polar head group is the same while the degree of saturation of the hydrocarbon
chains varies [71]. The spectroelectrochemical characteristic of asymmetric bilayers
composed of following lipids:
1. With two saturated hydrocarbon chains: DMPE and 1,2-dimyristoyl-sn-glycerol3-phoshoglycerol (DMPG) (DMPE-DMPG bilayer),
2. One monounsaturated hydrocarbon chain: 1-palmitoyl-2-oleoyl-sn-glycero-3phosphoethanolamine
(POPE)
and
1-plamitoyl-2-oleoyl-sn-glycerol-3phospho-(1
0 rac glycerol) (POPG) mixed with 1,2-diplamitoyl-sn-glycero-3phospho-(1
0 rac glycerol) (DPPG) [POPE-POPG:DPPG bilayer (0.95:0.05 mole
ratio)] and
3. Two monounsaturated hydrocarbon chains in the inner leaflet: 1,2-dioleoyl-snglycero-3-phosphoethanolamine
(DOPE)
[DOPE-POPG:DPPG
bilayer
(0.95:0.05 mole ratio)] is described below.
The electrochemical behavior of these bilayers shows that the potential window
of the bilayer adsorption on the metal surface decreases with an increase in the
degree of unsaturation of the hydrocarbon chains. Potential-dependent changes in
the structure of these lipid bilayers were investigated using in situ PM IRRAS.
Figure 3.10 shows the PM IRRA spectra of the three transverse asymmetric lipid
bilayers at different potentials applied to the Au(111) electrode.
In the DMPE-DMPG bilayer the maxima of absorption of the methylene
stretching modes depend on the potential applied to the Au(111) electrode
(Fig. 3.10a). In the bilayer adsorbed directly on the Au electrode surface the
ν as (CH 2 ) and ν s (CH 2 ) modes are centered at 2924 and 2851 cm
À1 , respectively. It
indicates that the hydrocarbon chains in both lipids exist in a liquid phase. Desorption of the lipid bilayer leads to a hypsochromic shift of the methylene stretching
modes and indicates a potential-driven phase transition of the hydrocarbon chains to
the gel phase [71]. As expected for lipid molecules containing monounsaturated
chain(-s) the positions of the methylene stretching modes indicate that the hydrocarbon chains exist in a liquid state. Similarly to the DMPC and DMPS bilayer, in
the DMPE-DMPG bilayer the intensities of the IR spectra in the CH stretching
modes region depend on the potential applied to the Au(111) electrode (Fig. 3.10a).
This result indicates potential dependent reorientation of the hydrocarbon chains in
the bilayer. The order parameter of the hydrocarbon chains (Sect. 2.4, Eq. 2.56) was
calculated. In the DMPE-DMPG bilayer adsorbed directly on the electrode surface
the order parameter S chain of the hydrocarbon chains is equal to 0.55, which gives the
average tilt of the hydrocarbon chains of 35
with respect to the surface normal.
Desorption of the lipid bilayer leads to an increase in the S chain to ca. 0.8 and decrease
in the chain tilt to 21
versus surface normal. In the PE-PG bilayers containing lipids
with unsaturated chains no significant spectral changes are observed during the
potential scans (Fig. 3.10b,c). In the POPE-POPG:DPPG bilayer the S chain is equal
to 0.63 (average tilt of the chains of 30
vs surface normal) while in the DOPEPOPG:DPPG bilayer the S chain is close to 0.51 (average tilt of the chains of
34
vs surface normal). Madrit et al. [34] reported that the orientation of the
hydrocarbon chains in the DMPE bilayer does not depend on the electrode potential.
3.3 In Situ PM IRRAS Studies of Films of Biomolecules Adsorbed on Electrode Surfaces 63
chains varies [71]. The spectroelectrochemical characteristic of asymmetric bilayers
composed of following lipids:
1. With two saturated hydrocarbon chains: DMPE and 1,2-dimyristoyl-sn-glycerol3-phoshoglycerol (DMPG) (DMPE-DMPG bilayer),
2. One monounsaturated hydrocarbon chain: 1-palmitoyl-2-oleoyl-sn-glycero-3phosphoethanolamine
(POPE)
and
1-plamitoyl-2-oleoyl-sn-glycerol-3phospho-(1
0 rac glycerol) (POPG) mixed with 1,2-diplamitoyl-sn-glycero-3phospho-(1
0 rac glycerol) (DPPG) [POPE-POPG:DPPG bilayer (0.95:0.05 mole
ratio)] and
3. Two monounsaturated hydrocarbon chains in the inner leaflet: 1,2-dioleoyl-snglycero-3-phosphoethanolamine
(DOPE)
[DOPE-POPG:DPPG
bilayer
(0.95:0.05 mole ratio)] is described below.
The electrochemical behavior of these bilayers shows that the potential window
of the bilayer adsorption on the metal surface decreases with an increase in the
degree of unsaturation of the hydrocarbon chains. Potential-dependent changes in
the structure of these lipid bilayers were investigated using in situ PM IRRAS.
Figure 3.10 shows the PM IRRA spectra of the three transverse asymmetric lipid
bilayers at different potentials applied to the Au(111) electrode.
In the DMPE-DMPG bilayer the maxima of absorption of the methylene
stretching modes depend on the potential applied to the Au(111) electrode
(Fig. 3.10a). In the bilayer adsorbed directly on the Au electrode surface the
ν as (CH 2 ) and ν s (CH 2 ) modes are centered at 2924 and 2851 cm
À1 , respectively. It
indicates that the hydrocarbon chains in both lipids exist in a liquid phase. Desorption of the lipid bilayer leads to a hypsochromic shift of the methylene stretching
modes and indicates a potential-driven phase transition of the hydrocarbon chains to
the gel phase [71]. As expected for lipid molecules containing monounsaturated
chain(-s) the positions of the methylene stretching modes indicate that the hydrocarbon chains exist in a liquid state. Similarly to the DMPC and DMPS bilayer, in
the DMPE-DMPG bilayer the intensities of the IR spectra in the CH stretching
modes region depend on the potential applied to the Au(111) electrode (Fig. 3.10a).
This result indicates potential dependent reorientation of the hydrocarbon chains in
the bilayer. The order parameter of the hydrocarbon chains (Sect. 2.4, Eq. 2.56) was
calculated. In the DMPE-DMPG bilayer adsorbed directly on the electrode surface
the order parameter S chain of the hydrocarbon chains is equal to 0.55, which gives the
average tilt of the hydrocarbon chains of 35
with respect to the surface normal.
Desorption of the lipid bilayer leads to an increase in the S chain to ca. 0.8 and decrease
in the chain tilt to 21
versus surface normal. In the PE-PG bilayers containing lipids
with unsaturated chains no significant spectral changes are observed during the
potential scans (Fig. 3.10b,c). In the POPE-POPG:DPPG bilayer the S chain is equal
to 0.63 (average tilt of the chains of 30
vs surface normal) while in the DOPEPOPG:DPPG bilayer the S chain is close to 0.51 (average tilt of the chains of
34
vs surface normal). Madrit et al. [34] reported that the orientation of the
hydrocarbon chains in the DMPE bilayer does not depend on the electrode potential.
3.3 In Situ PM IRRAS Studies of Films of Biomolecules Adsorbed on Electrode Surfaces 63