Surface pressure-molecular area

This chapter is basically divided in two parts, namely, the study of surface pressure-molecular area (jr — A) isotherms of phospholipids at ITIES and their effect on ion transfer. In the first part, the emphasis is put on topics which have been left out from Ref [5], i.e., Langmuir film techniques and theoretical modeling of jr — isotherms, as well as on the latest progress in the field, especially on experiments that combine Langmuir techni-... 

Prior to LB transfer, the surface pressure - molecular area (n-A) isotherms of dialkylsilane under various pH and temperature conditions were investigated. The pH condition of the subphase (water phase under the monolayer) is a crucial factor for the monolayer state. The condensed phase was formed directly without formation... 

Figure 17. Experimental system for the measurement of surface pressure-molecular area (n-A) isotherms and the horizontal touch cyclic voltammetry/ and schematic representations for the control of permeabilities through oriented monolayers formed at low and high applied surface pressures.

Figure 1. The surface pressure-molecular area isotherm of the DPPC monolayer at 22 °C (Figure 1A, top) with the infrared frequencies of the CH, antisymmetric stretching vibration, plotted against molecular area for the DPPC monolayer (Figure IB, bottom).

The monolayer of amphiphile spread on water is then compressed into a well-packed state. The profile of monolayer compression is recorded as surface pressure-molecular area (n-A) isotherm. Typical examples of n-A isotherms are shown in Fig. 4.33. The transverse axis of the isotherm represents the molecular area, which can be obtained by dividing the total surface area by the number of amphiphile molecules. The surface pressure is derived by subtracting the surface tension of the monolayer-covered water surface from that of pure water. This has dimensions corresponding to two-dimensional pressure. Surface pressure cannot be regarded as a normal three-dimensional pressure. For convenience, when discussing the phase of two-dimensional monolayer here, the surface pressure is treated as a two-dimensional pressure. 

To test this model, the surface properties of the 22-kDa fragment at an air—water interface have been examined. The air—water interface system has been used extensively to model the interaction of apoli-poproteins with lipid (Phillips and Sparks, 1980 Shen and Scanu, 1980 Camejo and Munoz, 1981 Phillips and Krebs, 1986). When the 22-kDa fragment was spread as monomolecular film in a Langmuir trough, the surface pressure-molecular area isotherm was calculated to be —16 A /... 

Adsorption at Air -Water Interface. The data for the adsorption of /3-casein at the clean air—water interface (Figures 2 and 3) can be combined to give surface pressure-molecular area (A) curves for the adsorbed protein films. For -casein the resultant tt-A curve is similar to that obtained by spreading the protein (12,16,18) therefore, this flexible protein has essentially the same conformation when spread or adsorbed. At low 7T, A = 2 m2/mg or 38 A2/residue, so there is sufficient space for the peptide backbone to lie in the plane of the interface with no loops or tails of residues protruding into either bulk phase. A conformational change occurs at ir = 7 dynes/cm where loop-tail formation commences (12). Monolayer coverage (A 0.4 m2/mg = 1610 A2/molecule) is complete when Cp 10"4 %. At this point the protein is in a close-packed,... 

For penetration experiments the insoluble monolayer is spread between the movable barriers at the surface of one region filled with pure buffer solution. The other region is filled by the solution of the dissolved surfactant. Afterward, the monolayer is brought to the desired state, e.g. surface pressure, molecular area, and swept onto the region containing the dissolved surfactant. Then the penetration kinetics experiments coupled with the BAM imaging were performed and the state of the penetrated monolayer in equilibrium was characterized. 

Fig. 3 Langmuir film balance schematic, a - monolayer of lipid in a quasi gas state, b -state of maximal compaction indicative of molecular area, c - monolayer collapse point reflecting monolayer rigidity. Tested monolayer was palmitic acid over H2O to produce a typical surface pressure/molecular area isotherm.

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