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226 Cell-Penetrating Peptides: Processes and Applications
systems. In this section a number of biomembrane models will be briefly presented.
For more comprehensive knowledge, the vast literature in the membrane field,
including Handbook of Nonmedical Applications of Liposomes (in four volumes) 16
is recommended. A brief summary relevant for CPP studies is given here.
Although the chemical composition of real biomembranes varies, their physical
properties are mainly governed by the amphiphilic nature of the phospholipids
forming a bilayer in water. In the model studies, well-characterized synthetic phospholipids
should be used, instead of a mixture of ill-defined lipids from some natural
source. Phospholipids with saturated fatty acid chains, with at least 14 carbons, form
membranes with a sharp thermal phase transition between the gel state and the liquid
crystalline state. Since the latter state exists in vivo, the system studies should be
carried out above room temperature.
By choosing phospholipids with a thermal phase transition below room temperature,
the nonbiological gel state is easily avoided. This is usually accomplished by
having unsaturated fatty acids, although this means a higher risk for lipid oxidation
to occur. Phospholipids with one of the chains (number 2) containing a single doublebond
will have good and relevant properties. Compounds with the structure 1-palmitoyl-2-oleoyl-phosphatidyl-X
(POP-X) have been frequently used. Here “X”
denotes the “head group,” giving rise either to the neutral, zwitterionic lipids (POPC,
POPE) or the negatively charged lipids (POPG, POPS, POPA, etc.).
Biological membranes are electrically charged, where the fraction of charged
phospholipids normally is low (order of 10 mol%). In order to mimic a more real
system, it is therefore common to study membranes produced from mixtures of the
two types of phospholipids. Phospholipids with the head group consisting of ethanolamine
(X = E) frequently occur in natural membranes. However, for model
systems this type of neutral phospholipid should not be used with high mol%, since
a diacyl-PE molecule (for sterical reasons) does not form lamellar bilayers, but
instead a hexagonal phase. For simple model studies there is no primary reason to
include cholesterol or any other natural lipid molecule in the membrane composition.
In a biomembrane with low curvature, a phospholipid molecule occupies about
0.7 nm 2 in each layer of the bilayer. Below the thermal phase transition, in the gel
state, the surface area will become smaller and the packing higher. The molecular
shape as well as the electrical charge may cause an asymmetrical distribution of
lipid components between the outer and inner monolayers of the membrane. The
thickness of the bilayer depends on properties of the lipids, but is of the order of
5 nm. The transverse diffusion (“flip-flop”) of lipids between the two layers is slow
compared to the much more effective lateral, two-dimensional diffusion within each
monolayer.
10.3.1 LIPOSOMES AND VESICLES
A monolayer of lipids can be formed at the air–water interface by the Langmuir
technique; this type of preparation can be used for surface studies. A multilamellar
arrangement of oriented bilayers will occur when lipids are deposited on a support
and then hydrated. Such a specimen may be useful when studying the orientational
dependence of a physical observable. The concept of liposome is used to any