Lipids in Membranes - Membrane Biophysics, Biological and Medical Physics, Biomedical Engineering - page 33

and to gain detailed understanding the reader is encouraged to consult earlier
works (for example [ 19 , 25 ]). Considering the organizational diversity, it is now
well accepted to classify lipid organization mainly into two generalized categories,
namely, lamellar and non-lamellar phases. These will be discussed in Sect. 3.2 , and
further detailed in regard to other related aspects in subsequent sections of this
chapter.
3.2 Various Lipid Phases in Membranes: Lamellar
and Non-Lamellar Phase Propensities
Lamellar phases that exist in excess water are subclassified into different states: fluid
( L α ), ripple ( P β ), gel ( L β ), and pseudo-crystalline or subgel ( L c ), depending on
the lipid head group and tail orientations and assemblies. Model diagrams of these
structures have been presented in Fig. 3.2 . It is generally accepted that the lamellar
phases are probably compatible with the barrier functions of biological membranes.
The famous fluid mosaic model used to describe bilayer barrier functions [ 27 ]is
based on the liquid-crystalline phase of the biological membranes.
Although lamellar phases appear to be the primary states of the lipid assem-
bly in biological membranes, there also exists a significant amount of lipids that
do not spontaneously form any of the states illustrated in Fig. 3.2 . These non-
lamellar structure-forming lipids can further be classified into two major subcat-
egories, namely inverted cubic ( Q II ) and hexagonal ( H II ) phases, considering the
lipid compositions and their crystal structures. Figure 3.3 shows a model diagram
representing the crystalline structure of a hexagonal (as an example) form of a non-
lamellar lipid phase. Both non-lamellar structures of inverted cubic and hexagonal
phases are observed in membranes under physiologically relevant conditions. The
formation of structures following non-lamellar phases was originally assigned to the
molecular geometries of lipids [ 9 ]. Naturally occurring phospholipids such as phos-
phatidyl ethanolamine, phosphatidyl ethylamine, N -methyl PE, phosphatidylserine,
phosphatidylglycerol, phosphatidic acid, cardiolipin, etc., are often responsible for
inducing non-lamellar phases. These non-lamellar phase-forming lipids are present
in both phospho- and glycolipid-based cell membranes. Of course, the non-lamellar
phase-forming propensity in certain lipids highly depends on the local ionic condi-
tions. Temperature-dependent phase preference is an important established fact (see
for example [ 26 ]). The inverted cubic and hexagonal phases are independent non-
lamellar phases that appear to be favored over lamellar phases when the temperature
of the lipid composition is raised. It has also been observed that the presence of
cubic phases may be a general feature of H II -forming lipids, which is supported by
the consideration of the spontaneous radius of curvature of lipid monolayers, referred
to as the intrinsic curvature hypothesis for bio-membrane lipid composition [ 7 ]. In
this study, Gruner has presented a pictorial representation for non-lamellar H II lipid
phases, which has been redrawn in Fig. 3.3 . The intrinsic curvature hypothesis [ 7 ]