lipid diphytanoyl phosphocholine (DPhPC) serve as a basis for comparison with the polymeric membranes. Phase-contrast microscopy is used to study how membranes evolve over time after their formation. Capacitance measurements as a function of the thinned membrane area (prepared from two separate solvent systems) are performed to clarify the importance of the Plateau-Gibbs border in electrical measurements. Finally, functional reconstitution of the two ion channels, alamethicin and gramicidin, is investigated. Imaging in transmitted phase-contrast mode provides visualization of thinned regions that contain monolayers or bilayers (in the case of diblock copolymer). The specific capacitance measurements yield 0.28 μF/cm2 with a corresponding thickness of 8.5 nm for PMOXA6-PDMS35-PMOXA6 (blocks of 6 PMOXA and 35 PDMS repeat units) formed from a solution of ethanol-decane, 0.55 μF/cm2 and 4.4 nm in chloroform-toluene, and 0.46 μF/cm2 and 5.4 nm for the diblock PMOXA6-PDMS17 in ethanol-decane. Alamethicin reconstitution in the block copolymers shows slower channel-forming kinetics with somewhat higher conductance values than found in DPhPC. Gramicidin in the block copolymer shows a slightly voltage-dependent conductance as a function of time, with little stochastic conductance state switching, in contrast to reconstitution in DPhPC where gramicidin switches states at ∼3 Hz.