Application of Polarization Modulation Infrared Reflection Absorption Spectroscopy in Electrochemistry by Izabella Brand

Author:Izabella Brand
Language: eng
Format: epub
ISBN: 9783030421649
Publisher: Springer International Publishing

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 nm2 per lipid molecule. It is larger than the cross-sectional area of the PC moiety (0.46 nm2) [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 nm2 [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 nm2 [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].

The two leaflets of the DMPC bilayer may be spectroscopically distinguished when per-deuterated phospholipids are introduced into one leaflet. Garcia-Areaz et al. [31]

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