Biomedical Engineering Reference
In-Depth Information
voltage ramps of up to 500 mV, and had lifetimes of up to 8 h. In
such a comparison, the potential difference across the SUM was
regarded as negligible in view of its very low resistance of about 5
m:. Incorporation of D-hemolysin in the SUM-supported lipid
bilayer allowed the recording of single channel currents, which
were of comparable height with those on conventional BLMs. TEL
monolayers were also deposited by the modified Langmuir-
Blodgett technique on a SUM interposed between two aqueous
phases, in place of phospholipid bilayers.
236
Recrystallization of a
S-layer on top of SUM-supported TEL monolayers increased the
lifetime of the membrane from about 7 to about 21 h, while leav-
ing the specific conductance unaltered. Gramicidin was incorpo-
rated in SUM-supported membranes composed of DPhyPC, TEL
and mixtures of DPhyPC and TEL, and single-channel currents
were recorded. The selectivity toward K
+
ions was found to be
higher than that toward Na
+
ions, in accordance with the behavior
at conventional BLMs. However, the conductivity at SUM-
supported membranes was significantly lower than that at conven-
tional BLMs. Attempts to span a pure TEL monolayer across the
aperture in a plastic sheet without a SUM support by the modified
Langmuir-Blodgett technique were unsuccessful.
8. Protein-Tethered Bilayer Lipid Membranes (ptBLMs)
In all the biomimetic membranes previously described and allow-
ing the incorporation of proteins, the protein orientation in the
membrane is purely casual. At most, if one of the two extremem-
brane domains of the protein is much bulkier than the other, incor-
poration in a tBLM occurs preferentially with the bulkier domain
turned toward the aqueous phase, in view of the limited spacious-
ness of the hydrophilic moiety of the tBLM. Moreover, the pack-
ing density of the reconstituted proteins in the lipid bilayer is not
well controlled. The need for a well-defined protein orientation
with respect to the electrode surface is particularly felt with redox
membrane proteins, in which the electrons involved in a chain of
redox centers are conveyed across the membrane in a well-defined
direction.
To overcome this problem, Knoll, Naumann and coworkers
have developed a novel methodology based on tethering proteins,
rather than lipids, to the electrode surface; the lipids are then al-
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