Biomedical Engineering Reference
In-Depth Information
about the influence of nanopores on the fusion of proteoliposomes
of similar size.
By comparing free-standing bilayers on Teflon chips with
pore diameters ranging from 25 to 250 Pm, the capacitance was
found to be about equal to 0.6 PF cm
-2
, a value typical of solvent-
containing BLMs.
103
These bilayers were stable for many hours
even at applied voltages up to 400 mV. The ion-channel activity of
alamethicin and D-hemolysin could be recorded with a high signal-
to-noise ratio, by using low-noise patch-clamp amplifier technolo-
gy. Evans' group reported a device consisting of a 100 Pm mi-
cromachined hole in a gold surface suspended over an aqueous
reservoir. An octadecanethiol
104
or perfluorothiol monolayer
105
was tethered to the gold surface, to impart a hydrophobicity similar
to that of the bilayer-forming solution to the hole rim. A lipid bi-
layer was then painted across the aperture with a plastic stick. In-
corporation of the channel-forming peptides gramicidin and ala-
methicin allowed the recording of their single-channel activities.
Polycarbonate membranes commercially used for ultrafiltra-
tion are a porous material, with irregularly arranged pores of uni-
form size of about 1 Pm. Favero et al.
106
covered one side of the
membrane with a gold layer that was subsequently hydrophobized
by tethering an alkanethiol monolayer on top of it. After gently
dropping an egg-PC solution in
n
-hexane on the gold-coated side
of the polycarbonate membrane, free-standing bilayers were spon-
taneously formed in the pores. By incorporating gramicidin D, the
following order of increasing conductance, NH
4
+
> K
+
>> Li
+
was
verified, in accordance with the known selectivity coefficients of
this peptide. By setting up an apparatus allowing continuous con-
ductance measurements under flowing conditions, the same au-
thors
107
tested the functional activity of an ionotropic glutamate
receptor incorporated in this microarray of BLMs. By injecting 0.1
PM glutamate in the flowing solution and by then removing it with
pure buffer, a notable increase in conductance was recorded, fol-
lowed by a return to the initial low value. Enhancement in con-
ductance by injecting the coagonist glycine, and conductance sup-
pression by injecting the antagonist Mg
2+
ions, were verified. Alt-
hough the feasibility of functional assays for membrane proteins
was demonstrated, ultrafiltration membranes have a number of
limitations. Thus, the stability of these membranes is about 10 h
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