Behavior of Lipids in Water

By definition, lipids are insoluble in water. Yet they exist in an aqueous environment, and their behavior toward water is therefore of critical importance biologically. 

Many types of lipid are said to be amphiphilic, meaning they consist of two parts—a nonpolar hydrocarbon region and a region that is polar, ionic, or both. (The term amphiphilic has tended to replace amphipathic, used formerly.) 

Question Which of the following lipids are amphiphilic fatty acids acylate ions TAGS cholesterol phosphoglycerides phosphosphingolipids glycosphingolipids  

Fatty acids, TAGs, and cholesterol are not amphiphilic what polarity they have is extremely weak. All the others possess at least one formal charge or an abundance of hydroxyl groups in one part of the molecule. 

The tendency for hydrocarbon chains to become remote from the polar solvent, water, is known as the hydrophobic effect (Chap. 4). Hydrocarbons form no hydrogen bonds with water, and a hydrocarbon surrounded by water facilitates the formation of hydrogen bonds between the water molecules themselves. The bulk water is more structured than it is in the absence of the hydrocarbon i.e., it has lost entropy (Chap. 10) and is thus in a thermodynamically less favorable state. This state is obviated by the hydrocarbon being organized so that it is remote from water, thus rendering the water molecules near to it less ordered. Thus the hydrophobic effect is said to be entropically driven. 

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Owing to the impossibility of compiling partition coefficients for a substance into every possible set of solvents/phases, a reference that is commonly accepted is the octanol/water partition coefficient, Kj0tW. This solvent (i.e., the 1-octanol) is a good reference choice as it mimics (to a reasonable extent) the solvent behavior of lipids in biota as well as that of humic substances in soils. Extensive tables of Kio w values are readily available (see for example, the CRC Handbook for Chemistry and Physics). Such data help predict the environmental fate of many substances (see Casey and Pittman, 2005). 

In the previous two sections we discussed the electrodeformation and electroporation of vesicles made of single-component membranes in water. In this section, we consider the effect of salt present in the solutions. The membrane response discussed above was based on data accumulated for vesicles made of phosphatidylcholines (PCs), the most abundant fraction of lipids in mammahan cells. PC membranes are neutral and predominantly located in the outer leaflet of the plasma membrane. The inner leaflet, as well as the bilayer of bacterial membranes, is rich in charged lipids. This raises the question as to whether the presence of such charged lipids would influence the vesicle behavior in electric fields. Cholesterol is also present at a large fraction in mammalian cell membranes. It is extensively involved in the dynamics and stability of raft-hke domains in membranes [120]. In this section, apart from considering the response of vesicles in salt solutions, we describe aspects of the vesicle behavior of fluid-phase vesicles when two types of membrane inclusions are introduced, namely cholesterol and charged lipids. 

TiTuch of our understanding of the phase behavior of insoluble - monolayers of lipids at the air-water interface is derived from Adam s studies of fatty acid monolayers (I). It is now clear that the phase behavior of phospholipid monolayers (2) parallels that of the fatty acids we make use of these structure variations in our study of the interactions of phosphatidylcholine (lecithin) monolayers with proteins. Because of the biological significance of the interfacial behavior of lipids and proteins, there is a long history of studies on such systems. When Adam was studying lipid monolayers, other noted contemporary surface chemists were studying protein monolayers (3) and the interactions of proteins with lipid monolayers (4). The latter interaction has been studied by many so-called 4 penetration experiments where the protein is injected into the substrate below insoluble lipid monolayers that are spread on the... 

To describe the behavior of a lipid in water, it is customary to use a phase diagram in which the physical state or molecular arrangement of the lipid or the system is shown in relation to the composition of the mixture (Fig. 1). Composition is usually expressed in percentage by weight (abscissa), and the ordinate is temperature. In two-component systems, called binary... 

We have studied the viscoelastic behavior of lipid thin films at an air/water interface from surface pressure (tt) vs. surface area (A) curves, since an observation of the k-A curve of a monolayer is one of the most convenient methods to elucidate the viscoelastic behavior of the monolayer. Recently, in spite of compression process, we observed an overshoot-hump, a zero surface pressure, and a flat plateau in the rt-A curve of a synthetic fatty acid [3]. These characteristic features in the curve were explained by using a kinetic model representing the formation of aggregates of the fatty-adds molecules at an air/water interface. 

Systematic studies of the phase behavior of lipid-surfactant systems have been performed particularly on mixtures of surfactants of the Ci2EOn type and phosphatidylcholines with different chain lengths and saturation [109-113]. The phase diagrams of these pseudo-binary mixtures in excess water are complicated and become even more so when the water content is varied. 

Figure 40. Schematic diagram describing the behavior of lipid/surfactant systems in the region of high water content. Arrow (1) describes the demicellization experiment, arrow (2) the solubilization, and arrows (3) and (4) the partitioning experiments.

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