Electrochemistry of Biomimetic Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 150

Figure 1. (A) Structure of a widely adopted thiolipid, called DPTL. 1 It consists of a
tetraethyleneoxy chain terminated at one end with a lipoic acid residue and cova-
lently linked at the other end to two phytanyl chains. (B) Structure of the corre-
sponding hydrophilic spacer (TEGL), in which the two phytanyl chains are replaced
by a hydroxyl group.
vestigated exclusively by non-electrochemical surface sensitive
techniques, such as those formed on insulating supports (e.g.,
glass, mica, quartz, silica, etc.), will be considered only briefly.
III. ELECTROCHEMICALIMPEDANCE
SPECTROSCOPY
Many membrane proteins are electrogenic , i.e., translocate a net
charge across the membrane. Consequently, it is possible to moni-
tor their function directly by measuring the current flowing along
an external electrical circuit upon their activation. Analogously,
their deactivation by some antagonist can be monitored by a drop
in current. The techniques of choice for these measurements are
EIS and potential-step chronoamperometry or chronocoulometry,
because the limited volume of the ionic reservoir created by a hy-
drophilic spacer in solid-supported biomimetic membranes cannot
sustain a steady-state current.
Electrochemical impedance spectroscopy applies an a.c. volt-
age of given frequency to the system under study and measures the
resulting current that flows with the same frequency. Both the am-
plitude of the a.c. current and its phase shift with respect to the a.c.
voltage are measured. The frequency is normally varied gradually
from 10 -3 to 10 5 Hz. To interpret measured impedance spectra, it is
necessary to compare them with the electrical response of an
equivalent circuit simulating the system under investigation. As a