Monolayer Model Membrane

Chapter 6 (Section 6.3) introduced the formation of the Langmuir monolayer at the air-liquid interface, and the role of intermolecular force balance in its assembly. When its building units are lipids or lipid-based amphiphiles, their molecular structure, configuration, and the interplay of the intermolecular forces within are a lot like those of biological membranes. The difference is that this is a monolayer while biological membranes have a bilayer morphology consisting of mainly lipids and proteins. The key to the employment of the Langmuir monolayer as a model lipid membrane lies here. The structural, compositional, and confi-gurational resemblances between the two provide the powerful insight that the Langmuir monolayer can mimic biological membranes with facile advantage (Brockman, 1999; Feng, 1999). Those valuable parameters including surface pressure, surface potential, surface density, and temperature are highly controllable, and the phase formation and transition on the surface of the monolayer can be tracked with nanoscale resolution in situ (Vollhardt and Fainerman, 2000; Brezesinski and Mohwald, 2003).

Figure 11.10 presents one possible design of a model lipid membrane. Lipid building units for the Langmuir monolayer can be selected based on the representativeness of the biological membrane to be studied. Membrane proteins can be employed, if necessary, but primarily based on the same criterion. For the

Figure 11.10. Design of a model lipid membrane to mimic a biological membrane.

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lipid molecule 2

lipid molecule 2

Figure 11.10. Design of a model lipid membrane to mimic a biological membrane.

foreign OTgürnt: foreign inorganic divalent ion foreign OTgürnt: foreign inorganic divalent ion study of ligand-acceptor interaction, either ligand or acceptor can be inserted as part of the Langmuir monolayer while the other component is introduced in subphase. Divalent cations such as calcium or magnesium ions, which are critical in many biological activities, polymeric or oligomeric biomolecules, and foreign organic and inorganic components to be studied also can be introduced in subphase. As these components interact with the monolayer, the force balance that was set for the given condition of the monolayer is subject to change. For example, divalent cations, inorganics, or biopolymers whose surfaces usually are highly charged will have a profound effect on the electrostatic interaction between the head groups of the lipid building units, while the hydrophobic moiety of biopolymers or organics will induce new van der Waals interaction with the alkyl groups of the monolayer. Acceptor that is complexed with ligand possibly causes steric repulsion. All these newly induced intermolecular forces will cause the whole system to be shifted into the new force balance. And this can be tracked both quantitatively and qualitatively by recording the change in the surface parameters mentioned above and in the surface phase. That in turn may allow extracting meaningful insights to understand the possible interaction mechanism of those additives with the biological membrane to be studied.

A model lipid membrane has been used for a variety of applications. This includes the physiological study of human lung surfactant (Zasadzinski et al., 2001), the development of drug discovery and delivery systems (Baksh et al., 2004), and the effect of foreign peptides on the biological membranes of human and other living organisms (Maget-Dana, 1999). One critical issue remains in the fact that most of the valid model membrane systems are assembled at the liquid-gas interface, which can raise the question of the validity of the results related to the interaction in the hydrocarbon chain regions. Further advances of this model membrane toward the area of bionanotechnology will depend on the reliable resolution of this correspondence issue between model and real systems.

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