Biology Reference
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350
345
340
335
330
325
320
0
5
10
15
20
Lipid/peptide ratio (mol/mol)
Figure 4.14 Effect of membrane on the fluorescence emission of tryptophan residue of
two cell-penetrating peptides. The wavelengths of the maximal tryptophan fluores-
cence emission of MPG-a (open label) and MPG-b peptides (solid label) were recorded
in the presence of liposomes composed of neutral (POPC, circle) or negatively charged
(POPG, triangle) phospholipids at different lipid/peptide molar ratios.
a small but significant shift, indicating that MPG-
a
also inserts in neutral bilayers
with a positioning of the tryptophan closer to the lipid/water interface
(
Fig. 4.14
). This kind of behavior was also observed for other
peptides.
59,79,88-90
Further analyses also enabled the determination of the
proportion of bound versus free peptides through the plot of binding
isotherms.
86
Thus the insertion of a single tryptophan residue in the sequence
of several carrier peptides led to comparative membrane insertion analyses.
87
4.2. Probing membrane insertion by tryptophan quenchers
Although the solvatochromism approach can provide information on
protein/membrane interactions, the possibility of insertion through the lipid
bilayer or at the interface is usually ruled out by quenching measurements
and FRET investigations.
91-93
Indeed, the combination of tryptophan
fluorescence with both improvements in vesicle formation and
phospholipid chemistry enabled the development of useful methods to
determine the depth of insertion of molecules through a lipid bilayer.
Acrylamide and potassium iodide (KI) are well known to interact
with tryptophan and to induce a specific quenching of fluorescence
intensity.
94-96
In fact, by combining these tryptophan quenchers with
LUVs, it is possible to sense the accessibility of the indole in phospholipid
bilayers and then the penetration of
the protein or peptide in a
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