Lipid monolayers at the air-water interface

The spreading pressure of the lipid film tends to increase the surface, i.e. it works in a direction opposite that of the surface tension (y). Thus the reduction of the surface tension (Ay) of water by a lipid is equivalent to the film pressure (11). 

It is remarkable that lipid molecules, which are spread on a water surface, remain at the surface for quite long times, even though the lipid is soluble to a certain extent in the bulk water. Most lipids form such insoluble monolayers on water. 

The most intensive period of monolayer research took place during the decade before the Second World War. Thus the basic interpretations of Il-A isotherms were done before the characteristic crystalline and liquid-crystalline structural features were known. Unfortunately much of the later monolayer work has also been limited to the early concepts only, without the knowledge on three-dimensional lipid phases being fully utilized. Reports of molecular arrangement in monolayers are therefore often misleading and the nomenclature used is inadequate. A structural description of monolayer phases will first be given based on relations with crystalline and liquid-crystalline phases. A hypothetical pressure-area (II-A) isotherm shown in Fig. 8.23 will be used and the nomenclature according to Harkins (1952) is applied. 

When the molecules are far apart they are considered to lie nearly flat on the surface and distributed one by one in a two-dimensional gas phase (Gaines, 1966). An ideal gaseous monolayer should follow the two-dimensional gas equation  

When the gaseous monolayer is compressed, a transition into the so-called liquid-expanded phase usually takes place. This phase has been the subject of many controversies. Most of this discussion, however, took place before the structure of the lamellar liquid crystalline phase was known. It should be mentioned in this connection that Phillips et al. (1969) used such correlations in their interpretation of the monolayer structure of dipalmitoylphosphatidyl-choline which will be further discussed in Section 8.10. 

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S. Kjelleberg, B. Norkrans, H. Lofgren and K. Larsson, Surface balance study of interaction between microorganisms and a lipid monolayer at the air-water interface, Appl. Environmental Microbiol. 31 (1976) 609-611. 

McConnell, H.M. Structures and transitions in lipid monolayers at the air-water interface. Ann. Rev. Phys. Chem. 1991, 42, 171. 

Pallas. N.R. Pethica, B.A. Liquid-expanded to liquid-condensed transition in lipid monolayers at the air/ water interface. Langmuir 1985, 1, 509. 

Kuhl TL, Majewski J, Howes PB, Kjaier K, von Nahmen A, Lee KYC, Ocko B, Israelachvili JN, Smith GS (1999) Packing stress relaxation in polymer-lipid monolayers at the air-water interface an X-ray grazing-incidence diffraction and reflectivity study. J Am Chem Soc 121 7682-7688... 

FIGURE 2.4 Organization of a lipid monolayer at the air-water interface. All fatty acid molecules are oriented with respect to water so that polar head groups interact with water and apolar chains are rejected in the air. The limit between air and water in such systems is referred to as an air-water interface. 

FIGURE 2.5 Intermolecular forces stabilizing a lipid monolayer at the air-water interface. The apolar chains interact through London forces. Hydrogen bonds stabilize the interaction of two vicinal carboxylic acid head groups as well as the interaction of water molecules with these polar groups. 

The molecular mechanisms accoimting for the organization of a lipid monolayer have been the subject of an intense debate. Historically, the Nobel Prize recipient Irvin Langmuir thought that the apolar chains of fatty acid have just more affinity for each other than for water. It is interesting to recall his molecular interpretation of the forces involved in the formation of lipid monolayers at the air-water interface ... 

The enhancement of the electric field by about a factor of ten is not specifically related to the surfactant used, as it is also observed in the context of charged lipid monolayers at the air-water interface. Wurpel and co-workers used A-phage DNA bound to a cationic lipid monolayer at the air-water interface to study screening effects of counterions by detecting water signals in the OD stretching region at different concentrations. ... 

The structure of biological and model membranes is frequently viewed in the context of the fluid mosaic model [4], Since biological membranes are composed of a mixture of various lipids, proteins, and carbohydrates the supra-structure or lateral organization of the components is not necessarily random. In order to model biological membranes, lipid assemblies of increasing complexity were studied. Extensive investigation of multicomponent monolayers (at the air-water interface) as well as bilayers have been reported. 

DPPC is prominent in the lipid bilayer making up the cell membrane and is also a major constituent of lung surfactant ( pulmonary surfactant). The lung membrane resembles a mixed surfactant monolayer at the air/water interface. Since the temperature in a lung is below the critical temperature for DPPC monolayers, the LE-LC transition may be of significance in the continuous compression and expansion loops that this membrane undergoes during respiration. We will say more about this in sec. 3.9. 

From a systematic study focused on fhe tt-A isofherm of protein-LMWE mixed monolayers (including fhe application of fhe additivity rule on miscibility and the quantification of inferacfions between monolayer components by excess free energy ( if has been concluded that, at a macroscopic level, these compounds form a pracfically immiscible monolayer at the air-water interface, af tt < Tlf At higher tt the collapsed protein is displaced from the interface by LMWE (monoglycerides, phospholipids, etc.). The existence of low profein interactions in disordered proteins ((3-casein and caseinate) facilitates the protein displacement by LMWE from fhe air-water interface. However, the lower surface acfivify of unsafurafed-LMWE explains the fact that this lipid has a lower capacity than saturated-LMWE for protein displacement. 

Wiesenthal T, Baekmark TR, Merkel R (1999) Direct evidence for a lipid alkyl chain orderng transition in poly(ethylene oxide) lipopolymer monolayers at the air-water interface obtained from infrared reflection absorption spectroscopy. Langmuir 15 6837-6844... 

Chapter 1 summarizes methods for the stabilization of artificial lipid membranes. They include synthesis of new types of polymerizable lipids and polymerization of membranes. Creation and characterization of novel poly(lipid) membrane systems, as well as their functionalization for biotechnological applications, are also described. Chapter 2 addresses experimental studies on the design and characterization of lipopolymer-based monolayers at the air-water interface. Thermodynamic and structural data collected with X-ray and neutron reflectrometry, infrared reflection absorption spectroscopy, and sum frequency generation spectroscopy provide... 

The interaction of /3-casein molecules with lecithin monolayers is governed by the surface activity (hydrophobicity) of the macromolecule. Hydrophobicity does not guarantee interaction of a protein with lecithin in dispersion (22), but it favors penetration of proteins into monolayers at the air-water interface. The whole molecule seems to penetrate in the case of /3-casein (Figure 7) this leads to the compression of the lipid molecules and perhaps the formation of a layer of relatively restricted lecithin molecules around the periphery of the protein molecules. The situation is quite different for a very polar protein such as lysozyme where... 

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