Biomedical Engineering Reference
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we have observed a 13-fold reduction in the gA channel lifetime. Whereas by replac-
ing
DC
18
:
1
PC
bilayer with DOPE bilayer bearing a relatively more negative cur-
vature with comparable thickness [
52
] we observe much less reduction in the gA
channel lifetime for both short and long gA channels, and the stepwise introduc-
tion of DOPE (0, 50, and 100 %) over
DC
18
:
1
PC
we observe that the gA chan-
nel lifetime decreases almost linearly with an increasing negative curvature (see
Fig.
5.13
). This observation suggests that bilayer thickness and the gA channel length
mismatch appears as a stronger gA channel regulator than the lipid curvature. On
the other hand, we observe that the shorter gA channels are about 13- and 23-fold
less stable than longer gA channels in
DC
18
:
1
PC
/squalene (thinner bilayer) and
DC
20
:
1
PC
/squalene (thicker bilayer) bilayers, respectively. We also observe that
shorter
g
A
−
(
channels become about 100- and 55-fold less
stable, respectively, when equal amounts of increase in the hydrophobic mismatch
between bilayer thickness and gA channel lengths for both
g
A
−
(
13
)
and longer
A
g
A
(
15
)
)
channels occur by replacing
DC
18
:
1
PC
with
DC
20
:
1
PC
in squalene-containing lipid
bilayers. With a further increase in bilayer thickness, by choosing
DC
22
:
1
PC
lipids
to form lipid bilayers, we observe no formation of linear
β
-helical gA dimers, prob-
ably due to extremely high values of the hydrophobic bilayer thickness gA channel
length mismatch (
d
0
−
13
)
and
A
g
A
(
15
l
). Nonetheless, even under this condition the gA channels
are still formed, although a different conformational mechanism is at work, namely
the gA monomers no longer form linear dimers as shown in the model diagram (see
Fig.
5.2
) but instead the monomers partially bind with each other through their whole
lengths [
62
]. Thus, the channel length is on the order of just a single gA monomer and
not on the order of the sum of two gA monomer lengths (see Fig.
5.2
). These exper-
imental results, taken together, suggest that the increase in hydrophobic mismatch
between bilayer thickness and the gA channel length by either increasing the lipid
acyl chain length or reducing the gA channel length appears as a very strong regulator
of gA channel stability. Lipid curvature is also an important regulator of gA channel
stability, but not as strong as (
d
0
−
l
). Another important parameter is the peptide
concentration required to form readily observable gA channels in lipid bilayers. The
appearance frequency of gA channels
f
gA
increases more rapidly (proportional to
the second power or greater) than the gA monomer concentration (
), although
the average gA channel lifetime remains almost unchanged. The required pep-
tide concentration is therefore an important parameter in determining the change
of bilayer deformation energy. We observe that about a 100-fold higher
[
M
gA
]
is
required in the
DC
20
:
1
PC
bilayer over the
DC
18
:
1
PC
bilayer while about a 10-fold
higher
[
M
gA
]
is required in the DOPE bilayer over the
DC
18
:
1
PC
bilayer. Here, we
also observe higher effects on the gA channel appearance frequency
f
gA
due to the
change of
d
0
−
[
M
gA
]
l
than those due to the lipid intrinsic curvature.
Alamethicin Channel Results
We have already presented a representative current trace obtained in the
DC
18
:
1
PC
/
n
-
decane bilayer in the presence of Alm peptide in Fig.
5.11
. To initiate the formation
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