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M. Melicherčík et al.
of peptide in the methylene and methyl stretching region. Interchain coupling is
significant enough even in fluid bilayers [21]. Therefore, using exclusively FTIR it
is difficult to decide what process is dominant in fluid state—interchain coupling or
trans/gauche isomerisation.
In contrast with spectroscopic methods that provide information about microscopic changes of the lipid bilayer in close proximity of the protein, macroscopic
methods, such are membrane compressibility measurements, are sensitive to changes of large membrane regions. The sensitivity and utility of measurements of volume
compressibility has been proved in several studies of the interaction of integral proteins with lipid bilayers, e.g. bacteriorhodopsin [4] or peptides like ACTH 24 [9] or
gramicidin S [10]. In the case of bacteriorhodopsin, it has been shown that one molecule of the peptide is able to change the structural state of the lipid bilayer of whole
large unilamellar vesicle (LUV). We applied this method also to the study of the
interaction of synthetic α-helical transmembrane peptides like L 24 with lipid bilayers [22]. We used precise measurement of density and ultrasound velocity to study
the physical properties of LUVs composed of a homologous series of n-saturated
phosphatidylcholines (PC) containing L 24 . PCs whose hydrocarbon chains contained
from 13 to 16 carbon atoms, thus producing phospholipid bilayers of different thicknesses and gel to liquid-crystalline phase transition temperatures. This allowed us
to analyze how the difference between the hydrophobic length of the peptide and
the hydrophobic thickness of the lipid bilayer influences the thermodynamical and
mechanical properties of the membranes. We showed that the incorporation of L 24
decreases the temperature and cooperativity of the main phase transition of all LUVs
studied. The presence of L 24 in the bilayer also caused an increase of the specific
volume and of the volume compressibility in the gel state bilayers. In the liquid crystalline state, the peptide decreases the specific volume at relatively higher peptide
concentration (mole ratio L 24 :PC = 1:50). The overall volume compressibility of the
peptide-containing lipid bilayers in the liquid crystalline state was in general higher
in comparison with pure membranes. There was, however, a tendency for the volume compressibility of these lipid bilayers to decrease with higher peptide content
in comparison with bilayers of lower peptide concentration. For one lipid composition, we also compared the thermodynamical and mechanical properties of LUVs
and large multilamellar vesicles (MLVs) with and without L 24 . As expected, a higher
cooperativity of the changes of the thermodynamical and mechanical parameters
took place for MLVs in comparison with LUVs. These results are in agreement with
previously reported DSC and
2
H NMR spectroscopy study of the interaction of the
L 24 and structurally related peptides with phosphatidylcholine bilayers. An apparent
discrepancy between
2
H NMR spectroscopy and compressibility data in the liquid
crystalline state may be connected with the complex and anisotropic nature of macroscopic mechanical properties of the membranes. The observed changes in membrane mechanical properties induced by the presence of L 24 suggest that around each
peptide a distorted region exists that involves at least two layers of lipid molecules.
Further information on the structure and dynamics of lipid bilayer as well as
on the molecular mechanisms of protein–lipid interactions, can be obtained by
molecular dynamics simulations (MD). This method is widely used for this purpose.
During the last three decades MD method has been applied to many short peptides
M. Melicherčík et al.
of peptide in the methylene and methyl stretching region. Interchain coupling is
significant enough even in fluid bilayers [21]. Therefore, using exclusively FTIR it
is difficult to decide what process is dominant in fluid state—interchain coupling or
trans/gauche isomerisation.
In contrast with spectroscopic methods that provide information about microscopic changes of the lipid bilayer in close proximity of the protein, macroscopic
methods, such are membrane compressibility measurements, are sensitive to changes of large membrane regions. The sensitivity and utility of measurements of volume
compressibility has been proved in several studies of the interaction of integral proteins with lipid bilayers, e.g. bacteriorhodopsin [4] or peptides like ACTH 24 [9] or
gramicidin S [10]. In the case of bacteriorhodopsin, it has been shown that one molecule of the peptide is able to change the structural state of the lipid bilayer of whole
large unilamellar vesicle (LUV). We applied this method also to the study of the
interaction of synthetic α-helical transmembrane peptides like L 24 with lipid bilayers [22]. We used precise measurement of density and ultrasound velocity to study
the physical properties of LUVs composed of a homologous series of n-saturated
phosphatidylcholines (PC) containing L 24 . PCs whose hydrocarbon chains contained
from 13 to 16 carbon atoms, thus producing phospholipid bilayers of different thicknesses and gel to liquid-crystalline phase transition temperatures. This allowed us
to analyze how the difference between the hydrophobic length of the peptide and
the hydrophobic thickness of the lipid bilayer influences the thermodynamical and
mechanical properties of the membranes. We showed that the incorporation of L 24
decreases the temperature and cooperativity of the main phase transition of all LUVs
studied. The presence of L 24 in the bilayer also caused an increase of the specific
volume and of the volume compressibility in the gel state bilayers. In the liquid crystalline state, the peptide decreases the specific volume at relatively higher peptide
concentration (mole ratio L 24 :PC = 1:50). The overall volume compressibility of the
peptide-containing lipid bilayers in the liquid crystalline state was in general higher
in comparison with pure membranes. There was, however, a tendency for the volume compressibility of these lipid bilayers to decrease with higher peptide content
in comparison with bilayers of lower peptide concentration. For one lipid composition, we also compared the thermodynamical and mechanical properties of LUVs
and large multilamellar vesicles (MLVs) with and without L 24 . As expected, a higher
cooperativity of the changes of the thermodynamical and mechanical parameters
took place for MLVs in comparison with LUVs. These results are in agreement with
previously reported DSC and
2
H NMR spectroscopy study of the interaction of the
L 24 and structurally related peptides with phosphatidylcholine bilayers. An apparent
discrepancy between
2
H NMR spectroscopy and compressibility data in the liquid
crystalline state may be connected with the complex and anisotropic nature of macroscopic mechanical properties of the membranes. The observed changes in membrane mechanical properties induced by the presence of L 24 suggest that around each
peptide a distorted region exists that involves at least two layers of lipid molecules.
Further information on the structure and dynamics of lipid bilayer as well as
on the molecular mechanisms of protein–lipid interactions, can be obtained by
molecular dynamics simulations (MD). This method is widely used for this purpose.
During the last three decades MD method has been applied to many short peptides