Block Copolymer Membranes

Self-assembly of reactive amphiphilic block copolymers may also be used to prepare nanostructured hydrogel membranes with exceptional permeability properties [158]. Although the block copolymer membranes (PMOXA-PDMS-PMOXA triblock) are two- to threefold thicker than conventional

H&fll fit

S-layer

antibody

S-layer

Figure 59. Schematic drawing illustrating the immobilization of IgG to carbodiimide-activated carboxyl groups of the S-layer protein of SUMs (A), to protein A covalently bound to the S-layer lattice (B), and after biotinylation to a streptavidin-modified SUM (C).

Figure 59. Schematic drawing illustrating the immobilization of IgG to carbodiimide-activated carboxyl groups of the S-layer protein of SUMs (A), to protein A covalently bound to the S-layer lattice (B), and after biotinylation to a streptavidin-modified SUM (C).

lipid bilayers, they can be regarded as mimetic of biological membranes and can further be used as a matrix or scaffold for membrane-spanning proteins [159]. Surprisingly it has been found that the proteins remain functional, despite the thickness of the membranes and even after polymerization of the reactive block copolymers [160]. The unique combination of block copolymers with membrane protein based channels allows the preparation of mechanically stable, defect-free membranes and nanovesicles [161] that have highly selective permeability and/or specific recognition sites. See Figure 61.

The block copolymer/protein hybrid shells of the nano-vesicles can be regarded as a semipermeable membranes separating their internal volume from the external solution. This property opens a convenient approach to trigger the gating transition of OmpF. Large polyelectrolyte ions, such as polystyrene sulfonate, will not permeate, and

Figure 60. (a) Transmission electron micrographs of a nanometric point pattern of CdS particles obtained by biomineralization on an S-layer with oblique lattice symmetry. Protein appears white, CdS particles dark. Bar, 60 nm. (b) Corresponding computer image reconstruction to (A). Bar = 10 nm. Courtesy of FEMS Microbiology Reviews.

Figure 60. (a) Transmission electron micrographs of a nanometric point pattern of CdS particles obtained by biomineralization on an S-layer with oblique lattice symmetry. Protein appears white, CdS particles dark. Bar, 60 nm. (b) Corresponding computer image reconstruction to (A). Bar = 10 nm. Courtesy of FEMS Microbiology Reviews.

enzyme product enzyme product

protein channel supply

Figure 61. Schematic view of a bioreactor/nanovescicle formed by a poly-2-methyloxazoline-polydimethylsiloxane-poly-2-methyloxazoline (PMOXA-PDMS-PMOXA) triblock copolymer with encapsulated enzyme. Channel proteins in the shells of the bioreactors control the exchange of substrates and products. Reprinted with permission from [158], C. Nardin and W. Meyer, Rev. Mol. Biotechn. 90, 17 (2002). © 2002, Elsevier Science.

protein channel supply

Figure 61. Schematic view of a bioreactor/nanovescicle formed by a poly-2-methyloxazoline-polydimethylsiloxane-poly-2-methyloxazoline (PMOXA-PDMS-PMOXA) triblock copolymer with encapsulated enzyme. Channel proteins in the shells of the bioreactors control the exchange of substrates and products. Reprinted with permission from [158], C. Nardin and W. Meyer, Rev. Mol. Biotechn. 90, 17 (2002). © 2002, Elsevier Science.

therefore the sodium counterions will be distributed inside and outside the nanovesicles according to Donnan equilibrium conditions, giving rise to a Donnan potential. If this potential exceeds the critical value necessary for closure of OmpF, the substrates can no longer enter the interior of the nanoreactor vesicle (i.e., the reactors are then deactivated). The closure is a reversible process, and decreasing the potential below 100 mV may reactivate the nanoreactor vesicles. This could be carried out by diluting the system with buffer or by increasing the Na+ concentration in the system.

Peptide nanochannels or nanotubes have also been formed by cyclo([L-Trp-D-Leu]3-L-Gln-D-Leu) in phospho-lipid multilayers obtained by air drying on a substrate [162]. Nanotubes formed from cyclo([L-Trp-D-Leu]4) have also been allowed to assemble into preformed monolayers of dodecanethiol or octadecyl sulfide deposited on gold films [163].

Bonding of the bilayer can be adjusted by coating of the membrane using a matching precoat, for instance high-quality alkyl monolayers. The stability of the precoat is of very high importance for the final stability of the total stack [164, 165]. See Figure 62.

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