3.3.1 Determination of Potential-Driven Structural Changes
in Lipid Bilayers: In Situ PM IRRAS
A biological cell membrane is the most important electrified interface in nature. Cell
membranes are composed of a fluid, dynamic lipid bilayer in which the polar head
groups face the cytoplasm and the extracellular space while the hydrophobic hydrocarbon chains are oriented toward each other. Such molecular scale order in a bilayer
assembly is known as a Y-type multilayer [26]. Various membrane associated
proteins are embedded in the fluid lipid matrix. Due to the compositional and
structural complexity of biological cell membranes, their models are commonly
used to study their composition, structure, dynamics and functions. Phospholipids
are the main component of the cell membranes and indeed they are predominantly
used in biomimetic studies. Next to phospholipids, sterols and glycolipids belong to
two important classes of membrane lipids [49]. The most commonly used models of
cell membranes are composed of phospholipids, however multicomponent bilayers
containing phospholipids, sterols and glycolipids have been also investigated.
Planar lipid bilayers are the most often used models of cell membranes. Vesicles
spreading [50–52], Langmuir-Blodgett and Langmuir-Schaefer transfer (LB-LS)
[31, 37, 53], or self-assembly methods are used to fabricate lipid bilayers on solid
surfaces. Depending on the bilayer fabrication method supported, tethered [54–56],
or floating lipid bilayers [33] are produced on a solid substrate. Layer by layer
deposition of each monolayer during the LB-LS transfer allows for the fabrication of
either symmetric or asymmetric lipid bilayers on electrode surfaces [30, 31, 34, 35,
37, 39, 44]. The spectroelectrochemical characterization of LB-LS transferred planar
lipid bilayers is described below. The electrochemical behavior of LB-LS planar
lipid bilayers supported directly on metallic electrode surfaces is similar. Representative capacitance potential curves of 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) bilayer on Au(111) electrode surface are shown in Fig. 3.6. A
lipid bilayer containing phosphatidylcholine (PC) is selected, because PC is the most
common component of cell membranes.
The capacitance-potential curves of various lipid bilayer show similarities. In
general, at potentials close to the potential of zero charge (pzc) of a gold electrode in
a given electrolyte solution, the capacity of a lipid bilayer reaches a minimum. The
capacitance minimum of the LB-LS transferred DMPC bilayer is equal to
6.3 μF cm
À2 (Fig. 3.6). Depending on the bilayer fabrication method and lipid
composition the capacitance minimum varies between 2 [30] and 10 μF cm
À2
[36]. On the gold electrode surface, a large positive potential shift from the pzc is
not possible due to the oxidation of the electrode material. However, at positive
surface charge accumulated on the gold electrode an increase in the capacitance in
some bilayers is observed [36]. A negative potential shift from the pzc leads to an
increase in the capacitance and appearance of a pseudocapacitive peak. This peak
(E tr in Fig. 3.6) is associated with a phase transition in the lipid bilayer. A further
negative potential shift leads to an increase in the capacitance to values which are
characteristic for the unmodified Au(111) electrode (Fig. 3.6). It indicates desorption
3.3 In Situ PM IRRAS Studies of Films of Biomolecules Adsorbed on Electrode Surfaces 55
in Lipid Bilayers: In Situ PM IRRAS
A biological cell membrane is the most important electrified interface in nature. Cell
membranes are composed of a fluid, dynamic lipid bilayer in which the polar head
groups face the cytoplasm and the extracellular space while the hydrophobic hydrocarbon chains are oriented toward each other. Such molecular scale order in a bilayer
assembly is known as a Y-type multilayer [26]. Various membrane associated
proteins are embedded in the fluid lipid matrix. Due to the compositional and
structural complexity of biological cell membranes, their models are commonly
used to study their composition, structure, dynamics and functions. Phospholipids
are the main component of the cell membranes and indeed they are predominantly
used in biomimetic studies. Next to phospholipids, sterols and glycolipids belong to
two important classes of membrane lipids [49]. The most commonly used models of
cell membranes are composed of phospholipids, however multicomponent bilayers
containing phospholipids, sterols and glycolipids have been also investigated.
Planar lipid bilayers are the most often used models of cell membranes. Vesicles
spreading [50–52], Langmuir-Blodgett and Langmuir-Schaefer transfer (LB-LS)
[31, 37, 53], or self-assembly methods are used to fabricate lipid bilayers on solid
surfaces. Depending on the bilayer fabrication method supported, tethered [54–56],
or floating lipid bilayers [33] are produced on a solid substrate. Layer by layer
deposition of each monolayer during the LB-LS transfer allows for the fabrication of
either symmetric or asymmetric lipid bilayers on electrode surfaces [30, 31, 34, 35,
37, 39, 44]. The spectroelectrochemical characterization of LB-LS transferred planar
lipid bilayers is described below. The electrochemical behavior of LB-LS planar
lipid bilayers supported directly on metallic electrode surfaces is similar. Representative capacitance potential curves of 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) bilayer on Au(111) electrode surface are shown in Fig. 3.6. A
lipid bilayer containing phosphatidylcholine (PC) is selected, because PC is the most
common component of cell membranes.
The capacitance-potential curves of various lipid bilayer show similarities. In
general, at potentials close to the potential of zero charge (pzc) of a gold electrode in
a given electrolyte solution, the capacity of a lipid bilayer reaches a minimum. The
capacitance minimum of the LB-LS transferred DMPC bilayer is equal to
6.3 μF cm
À2 (Fig. 3.6). Depending on the bilayer fabrication method and lipid
composition the capacitance minimum varies between 2 [30] and 10 μF cm
À2
[36]. On the gold electrode surface, a large positive potential shift from the pzc is
not possible due to the oxidation of the electrode material. However, at positive
surface charge accumulated on the gold electrode an increase in the capacitance in
some bilayers is observed [36]. A negative potential shift from the pzc leads to an
increase in the capacitance and appearance of a pseudocapacitive peak. This peak
(E tr in Fig. 3.6) is associated with a phase transition in the lipid bilayer. A further
negative potential shift leads to an increase in the capacitance to values which are
characteristic for the unmodified Au(111) electrode (Fig. 3.6). It indicates desorption
3.3 In Situ PM IRRAS Studies of Films of Biomolecules Adsorbed on Electrode Surfaces 55