DPPC hydrocarbon chains

Figure 5. The average angle of orientation of the DPPC hydrocarbon chains in the in-situ film at the A/W interface as, a function of the monolayer molecular area (solid circles). The pressure-area curve of DPPC (open circles) is superimposed on the figure.

Figure 3.67. AFM image of a dipalmitoylphosphatidylcholine (DPPC) monolayer transferred onto a quartz plate at a surface pressure of 30 mN m. On this hydrophilic substrate the phospholipids have their head groups on the surface. Therefore, the bright spots should correspond to the end-methyl groups of the DPPC hydrocarbon chains. This is corroborated by the finding that the area per bright spot (averaged over many images) corresponds to half of the value for the area per phospholipid molecule as found from the r(A) isotherm at 30 mN m. (Courtesy of X. Zhai and J.M. Kleijn" )...

Figure 2 Snapshot from an MD simulation of a multilamellar liquid crystalline phase DPPC bilayer. Water molecules are colored white, lipid polar groups gray, and lipid hydrocarbon chains black. The central simulation cell containing 64 DPPC and 1792 water molecules, outlined m the upper left portion of the figure, is shown along with seven replicas generated by the periodic boundary conditions. (From Ref. 55.)...

Helm et al. [443] subsequently studied dipalmitoyl phosphatidylcholine (DPPC). They concluded that the condensed two-dimensional phase normally involves a tilt of the hydrocarbon chains of about 30°... 

Finally, a number of studies have employed viologens in the bilayer, either added as an amphiphilic reagent to DPPC,340 or as the sole component of the vesicle walls.337,341 342 In the latter cases, double bonds in the hydrocarbon chain of the viologen have been polymerized to give added stability and rigidity to the bilayer structure. 

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. 

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,... 

Figure 3-37 Raman spectra of DPPC in the CH stretching region (a) measured with an AOTF followed by Fourier self-deconvolution (b) measured with a dispersive scanning monochromator at 5 cm-1 resolution. The spectra show the features due to methyl and methylene vibrations arising from both the hydrocarbon chain and headgroup portions of the lipind. (Reproduced with permission from Ref. 101.)...

With increasing temperature, the steric and van der Waal s interchain interactions that favor a crystalline-like packing of the DPPC molecules are progressively overcome by thermally induced rotational excitations of the hydrocarbon chains. Thus,... 

Figure 2 Organization of the lamellar bilayer phases of DPPC in the fiuid (Lx), ripple (Pp), gel (Lp/) and pseudo-crystalline (Lc) states. A top view of the packing of the hydrocarbon chains is shown in the last column. (From Reference 12.)...

Figure 8 shows a plot of a fluid-like DPPC bilayer, in which a small fraction of the lipids are replaced by a pyrene-containing PyrPC probe, see Fig. 1 (26). The study has demonstrated that the perturbations in the vicinity of the probe are substantial, as the conformational order parameter (Scd) of lipid hydrocarbon chains close to the probe may change as much as about 100%. However, what is also found is the short range of perturbations, because the perturbations are negligible beyond a distance of about 1.5 nm. In practice, this finding implies that about 20-30 lipids around the probe are affected by the marker, but the global properties averaged across the membrane are affected only little.

Information on the conformational state of the hydrocarbon chains and their orientation has been obtained from external infrared reflection absorption spectroscopy (IRRAS). The first systematic IRRAS studies on phospholipid Langmuir monolayers were reported by Dluhy et al ) (see, for instance fig. 3.62). For DPPC monolayers in the LE phase the positions of the conformation-sensitive symmetric and anti-symmetric C-H stretching bands in the IRRAS spectra were found to be at the same positions as for bilayer systems of DPPC above the Kralft temperature. In the LC phase the frequencies of these bands indicate that the hydrocarbon chains of the lipid molecules are in the all-trans ) conformation (i.e. zig-zag) and analysis of polarized IRRAS spectra show that their average tilt is ca 35° relative to the monolayer normal. This is in reasonable agreement with the tilt angle of 30° obtciined from X-ray diffraction on DPPC monolayers (30°). 

Figure 41.1 shows the gel-to-liquid crystalline phase transition temperatures (Tm) of DPPC-cholesterol mixtures as a function of the cholesterol-lipid molar ratio. The Tm of fully hydrated DPPC is 42°C (Crowe and Crowe, 1988 Vist and Davis, 1990 McMullen et al., 1993 Ohtake et al., 2004). Upon the addition of cholesterol, the transition enthalpy decreases continuously imtil it is no longer observable at 50 mol% cholesterol. The disappearance of the melting transition has been attributed to strong interactions between cholesterol and DPPC (McCoimell, 2003). Upon dehydration, the Tm for DPPC increases from 42 to 105°C (Crowe and Crowe, 1988 Ohtake et al., 2004). This Tm increase is caused by the reduction in the spacing between the phospholipids, which allows for increased van der Waals interactions between the lipid hydrocarbon chains (Koster et al., 1994). Between 10 and 70 mol% cholesterol, two endothermic transitions are observed, both lower than the Tm of the pure phospholipid (Figure 41.1). High-sensitivity DSC studies on fully hydrated DPPC-cholesterol systems reported endotherms consisting of two components, suggesting the existence of domains enriched/depleted in cholesterol (Vist and Davis, 1990 McMullen et al., 1993). The two peaks present in our freeze-dried systems also suggest the...

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