Lipids in Membranes - Membrane Biophysics, Biological and Medical Physics, Biomedical Engineering - page 38

regard, while X-ray crystallography determines the crystal structures corresponding
to the various phases.
Energetics of the lipid molecule assembly into various lipid phases is case spe-
cific, and it was shown [ 7 ] that the energetic contributions to the total free energy
determining the lipid membrane conformation come from the properties of lipid
monolayers and the participating lipids. The lipid monolayer's elastic property and
the lipid's head group and carbon chain geometry are primarily considered to be
the main factors in the so-called intrinsic curvature hypothesis for bio-membrane
lipid composition [ 7 ]. A very important lipid property, the lipid's charge profile, is
totally ignored in this model. We have made substantial contributions to the field in
this regard. In later chapters the reader will be given detailed information about the
lipid charge effects. We just wish to mention here that the lipid membrane's elastic
properties fail to explain most of the regulatory membrane effects. However, the
lipid's charge profiles do so. In the next sections of this chapter, we will discuss
the lamellar/non-lamellar phase properties in view of mostly experimental studies of
specific lipid systems. It has been found that the non-lamellar lipid assembly brings
not only special membrane structures, but also often induces regulatory effects on
other agents residing in the membrane environment. Perhaps the best example of
an enzyme activated by the H II phase-forming lipids is protein kinase C. Numerous
publications on non-lamellar phase-forming lipids and the mechanisms are useful in
this regard (e.g. [ 18 ] and references therein; also [ 14 ]). Our main interest here lies in
the lipid phases and the regulation of these phase properties. A group of AMPs has
been observed to be regulating the lipid's lamellar/non-lamellar preferential phase
propensities. Interactions of AMPs with lipid membranes, using different techniques
such as differential scanning calorimetry (DSC), 31 P -nuclear magnetic resonance
(NMR) spectroscopy, X-ray diffraction, etc., elucidate how the lipid phase proper-
ties are regulated due to the effects of membrane-residing ingredients, such as various
naturally occurring MPs, and artificial ones often used during treatment.
In the next few sections of this chapter, we provide more details regarding the
physical interactions involving various classes of AMPs having varied structures
and molecular mechanisms of actions, etc., with phospholipid bilayers with different
membrane-forming components.
3.3 Lipid-Specific Phase Diagram: A Thermotropic
Perspective
In the previous section, we have addressed the various lipid phases and the consequent
mechanical energy components arising from the corresponding physical structure of
organized lipid monolayers and lipids themselves. Here, we describe a lipid phase
diagram, addressing the various lipid phases as a function of both temperature, rep-
resenting the thermodynamic environment, and the hydrocarbon chain lengths of
certain lipid types, determining the membrane's physical properties, in particular its