Saturated fatty acids monolayers

It was estabhshed ia 1945 that monolayers of saturated fatty acids have quite compHcated phase diagrams (13). However, the observation of the different phases has become possible only much more recendy owiag to improvements ia experimental optical techniques such as duorescence, polarized duorescence, and Brewster angle microscopies, and x-ray methods usiag synchrotron radiation, etc. Thus, it has become well accepted that Hpid monolayer stmctures are not merely soHd, Hquid expanded, Hquid condensed, etc, but that a faidy large number of phases and mesophases exist, as a variety of phase transitions between them (14,15). 

As has been indicated recently [27], the relaxation process during the compression of the monolayers of saturated fatty acids is rather slow and usually incomplete. Thus, the experimental Jt-A curves obtained under the usual continuous compression include the nonequilibrium effects. 

The phospholipid monolayers examined in this study were all saturated, symmetric, 1,2-diacyl-j -glycero-3-phosphate-based lipids. Four different lipid headgroups attached to the phosphate were examined choline, ethanolamine, glycerol and serine. Each lipid features a glycerol backbone, two saturated fatty acid chains and a phosphatidyl headgroup. 

It is not clear why LA and none of the saturated fatty acids that were studied disrupted endothelial barrier function. The injurious effects of LA on cultured endothelial cells may be mediated, in part, by the induction of peroxisomes and, thus, by excessive hydrogen peroxide formation. In addition, enrichment of endothelial lipids with selective fatty acids can modify specific cellular lipid pools and alter the morphology of cultured cell monolayers. Such fatty acid-mediated compositional changes may be sufficient to alter membrane properties, e.g., fluidity and activities of membrane-bound enzymes. One may speculate from these and other data that high dietary intakes of certain unsaturated fatty acids, such as LA, might not be entirely safe. 

Hughes, A. H., and Rideal, E. K. 1933. On the rate of oxidation of monolayers of im-saturated fatty acids. Proc. Roy. Soc. (London) A140, 253. 

Using polarised microscopy, Veale et al (33,34) have shown that monolayers of a number of saturated fatty acids with chain lengths of up to 23 are deposited epitaxially from a pure water subphase. Epitaxy is inhibited (perhaps only partially) with longer chain lengths and in the presence of cadmium. 

Fundamental membrane research has benefited greatly from the study of monolayers. One of the most important discoveries from this sort of research is the very existence of two-dimensional phases and phase transitions. Generally, studies of the sort that can be carried out with monolayers and bilayers cannot be directly extended to living cells, but some exceptional cases have shown that the extrapolation is valid. For example, it is known from monolayer studies that the presence of unsaturated hydrocarbon chains in lipid monolayers prevents some phase transitions from occurring as the temperature is lowered. Certain mutants of Escherichia coli are unable to synthesize fatty acids and hence can be manipulated through the compounds they are provided as nutrients. Abnormal levels of saturated hydrocarbon can... 

The occurrence of cis-double bonds hampers dense packing. Trans-double bonds do not have this effect elaldic acid (which has such a bond) packs like stearic acid. The effect of the cis-bond in the hydrocarbon chain is shown in fig. 3.1 lb, where it is observed that in the condensed phase the molecular area of Cj COOH increases from 0.28 nm for the fully saturated hydrocarbon chain (stearic acid), via 0.40 nm for the single unsaturated chain (oleic acid) to 0.49 nm for the doubly conjugated unsaturated chain (linoleic acid). In line with this, the collapse point, i.e. the value for where the monolayer breaks down to form a multilayer. Increases with decreasing degree of saturation. The pressure corresponding to the collapse point is lower when the fatty acid contains more double bonds (see the arrows in the figure). 

Philips et al. (18) reported that in mixed monolayers of dioleoylleci-thin—distearoyllecithin, the area/molecule showed expansion from the additivity rule because the unsaturated fatty acid chains increased the kinetic motion of saturated chains. However, in contrast to the studies here on Ci6 + Ci8 alcohols, they found no expansion in the mixed mono-layers of dipalmitoyllecithin-distearoyllecithin. 

The LB technique may also be combined with solid-state chemistry methods to produce novel molecular architectures. For example, a network of conductive polypyrrole (molecular wires ) may be obtained in a fatty acid matrix [37, 38]. First, monolayers of the iron salt of a long-chain fatty acid (e.g., ferric pahnitate) are assembled on an appropriate substrate. The multilayer film is then exposed to saturated HCl vapor at room temperature for several minutes. During this process, a chemical reaction transforms the fatty acid salt into layers of ferric chloride separated by layers of fatty acid. In the third and final step, the film is exposed to pyrrole vapor and a reaction occurs between the pyrrole and the ferric chloride, producing polypyrrole distributed within the multilayer assembly. 

During the past quarter century, considerable studies have been carried out on the reactions in monomolecular films of surfactant, or monolayers. Figure 1 shows the surface pressure-area curves for dioleoyl, soybean, egg, and dipalmitoyl lecithins [1]. For these four lecithins, the fatty acid composition was determined by gas chromatography. The dioleoyl lecithin has both chains unsaturated, soybean lecithin has polyunsaturated fatty acid chains, egg lecithin has 50% saturated and 50% unsatmated chains, and dipalmitoyl lecithin has both chains fully saturated. It is evident that, at any fixed surface pressure, the area per molecule is in the following order ... 

It can be assumed that the area per molecule represents the area of a square at the interface. Thus, the square root of the area per molecule gives the length of one side of the square, which represents the intermolecular distance. Figure 2 schematically illustrates the area per molecule and intermolecular distance in these four lecithins. The corresponding intermolecular distances were calculated to be 9.5, 8.8, 7.1, and 6.5 A, respectively, at a surface pressure of 20 mN/m [2]. Thus, one can conclude that a change in the saturation of the fatty acid chains produces subangstrom changes in the intermolecular distance in the monolayer. 

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