Biomedical Engineering Reference
In-Depth Information
potential of the ion is higher to that where it is lower (passive
transport). A widely investigated ion channel is gramicidin, a line-
ar pentadecapeptide with helical structure that turns its hydropho-
bic groups toward the exterior of the helix and its hydrophilic car-
boxyl groups toward the interior. The length of a gramicidin chan-
nel is of 1.3 nm, about one half the thickness of a biomembrane.
To span a biomembrane, two helical monomers of gramicidin form
a helical dimer, with the N-terminuses of the dimer interacting in
the center of the membrane. The resulting hydrophilic pore allows
the transport of monovalent cations, such as alkali metal ions,
across the membrane. The monomer of gramicidin incorporated in
a mercury-supported phospholipid monolayer was reported by
Nelson to act as an ion channel toward Tl
+
ion, thus allowing its
penetration across the monolayer and its electroreduction to thalli-
um amalgam.
67-71
From a cyclic voltammetric and a chronoam-
perometric investigation of Tl
+
ion electroreduction through the
gramicidin channel, Nelson proposed a Chemical-Electrochemical
(CE) mechanism, in which the rate-determining step is a preceding
homogeneous chemical step associated with Tl
+
entry into the
channel, prior to Tl
+
electroreduction.
69,70
This electrode process at
a mercury-supported phospholipid monolayer incorporating gram-
icidin was employed by Nelson as a model system to probe the
effect of lipid charge, solution composition and incorporation of
biologically active compounds on ion channel transport. Thus, it
was shown that a negatively charged DOPS monolayer increases
the rate of Tl
+
transport with respect to a neutral DOPC monolayer,
while polyvalent cations, such as Mg
2+
and Dy
3+
, have an opposite
effect.
70
This is ascribed to the electrostatic interaction of this
charge with Tl
+
ions in the immediate vicinity of the lipid film,
with a resulting influence on their local concentration. The behav-
ior of the same system by potential-step chronocoulometry was
interpreted by Becucci et al.
72
on the basis of a mechanism that
includes the diffusion of thallous ions toward the lipid film and a
potential-independent heterogeneous step consisting of the dehy-
dration of the ion and its binding to a site located at the mouth of
the ion channel. The further step involving the surmounting of a
potential energy barrier located somewhere in the middle of the
channel is considered to be in quasi-equilibrium, and hence its
potential dependence is of the Nernstian type. Rueda and cowork-
ers
73
, by investigating this system by EIS, postulated an additional
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