Lipids in Membranes - Membrane Biophysics, Biological and Medical Physics, Biomedical Engineering - page 47

ilarly, at a high Alm level, the sharp drop in T H occurs at relatively higher Alm/lipid
ratios (over 0.6%) with 1.36% GS but lower Alm-lipid ratios (over 0.3%) with
0.2% (LA) 12 than that (Alm-lipid ratio over 0.5%) without GS/(LA) 12 . Here also,
similarly to the low level, non-transmembrane and transmembrane peptides ensure a
higher and a lower, respectively, amount of Alm needed to create sharp drops in T H .
This comparable study suggests that in binary mixtures between a high level semi-
transmembrane peptide Alm and other AMPs, the transmembrane-like (LA) 12 exerts
higher effects on Alm than the non-transmembrane-like GS in destabilizing a lamel-
lar phase in cell membranes. An important message is that the effects of any AMP
in the presence of multi-component AMPs in lipid dispersions on different thermo-
dynamic bilayer parameters heavily fluctuate under the influences of varied levels of
other AMPs.
Our investigation suggests that Alm clearly shows different concentration-
dependent qualitative effects on the thermodynamic properties of DEPE dispersions.
It also hints at support for the replacement of the H II phase with the cubic phase at its
higher level (over 0.5%) by showing a huge drop in T H , which is comparable to the
effects of the level (1%or higher) of Alm at which previous structural studies showed
the Alm-induced appearance of the cubic phase in the lipid's liquid crystalline state
[ 11 ]. Importantly, we find that non-transmembrane GS and transmembrane (LA) 12
both generally modulate the Alm-induced possible formation of the cubic phase.
(LA) 12 favors Alm for inducing the possible cubic phase at relatively lower concen-
tration, but GS requires higher Alm levels—both, however, heavily reduce the phase
transition temperature. The transmembrane property of a peptide may also appear
as an important factor, either for its direct effects or for its ability to modulate the
effects of another peptide on the thermotropic lipid phase behavior. Although Alm
up to 0.5%mole shows no dramatic effects on the lipid's thermodynamic properties,
an almost negligible amount of both GS and (LA) 12 (which also show no consider-
able effects on their own) in combination with 0.5% Alm causes a drastic drop in
T H (perhaps due to the promotion of the cubic phase at a much lower temperature
than the H II phase). This suggests that any AMP, no matter how large or small the
amount, may heavily perturb the effect(s) of another AMP in a lipid bilayer system.
Generally, we find that in binary mixtures of AMPs, [e.g. Alm and either GS or
(LA) 12 ], their combined effects show neither plain additiveness of their independent
effects, nor do they antagonize each other's effects. The resultant effect rather mimics
the effect(s) of one component AMP (primary effects), while the other appears as a
modulator (secondary effects) of the effects of the first component AMP, and vice
versa depending on their relative concentrations.
Although Alm at high concentrations independently or in a binary presence with
any other AMP (ultra low-high concentration) promotes a cubic phase instead of (or
in addition to) the H II phase in the liquid crystalline phases of DEPE dispersions,
Alm at an ultra-low ion channel-forming concentration (10 − 8 M) alone certainly
cannot disintegrate the lipid structure. However, another electrophysiology study
[ 3 ] suggests that GS at ultra-low concentrations can also perturb the Alm channel
properties, probably by changing physical bilayer properties such as its elasticity,