Electrochemistry of Biomimetic Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 243

7. S-Layer Stabilized Bilayer Lipid Membranes (ssBLMs)
Monomolecular crystalline arrays of protein subunits, called S-
layers, are common surface structures of archea and bacteria. 233-237
They constitute the outermost component of the cell envelope of
these procaryotic organisms. S-layer subunits can be aligned in
lattices with oblique, square or hexagonal symmetry. Since S-
layers are monomolecular assemblies of identical protein subunits,
they exhibit pores of identical size and morphology. Most S-layers
have a rather smooth outer surface and a more corrugated inner
surface. In gram-negative archea, S-layers are the only component
external to the cytoplasmic membrane; pillar-like extensions of the
subunits of the S-layers may even penetrate into the polar head
region of the membrane. Conversely, in gram-positive bacteria, the
rigid wall component in direct contact with the cytoplasmic mem-
brane is primarily composed of peptidoglycan, a polymer consist-
ing of sugars and amino acids that forms a mesh-like layer. A
group of non classical cell wall polymers, called secondary cell
wall polymers (SCWPs), are covalently bound to muramic acid
residues of peptidoglycan and attached non-covalently, presuma-
bly by a lectin-type interaction, to the S-layer proteins. SCWPs are
composed of disaccharide repeating units and contain pyruvate
ketals that provide a negative charge. In gram-negative bacteria, a
thin peptidoglycan layer is interposed between an inner membrane
and an outer membrane, with an S-layer non-covalently bound to
the latter.
Since S-layer subunits of most bacteria interact with each oth-
er through non-covalent forces, they can be set free with high con-
centrations of agents that break hydrogen bonds, such as guanidine
hydrochloride or urea. Once the S-layer lattice of a bacterial cell is
completely disintegrated and the disintegrating agent is removed
by dialysis, the S-layer subunits have the unique capability to reas-
semble spontaneously in suspension, at the liquid/air interface, on
solid surfaces, on spread lipid monolayers and on liposomes. Re-
crystallization starts at several distant nucleation points on the sur-
face and proceeds until neighboring crystalline areas meet. It this
way, a closed mosaic of differently oriented monocrystalline do-
mains is formed. Recrystallization of isolated S-layer proteins on
differently charged solid supports reveals an electroneutral outer
and a net negative or net positive inner S-layer surface.