groups are less hydrated than in the desorbed state [35]. The presence of a weak
mode around 1712 cm
À1 indicates a formation of intramolecular hydrogen bonds to
the ester carbonyl groups in DMPS. In the DMPE bilayer the ν(C¼O) mode is
narrow and centered at 1744–1740 cm
À1 (Fig. 3.8c). The position of this mode does
not change with potential, indicating that independently of the adsorption state of the
bilayer, the ester groups in DMPE molecules are dehydrated [34].
The orientation and hydration of lipid molecules in DMPC and DMPS bilayers
are depended on the potential applied to the Au electrode. The potential-driven
adsorption-desorption process of the DMPC bilayer is connected with changes in the
hydration of the polar head group and the tilt of the hydrocarbon chains. In the
bilayer adsorbed on the Au surface the tilt of hydrocarbon chains (25
) gives the area
of 0.48 nm
2 per lipid molecule. It is larger than the cross-sectional area of the PC
moiety (0.46 nm
2 ) [39]. Thus, water molecules have a direct access to the polar head
groups and the polar head group region of the DMPC bilayer is well hydrated
(Fig. 3.8a). Desorption of the bilayer leads to a dehydration of the polar head groups
in the DMPC bilayer. A decrease in the area available per lipid molecule imposes the
up-right orientation of the hydrocarbon chains. In the DMPS bilayer adsorbed on the
Au surface the hydrocarbon chains are oriented up-right (Table 3.2) [35]. In the
presence of Na
+ ions in the electrolyte solution the cross-sectional area of the PS is
equal to 0.51 nm
2 [68]. Electrostatic interactions between PS head groups and
cations from the electrolyte solution facilitate a close packing of the DMPS molecules in the bilayer. In addition, the charge accumulated on the metal electrode
contributes to the total interaction energy between the model membrane and the solid
support. Attractive electrostatic interactions between negatively charged PS head
groups and the positively charged electrode favor a tight packing of the lipid
molecules in the membrane. Repulsions of the negatively charged DMPS molecules
from the Au electrode appear at negative potentials. They destabilize the molecular
scale order and lead to desorption of the bilayer form the Au surface. The packing
and order of lipids molecules in the DMPE bilayer is not affected by potentials
applied to the Au electrode [34]. The hydrocarbon chains are oriented up-ward and
the polar head groups are weakly hydrated. The PE group has the cross-sectional
area of 0.38 nm
2 [62]. It corresponds exactly to the cross-sectional area of two fully
stretched and vertically oriented hydrocarbon chains. The size matching between the
polar and non-polar parts of the DMPE molecules facilitates a tight packing of lipid
molecules in the bilayer and ensures immunity to potential-driven reorientations in
the membrane.
A single component lipid bilayer does not reflect the composition and structure of
biological cell membranes. For example, in cell membranes of eukaryote next to
phosphoglycerides, sterols and glycolipids are present. Bacterial cell membranes
have even more complex composition. Cell membranes of Gram-negative bacteria
have two membranes: the inner membrane and the outer membrane [69]. Lipids
present in cell membranes have different composition and concentration in the
extracellular and endoplasmic leaflets of the membrane. LB-LS transfer was successfully used to prepare supported asymmetric lipid bilayers on the Au electrode
surface [30, 31, 33, 70].
3.3 In Situ PM IRRAS Studies of Films of Biomolecules Adsorbed on Electrode Surfaces 61
mode around 1712 cm
À1 indicates a formation of intramolecular hydrogen bonds to
the ester carbonyl groups in DMPS. In the DMPE bilayer the ν(C¼O) mode is
narrow and centered at 1744–1740 cm
À1 (Fig. 3.8c). The position of this mode does
not change with potential, indicating that independently of the adsorption state of the
bilayer, the ester groups in DMPE molecules are dehydrated [34].
The orientation and hydration of lipid molecules in DMPC and DMPS bilayers
are depended on the potential applied to the Au electrode. The potential-driven
adsorption-desorption process of the DMPC bilayer is connected with changes in the
hydration of the polar head group and the tilt of the hydrocarbon chains. In the
bilayer adsorbed on the Au surface the tilt of hydrocarbon chains (25
) gives the area
of 0.48 nm
2 per lipid molecule. It is larger than the cross-sectional area of the PC
moiety (0.46 nm
2 ) [39]. Thus, water molecules have a direct access to the polar head
groups and the polar head group region of the DMPC bilayer is well hydrated
(Fig. 3.8a). Desorption of the bilayer leads to a dehydration of the polar head groups
in the DMPC bilayer. A decrease in the area available per lipid molecule imposes the
up-right orientation of the hydrocarbon chains. In the DMPS bilayer adsorbed on the
Au surface the hydrocarbon chains are oriented up-right (Table 3.2) [35]. In the
presence of Na
+ ions in the electrolyte solution the cross-sectional area of the PS is
equal to 0.51 nm
2 [68]. Electrostatic interactions between PS head groups and
cations from the electrolyte solution facilitate a close packing of the DMPS molecules in the bilayer. In addition, the charge accumulated on the metal electrode
contributes to the total interaction energy between the model membrane and the solid
support. Attractive electrostatic interactions between negatively charged PS head
groups and the positively charged electrode favor a tight packing of the lipid
molecules in the membrane. Repulsions of the negatively charged DMPS molecules
from the Au electrode appear at negative potentials. They destabilize the molecular
scale order and lead to desorption of the bilayer form the Au surface. The packing
and order of lipids molecules in the DMPE bilayer is not affected by potentials
applied to the Au electrode [34]. The hydrocarbon chains are oriented up-ward and
the polar head groups are weakly hydrated. The PE group has the cross-sectional
area of 0.38 nm
2 [62]. It corresponds exactly to the cross-sectional area of two fully
stretched and vertically oriented hydrocarbon chains. The size matching between the
polar and non-polar parts of the DMPE molecules facilitates a tight packing of lipid
molecules in the bilayer and ensures immunity to potential-driven reorientations in
the membrane.
A single component lipid bilayer does not reflect the composition and structure of
biological cell membranes. For example, in cell membranes of eukaryote next to
phosphoglycerides, sterols and glycolipids are present. Bacterial cell membranes
have even more complex composition. Cell membranes of Gram-negative bacteria
have two membranes: the inner membrane and the outer membrane [69]. Lipids
present in cell membranes have different composition and concentration in the
extracellular and endoplasmic leaflets of the membrane. LB-LS transfer was successfully used to prepare supported asymmetric lipid bilayers on the Au electrode
surface [30, 31, 33, 70].
3.3 In Situ PM IRRAS Studies of Films of Biomolecules Adsorbed on Electrode Surfaces 61