Biomedical Engineering Reference
In-Depth Information
The capacitance and resistance of DPTL/DPhyPC tBLMs on
gold microelectrodes of circular shape, with diameters ranging
from 4000 to 8 Pm, was examined as a function of the electrode
size. 182 For the larger electrodes, the capacitance is directly propor-
tional to the electrode area, while the resistance is inversely pro-
portional to it. For the smaller electrodes, the capacitance decreas-
es linearly and the resistance increases linearly with a decrease in
the electrode diameter. This indicates that the capacitance and re-
sistance of the larger electrodes are dominated by the electrode
area, while the circumference seems to have the dominant role at
smaller electrodes. Consequently, for small electrode sizes, a slight
disorder of the bilayer structure at the edge of the electrode has a
major influence on the electrical properties of the membrane. This
conclusion is supported by the consideration that the micro-tBLMs
used for the recording of single channel currents have resistances
ranging from 1.5 to 15 G:. These resistances are high enough to
reduce the level of the background noise down to the range of a
few pA. However, the corresponding specific resistances are much
less than those attained with identical tBLMs anchored to macro-
scopic gold electrodes of areas of the order of 1u10 -2 cm 2 . In fact,
the specific resistance of the latter tBLMs amounts to about 15
M: cm 2 . If a 100u100 Pm 2 micro-tBLM had such a specific re-
sistance, it should have a resistance of 150 G:. The loose packing
of micro-tBLMs may explain why they may incorporate relatively
bulky proteins much more easily than macro-tBLMs, without hav-
ing to dilute the thiolipid with a short spacer.
( b ) Mercury-supported thiolipid-based tBLMs
As distinct from Au-supported thiolipid-based tBLMs, Hg-
supported thiolipid-based tBLMs do not require the use of thio-
lipid-spacer mixtures to incorporate channel-forming proteins.
Thanks to the fluidity imparted to the thiolipid monolayer by the
liquid mercury surface, these tBLMs may incorporate bulky pro-
teins, such as OmpF porin from Escherichia coli 183 and the HERG
potassium channel, 184 in a functionally active state. Upon incorpo-
rating gramicidin 2 or valinomicin, 3 the tetraethyleneoxy (TEO)
moiety of DPTL in aqueous KCl solution undergoes a conforma-
tional change ascribable to its elongation, as the applied potential
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