Unsaturated fatty acids, monolayers

The products of lipid oxidation in monolayers were also studied. Wu and coworkers (41) concluded that epoxides rather than hydroperoxides might be the major intermediates in the oxidation of unsaturated fatty acids adsorbed on silica, presumably because of the proximity of the substrate chains on the silica surface. In our work with ethyl oleate, linoleate and linolenate which were thermally oxidized on silica, the major decomposition products found were those typical of hydroperoxide decomposition (39). However, the decomposition patterns in monolayers were simpler and quantitatively different from those of bulk samples. For example, bulk samples produced significantly more ethyl octanoate than those of silica, whereas silica samples produced more ethyl 9-oxononanoate than those of bulk. This trend was consistent regardless of temperature, heating period or degree of oxidation. The fact that the same pattern of volatiles was found at both 60°C and 180°C implies that the same mode of decomposition occurs over this temperature range. 

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. 

Surface area depends on 1r, pH, ionic strength, and temperature, but wide variations in surface area are most readily attained with shorter-chain and unsaturated fatty acids (9). Since these fatty acids do not form stable monolayers, they are difficult to study using conventional procedures. So we took advantage of their instability and used desorption kinetics to estimate the apparent surface pKa, an indicator of charge density and field strength (10). 

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. 

Monolayers of stearic acid have been used as models to study the interaction of negatively charged surfaces with calcium ions. It is known since 1937 that these mono-layers are more condensed and are rigid when calcium ions are present in the aqueous substrate [5]. This behaviour may not be reproduced by the monolayers of the unsaturated fatty acid or of the acidic phospholipids [6]. Furthermore, at a given surface density of the lipids, two very similar carboxylic acids oleic and stearic acids, bind different amounts of calcium ions [7]. 

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... 

Influence of Intermolecular Spacing on Enzymic Hydrolysis of Lecithin Monolayers. When snake venom phospholipase A is injected under a lecithin monolayer, it splits lecithin into lysolecithin and free fatty acid. The change in polar groups of the monolayer results in a change of surface potential. However, if prior to injection of enzyme into the subsolution, a lecithin monolayer is compressed to such a surface pressure that the active site of the enzyme is unable to penetrate the monolayer, hydrolysis does not proceed. For monolayers of dipalmitoyl, egg, soybean, and dioleoyl lecithins the threshold surface pressure values at which hydrolysis does not proceed are 20, 30, 37, and 45 dynes per cm., respectively (40). This is also the same order for area per molecule in their surface pressure-area curves, indicating that enzymic hydrolysis of lecithin monolayers is influenced by the unsaturation of the fatty acyl chains and hence the intermolecular spacing in monolayers (40). 

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). 

One example has been given by Rakshit et al. ) who used [3.4.17] to obtain the Helmholtz energy for monolayers of CjjCOOH. CigCOOH, Cj COOH and C gCOOH, substituted fatty acids and unsaturated acids. The integration was car ried out from (almost) infinite area to r =. the equilibrium spreading pressure. 

A is the surface area, and t is time. A sigmoidal curve, reminiscent of a titration curve, was obtained when the initial rate of monolayer contraction, Rif was plotted as a function of pH (23). The apparent pKa, estimated from the midpoint of the sigmoidal curve, depended on the chain length and the degree of unsaturation of the fatty acid. The apparent pKa always exceeded the pKa for soluble carboxylic acids. 

In addition to acyl chains, the phospholipid headgroups should contribute to the stability of phospholipid bilayers as reflected by the considerably higher melting points (200°C) of pure phospholipids than of fatty acids (60°C). Also, the type of headgroup is important for the properties of a phospholipid membrane. For example, monolayers of phosphatidylethanolamine are less expanded than those derived from phosphatidylcholine and the melting point of the solid, pure phospholipid is lower for the former than for the latter, irrespective of the length or unsaturation of the fatty acids (Williams and Chapman, 1970). 

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 ... 

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