It depends on the concentration and net charge of the ganglioside present in the outer
leaflet of the model membrane [39, 70, 72]. The orientation of DMPC molecules is
affected by the addition of gangliosides to the lipid bilayer. Use of a per-deuterated
phospholipid (d 54 -DMPC) allowed the determination of the orientation of hydrocarbon chains in phospholipid and ganglioside molecules, respectively [30, 39, 70,
72]. The hydrocarbon chains in d 54 -DMPC molecules are almost vertically oriented
with respect to the bilayer surface (tilt of the hydrocarbon chains 14
–18
)
[39, 72]. The hydrocarbon chains in gangliosides are inclined by ca. 38
with respect
to the surface normal [70]. In three-component asymmetric bilayers the ester carbonyl group gives a broad IR absorption mode, which is composed of two modes
centered at 1745 and 1730 cm
À1 [30, 39]. The ν(C¼O) at 1730 cm
À1 is significantly
stronger than the mode at 1745 cm
À1 , indicating that the ester carbonyl groups are
well hydrated. In the 1150–1000 cm
À1 spectral region a broad absorption mode
appears in the PM IRRA spectra of DMPC:cholesterol:ganglioside lipid bilayers
[30, 73]. It is composed of several overlapped absorption bands which arise from the
ν s PO
À
2
À
Á
and ν(CO[P]) stretching modes in DMPC and ν as (COC) stretching modes
in ganglioside molecules. This mode was deconvoluted into five IR absorption bands
centered at 1119 Æ 2, 1096 Æ 2, 1088 Æ 2, 1071 Æ 1, 1062 Æ 1, 1054 Æ 2, 1042 Æ 2
and 1020 Æ 2 cm
À1 [73]. The PM IRRA spectra of the lipid bilayer adsorbed directly
on the Au surface display an enhancement of the IR absorption bands appearing at
e ν < 1065 cm
À1 compared to the solution spectrum of randomly distributed molecules in a bilayer thick film. Integral intensities of these modes are by a factor
2.5–1.5 higher, than in the spectrum of randomly oriented molecules. A large
enhancement of the ν as (COC) modes in the bilayer adsorbed on the Au surface
indicates a parallel orientation of the transition dipole vectors of these normal
vibrations with respect to the electric field vector. Quantum chemical calculations
of the IR spectra of the polar head group region of the GM 1 ganglioside was done
[74]. Results of these calculations show that the transition dipole vectors of the
ν as (COC) modes ~1130, 1065, 1057 and 1040 cm
À1 have a parallel orientation to the
plane of the two inner β1-1
0 Gluc-β1-3Gal sugar rings in the GM 1 ganglioside [74]. In
situ PM IRRAS was used to describe the molecular scale picture of potential-driven
changes in the asymmetric lipid bilayers containing gangliosides in the outer leaflet.
Figure 3.11 shows the capacitance potential curves of the bilayer and pictures
showing the molecular scale order in the studied bilayer.
In a freshly prepared bilayer the hydrocarbon chains in the phospholipid molecules are oriented almost vertically to the Au surface [62]. The PC head groups adopt
a space saving zig-zag orientation as schematically shows in Fig. 3.11a [30, 70]. The
analysis of the ν as (COC) mode indicates that the inner β1-1Glu-β1-3Gal-β1-4GalNAc saccharide residues in the GD 1a ganglioside tend to be oriented vertically
to the Au substrate. Desorption of the lipid bilayer from the electrode surface is
connected with a decrease in the intensities of the ν as (COC) modes and a more
3.3 In Situ PM IRRAS Studies of Films of Biomolecules Adsorbed on Electrode Surfaces 65
leaflet of the model membrane [39, 70, 72]. The orientation of DMPC molecules is
affected by the addition of gangliosides to the lipid bilayer. Use of a per-deuterated
phospholipid (d 54 -DMPC) allowed the determination of the orientation of hydrocarbon chains in phospholipid and ganglioside molecules, respectively [30, 39, 70,
72]. The hydrocarbon chains in d 54 -DMPC molecules are almost vertically oriented
with respect to the bilayer surface (tilt of the hydrocarbon chains 14
–18
)
[39, 72]. The hydrocarbon chains in gangliosides are inclined by ca. 38
with respect
to the surface normal [70]. In three-component asymmetric bilayers the ester carbonyl group gives a broad IR absorption mode, which is composed of two modes
centered at 1745 and 1730 cm
À1 [30, 39]. The ν(C¼O) at 1730 cm
À1 is significantly
stronger than the mode at 1745 cm
À1 , indicating that the ester carbonyl groups are
well hydrated. In the 1150–1000 cm
À1 spectral region a broad absorption mode
appears in the PM IRRA spectra of DMPC:cholesterol:ganglioside lipid bilayers
[30, 73]. It is composed of several overlapped absorption bands which arise from the
ν s PO
À
2
À
Á
and ν(CO[P]) stretching modes in DMPC and ν as (COC) stretching modes
in ganglioside molecules. This mode was deconvoluted into five IR absorption bands
centered at 1119 Æ 2, 1096 Æ 2, 1088 Æ 2, 1071 Æ 1, 1062 Æ 1, 1054 Æ 2, 1042 Æ 2
and 1020 Æ 2 cm
À1 [73]. The PM IRRA spectra of the lipid bilayer adsorbed directly
on the Au surface display an enhancement of the IR absorption bands appearing at
e ν < 1065 cm
À1 compared to the solution spectrum of randomly distributed molecules in a bilayer thick film. Integral intensities of these modes are by a factor
2.5–1.5 higher, than in the spectrum of randomly oriented molecules. A large
enhancement of the ν as (COC) modes in the bilayer adsorbed on the Au surface
indicates a parallel orientation of the transition dipole vectors of these normal
vibrations with respect to the electric field vector. Quantum chemical calculations
of the IR spectra of the polar head group region of the GM 1 ganglioside was done
[74]. Results of these calculations show that the transition dipole vectors of the
ν as (COC) modes ~1130, 1065, 1057 and 1040 cm
À1 have a parallel orientation to the
plane of the two inner β1-1
0 Gluc-β1-3Gal sugar rings in the GM 1 ganglioside [74]. In
situ PM IRRAS was used to describe the molecular scale picture of potential-driven
changes in the asymmetric lipid bilayers containing gangliosides in the outer leaflet.
Figure 3.11 shows the capacitance potential curves of the bilayer and pictures
showing the molecular scale order in the studied bilayer.
In a freshly prepared bilayer the hydrocarbon chains in the phospholipid molecules are oriented almost vertically to the Au surface [62]. The PC head groups adopt
a space saving zig-zag orientation as schematically shows in Fig. 3.11a [30, 70]. The
analysis of the ν as (COC) mode indicates that the inner β1-1Glu-β1-3Gal-β1-4GalNAc saccharide residues in the GD 1a ganglioside tend to be oriented vertically
to the Au substrate. Desorption of the lipid bilayer from the electrode surface is
connected with a decrease in the intensities of the ν as (COC) modes and a more
3.3 In Situ PM IRRAS Studies of Films of Biomolecules Adsorbed on Electrode Surfaces 65