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228 Cell-Penetrating Peptides: Processes and Applications
solvent will disappear in that way. An alternative procedure is the so-called electroformation
method, where a low-frequency AC field stimulates GUV formation. The
stability of GUVs has been reported to be good, probably comparable to that of
LUVs. A disadvantage with GUVs is that the ionic strength of the medium must be
restricted.
10.3.2 MICELLES, BICELLES, AND SOLVENT MIXTURES
Although phospholipid vesicles have many properties in common with biological
membranes, they are not convenient for certain biophysical studies. For instance, as
will be discussed later, no true high-resolution NMR spectrum will be recorded from
a peptide (CPP) when it is bound even to an SUV, due to the high molecular weight
and slow mobility of the complex in solution. For that reason other amphiphilic
biomembrane mimetic systems have also been applied.
10.3.2.1 Micelles
Micelles can be produced by various amphiphiles, dissolved in an aggregated form
above the critical micellar concentration (CMC) in aqueous media. The most common
micelles are formed from sodium dodecyl sulfate (SDS), which forms small
spherical micelles with a mean aggregation number of about 60 and a total molecular
weight of about 20 kD. The CMC is about 10 mM. Since SDS is negatively charged,
micelle stability becomes dependent on ionic strength. The micelle is very dynamic
and the lifetime of the SDS molecules in the aggregate is short (< msec) compared
to that of phospholipids in vesicles (order of hours).
10.3.2.2 Bicelles
A binary mixture of two phospholipids in water may form micelles with different
shapes and properties. 19-22 The major component is a molecule able to form bilayers.
Usually dimyristoyl-phosphatidylcholine (DMPC) is used. This zwitterionic molecule
is mixed with a variable amount of zwitterionic dihexanoyl-phosphatidylcholine
(DHPC), with lipid chains too short to be bilayer competent. The DMPC molecules
will form a bilayer where the rim is covered by the DHPC molecules. The bicelle
may be doped with charged lipids, like DMPG. 23 Bicelles have no inner aqueous
compartment; like micelles, they are dynamic structures. Depending on lipid composition,
concentration, temperature, etc., the bicellar samples can have either isotropic
or anisotropic properties. The isotropic particles are small enough to allow
high-resolution NMR. 24
10.3.2.3 Solvent Mixtures
Many peptides when free in water have a low degree of secondary structure (randomcoil
state). By using mixtures of water and fluoroalcohols as solvents, a certain
structural stabilization can take place, usually in the form of an α-helix. The general
explanation for secondary structure induction in such solvent mixtures is that a