DPPC monolayers

Fig. XV-8. Fluorescence micrographs of crystalline domains of an S-DPPC monolayer containing 2% cholesterol and compressed to the plateau region. [From H. McConnell, D. Keller, and H. Gaub, J. Phys. Chetn., 40, 1717 (I486) (Ref, 49). Copyright 1986, American Chemical Society.]...

Fig. XV-14. Surface pressure-area isotherms at 298 K for a DPPC monolayer on phos-photungstic acid (10 Af) at the pH values shown with 10 A/ NaCl added. (From Ref. 123.)...

Phospholipids are amphiphilic compoimds with high surface activity. They can significantly influence the physical properties of emulsions and foams used in the food industry. Rodriguez Patino et al. (2007) investigated structural, morphological, and surface rheology of dipalmitoylpho-sphatidylcholine (DPPC) and dioleoyl phosphatidylcholine (DOPC) monolayers at air-water interface. DPPC monolayers showed structural polymorphisms at the air-water interface as a function of surface pressure and the pH of the aqueous phase (Fig. 6.18). DOPC monolayers showed a... 

FIGURE 6.18 AFM images of DPPC monolayer structures formed at air-water interface at a temperature of 20 C and a surface pressure of 7 mN/m. Images were collected under three different pH levels. The total image size is shown on the images. Reprinted with permission from Rodriguez Patino et al. (2007). 

The Yl/A isotherms of the racemic and enantiomeric forms of DPPC are identical within experimental error under every condition of temperature, humidity, and rate of compression that we have tested. For example, the temperature dependence of the compression/expansion curves for DPPC monolayers spread on pure water are identical for both the racemic mixture and the d- and L-isomers (Fig. 13). Furthermore, the equilibrium spreading pressures of this surfactant are independent of stereochemistry in the same broad temperature range, indicating that both enantiomeric and racemic films of DPPC are at the same energetic state when in equilibrium with their bulk crystals. 

Figure 8.23 Left XPS survey spectrum recorded on palladium. Right Nitrogen IS spectrum of a surfactin/DPPC monolayer on mica. The vertical scale shows the number of counts per second. Adapted from ref. [358],...

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).

Figures 4A and 4B present experimental spectra that illustrate the principle that the incoming E field distribution helps govern the type of spectra obtained. In Figure 4A, spectra of a DPPC monolayer are presented which were obtained at 60 angle of incidence with s-polarized radiation. As in previous studies where the experimental angle of incidence was 30° (2-6), the observed spectra have negative absorbances. In Figure 4B, however, the spectra of the monolayer taken with p-polarized radiation show positive absorbance bands, as predicted from theory (Figure 3).

Figure 4. Experimental spectra of the DPPC monolayer at differing molecular areas. The angle of incidence of the incoming radiation was 60° relative to the surface normal. Figure 4A (top) shows s-polarized spectra Figure 4B (bottom) shows p-polarized spectra. The precise monolayer molecular areas at which each spectrum was obtained are given in the figures.

With the development of the external reflectance IR technique for observing monolayers in-situ at the A/W interface, we now have the ability, for the first time, to directly compare the structure of the monolayer film at the A/W interface with the monolayer transferred to a solid substrate. In order to determine whether these transfer artifacts occur for the DPPC monolayer, we have studied the structure of DPPC when transferred to Ge ATR crystals. Figure 6 is the pressure-area curve of the DPPC monolayer on which are indicated the points at which film transfer was made. Specific surface pressures of transfer were chosen in order to insure that transferred monolayers were studied in the LE, LE-LC and LC-SC regions, to provide a basis of comparison with the in-situ monolayers. 

In Figure 7 a comparison is made of the frequency of the CHj antisymmetric stretching vibration as a function of molecular area for DPPC monolayer films at the A/W and A/Ge interfaces. As described above, the frequency of (his vibration is related to the overall macromolecular conformation of the lipid hydrocarbon chains. For the condensed phase monolayer (-40-45 A2 molecule 1), the measured frequency of the transferred monolayer film is virtually the same as that of the in-situ monolayer at the same molecular area, indicating a highly ordered acyl chain, predominately all-trans in character. For LE films as well as films transferred in the LE-LC phase transition region, however, the measured frequency appears independent (within experimental uncertainty) of the surface pressure, or molecular area, at which the film was transferred. The hydrocarbon chains of these films are more disordered than those of the condensed phase transferred films. However, no such easy comparison can be made to the in-situ monolayers at comparable molecular areas. For the LE monolayers (> ca. 70 A2 molecule 1), the transferred monolayers are more ordered than the in-situ film. In the LE-LC phase transition region ( 55-70 A2 molecule 1), the opposite behavior occurs. 

Figure 7. The calculated frequency of the CH2 antisymmetric stretching vibration in the transferred DPPC monolayer films (solid circles) plotted against the molecular area at which the film was transferred. The frequency of this vibration for the in-situ monolayer film at the A/W interface is superimposed on the plot (open circles).

It has also proven possible to directly compare the structure of the monolayer film at the A/W interface with the structure of the monolayer film transferred onto a solid substrate using conventional L-B methods. For DPPC monolayer films transferred to Ge ATR crystals at low-to-intermediate pressures, the transferred monolayer films have a constant conformational order independent of the transfer pressure, and an orientational distribution that is more ordered than that of the in-situ monolayer. For those monolayer films transferred at high surface pressures, the hydrocarbon chains have a similar conformational order but are more oriented than the in-situ monolayer at the same surface pressure,... 

McConlogue, C.W. Vanderlick, T.K. A close look at domain formation in DPPC monolayers. Langmuir 1997,13, 7158. 

Fig. 3. (A) Far-field confocal micrograph (35 pm x 35 pm) of a mica-supported DPPC monolayer showing LE-LC phase coexistence, deposited at a surface pressure of 9mN/m. (B) Atomic force micrograph of the film depicted in (A). Bright features denote topographically higher substructure of the film. (C) Near-held fluorescence image of the him shown in (A). (D) Near-held topology image collected simultaneously with the image depicted in (C). Reproduced with permission from Ref. [18]. Copyright 1998 Biophysical Society.

Fig. 4. (A) Near-field fluorescence image of a DPPC monolayer at the air-sucrose solution interface under low surface pressure. (B) Near-field fluorescence image of a DPPC monolayer at the air-sucrose solution interface under high surface pressure. (C) Near-field fluorescence image of a DPPC monolayer at the air-sucrose solution interface under high surface pressure, collected using the optical feedback approach. Reproduced with permission from Ref. [19]. Copyright 1999 Blackwell Publishing.

In addition to the long-chain fatty acid molecules described above, a large number of studies have appeared that use IRRAS to study phospholipid monolayers as models of biomembrane interfaces. Mitchell and Dluhy [26] reported the first IRRAS spectra of 1,2-distearoyl-5 -glycero-3-phosphocholine (DSPC), l,2-dimyristoyl-i -glycero-3-phosphocholine (DMPC), and 1,2-di-palmitoyl-,v/ -glycero-3-phosphocholine (DPPC) monolayers at the air-water... 

Figure 1 Compression isotherm of pure DPPC monolayer.

If, as suggested by FTIR data, the situation here involves two, essentially separate, phases of OA and DPPC, one might expect a two-stage collapse in isotherms of mixed monolayers of these two components. Although it has been concluded that in the gel phase, complete separation of oleic acid and DPPC is unlikely (Lewis and Hadgraft, 1990), there is evidence from biphasic-type collapse pressures of OA/DPPC monolayers that a fair degree of phase separation does occur. It is well established that some degree of phase separation is likely in most biphasic systems, and there are several excellent review articles on this theme (e.g., Jain, 1983). 

There are many cases in which other techniques have been applied to biphasic systems in order to establish the nature of mixing. For example, fluorescence microscopy of DPPC monolayers containing 2% of a fluorescent probe have shown the coexistence of solid and fluid phases of DPPC at intermediate pressures (Weis, 1991). Similar results have been achieved with a variety of other phospholipids using the same technique (Vaz et al., 1989). The recent application of laser light scattering to this area (Street et al., unpublished data) has yet to produce any conclusive evidence, but the future for this particular technique is also promising. It also provides information about the viscoelastic properties of the monolayer and how these are affected by the inclusion of penetration enhancers. 

The lipid lamellae within the stratum corneum are thought to consist of a complex mixture of compounds but to contain predominantly cholesteryl sulfate (5%), free fatty acids (15%), cholesterol (25%), and ceramides (50%) (Abraham and Downing, 1990). Unlike DPPC monolayers, those formed from stratum corneum lipids do not undergo any obvious phase transitions during compression) therefore the information available from the resulting isotherms is more limited. The behavior of monolayers consisting of these types of compounds has been investigated in the presence and absence of Azone. 

Neutron reflectometry studies on mixed DPPC/oleic acid monolayers have been conducted using the CRISP reflectometer at RAL. First, the stmcture of DPPC monolayers was determined by measuring reflectivity profiles from three different isotopic forms of the DPPC monolayer system. This was achieved using hydrogenated (h-DPPC) and chain perdeuterated (d-DPPC) phospholipids and two different subphases of D2O and ACMW. The monolayers were studied at three surface coverages of approximately 50, 60, and 70 A /molecule. Examination of the surface pressure-area isotherm reveals that the main LE/LC phase transition for DPPC monolayers occurs over this range of molecular area (Lewis and Hadgraft, 1990). 

It was also possible to analyze data from the DPPC/OA monolayers using the optical matrix method. A one-layer model represents an oversimplification of the complex structure of this monolayer, and the quality of the fits to experimental data is generally poorer than for the DPPC monolayer. Despite these problems it has been possible to estimate the relative volume fraction of DPPC, OA, and water in these mixed monolayers. It is apparent at all surface coverages that the volume fraction of DPPC is approximately twice that of OA, as expected, and that the volume fraction of water in the monolayer increased steadily as the monolayer was expanded to 70 A /molecule. 

For DPPC monolayers spread on water there are two species in the system, producing three terms in Eq. 17 (two self-terms, /i o and h and one cross-term, hp ). 

Under some experimental circumstances, equations of the form of Eq. 17 can be simplified to gain information more easily (see later). For example, if null reflecting water is used as the subphase, then in Eq. 17 we have = 0, and hence for the DPPC monolayers we have... 

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