Electrochemistry of Biomimetic Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 245

olayer of SCWPs, suitably thiolated at one end, can be anchored to
a gold support, and a S-layer can be spontaneously recrystallized
on top of it; finally, a lipid bilayer can be self-assembled on top of
the S-layer. Optionally, the lipid bilayer so formed can be further
stabilized by recrystallizing an additional S-layer on top of it. Al-
ternatively, a loose monolayer of lipidated SCWP molecules can
be bound to a S-layer previously recrystallized on a substrate; a
lipid bilayer self-assembled on top of the SCWP monolayer is then
firmly anchored by the lipid moieties of the SCWPs, which pene-
trate the proximal lipid leaflet. Optionally, the lipidated SCWP
molecules present on the outer lipid leaflet can be bound to a fur-
ther S-layer. All these architectures have a stabilizing effect on the
associated lipid bilayer, leading to an improvement in its lifetime
and robustness. The assembly of S-layer structures from solution
to a solid substrate, such as an Au-coated glass slide, can be fol-
lowed by SPR or by a QCM-D. 233 The self-assembling process is
completed after approximately 45 min. The mass increase fol-
lowed by QCM-D corresponds to a thickness of about 8-9 nm, in
agreement with the value estimated by SPR.
A well-characterized S-layer protein, SbqA from Bacillus
sphaericus CCM 2177, was used as an ultrathin crystalline, water
containing hydrophilic layer between a gold electrode and a lipid
bilayer. 234 The SbqA protein recrystallizes in monomolecular
square lattices with the neutral outer surface exposed to the aque-
ous phase and the negatively charged inner surface attached to the
gold electrode. A morphological unit, about 170 nm in diameter,
consists of four protein monomers. The pores are of identical size
and morphology, with a diameter of about 3.5 nm. A SqbA-coated
gold substrate was pressed against a polyethylene plastic sheet
attached vertically to the open side of a chamber, as shown in Fig.
22a . An orifice, about 1u10 3 cm 2 in area, was made through the
plastic sheet by punching it with a perforating tool. Alternatively, a
piece of SUM was interposed between the gold surface and the
plastic sheet ( Fig. 22b ). SUM (S-layer ultrafiltration membrane) is
an isoporous structure with very sharp molecular exclusion limits,
manufactured by depositing under pressure up to three stacked S-
layer-carrying cell wall fragments on a commercial polyamide
microfiltration membrane (MFMs), with an average pore size of
approximately 0.4 Pm. The 1-4 Pm sized S-layer fragments are
deposited on the microfiltration membrane as shingles on a roof.