Biomedical Engineering Reference
In-Depth Information
consider
E
to be the free energies per lipid molecule in the inverse
hexagonal and lamellar phases respectively, all phase boundaries representing the
lamellar-inverse hexagonal phase transition are determined by the free energy change
(
(
H
)
and
E
(
L
)
E
between two subsequent phases
contains the following important energy contributions:
E
=
E
(
H
)
−
E
(
L
)
). The free energy change
E
=
E
elastic
+
E
hydration
+
E
van der Waals
+
E
electrostatic
(3.3)
Although the elastic energy (which follows from Eq.
3.1
), hydration and van der
Waals energy contributions have been well described (see Ref. [
16
] and many other
references therein), the electrostatic component is still missing. Another energy term
(
E
interstitial
), the 'energy of voids in inverse hexagonal interstices' in a reentrant
transition, is also needed to augment the four energy terms in Eq.
3.3
. This rather
passive energy contribution plays an important role in setting up the energy scale,
and determines the temperature of the inverse hexagonal-to-lamellar transition and
the temperature range of the reentrant transition. Although the van der Waals energy
component is understood to be producing negligible effects [
16
], a careful examina-
tion of relevant modeling efforts suggests that both the van derWaals and electrostatic
interactions appear to be dominant contributions to the membrane energetics, which
is responsible for certain lipid phase properties, especially in the case when the mem-
brane hosts different kinds of membrane proteins or antimicrobial peptides [
2
]. This
will be elaborated on in later chapters.
3.4 Modulation of the Phase Properties of Lipids
by Antimicrobial Peptides
We have so far addressed the various aspects of lipid phases in independent membrane
environments. The lamellar and non-lamellar lipid phases are found to be modulated
by the disproportionate presence of the natural constituents that are primarily respon-
sible for constructing membranes. However, cell membranes also host various other
components such as natural or artificial (used mainly during treatment) membrane
proteins, antimicrobial peptides, etc. Those proteins or peptides are not often found
to perform independent activities, but rather are found to be engaged in complex
mechanisms involving lipids and other membrane constituents, which altogether
have modulating effects on lipid phase properties. As a result, the phase diagrams
may experience substantial modifications due to an alteration of the lipid phase dis-
tributions. In this section, we discuss the effects of a few antimicrobial peptides on
the lamellar and inverse hexagonal phase properties. We also specifically address the
effects of the antimicrobial peptides on the transition temperatures between different
lipid phases and different subphases or states within major well-defined phases.
Gramicidin S, a cyclic peptide, has been found to disrupt the structural integrity
of specific lipid bilayer membranes by promoting the formation of cubic or other
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