Amphipathic chemicals

The lipids found in biological systems are either hydrophobic (containing only nonpolar groups) or amphipathic, which means they possess both polar and nonpolar groups. The hydrophobic nature of lipid molecules allows membranes to act as effective barriers to more polar molecules. In this chapter, we discuss the chemical and physical properties of the various classes of lipid molecules. The following chapter considers membranes, whose properties depend intimately on their lipid constituents. 

The impact of salt concentration on the formation of micelles has been reported and is in apparent accord with the interfacial tension model discussed in Sect. 4.1, where the CMC is lowered by the addition of simple electrolytes [ 19,65, 280,282]. The existence of a micellar phase in solution is important not only insofar as it describes the behavior of amphipathic organic chemicals in solution, but the existence of a nonpolar pseudophase can enhance the solubility of other hydrophobic chemicals in solution as they partition into the hydrophobic interior of the micelle. A general expression for the solubility enhancement of a solute by surfactants has been given by Kile and Chiou [253] in terms of the concentrations of monomers and micelles and the corresponding solute partition coefficients, giving... 

Figure 1 Chemical structures of some amphipathic weak bases that have been loaded and stabilized in liposomes using trialkylammonium salts of polyanionic trapping agents in our lab. (A) Doxorubicin, (B) epirubicin, (C) vinorelbine, (D) vincristine, (E) vinblastine, (E) topotecan, (G) irinotecan, (H) swainsonine, (I) 2-diethylami-noethyl-ellipticinium, (J) 6-(3-aminopropyl)ellipticine, and (K) LAQ824.

A special group of lipids that possess both hydrophilic and hydrophobic (lipophilic) parts are termed as amphiphiles or amphipathics and are also referred to as surfactants. They adsorb at surfaces or interfaces and change the interfacial free energy associated with the building of an interface. A surfactant molecule consists of two distinct chemical groups (i) the head which is hydrophilic (water-loving) and (ii) the tail which is hydrophobic (water-fearing). 

The colloidal structures we examine in this chapter are formed as a result of physical interactions among amphipathic molecules, rather than by covalent bonding. This sort of physical association has been recognized for a long time, although contemporary students may be relatively (or totally ) unaware of it. It is interesting to note that in the early days of polymer chemistry, macromolecules were believed to be physically associated rather than covalently bonded structures. The birth of modern polymer chemistry can be traced to the acceptance of the covalent character of these substances. Associated structures do exist, however, and we see by the end of this chapter that their investigation is a very lively area of chemical research. 

The chemical is removed before it can properly reach the cytoplasm or important organelles. The substrates for this transporter are structurally diverse but tend to be organic, weakly basic (cationic), or uncharged hydrophobic or amphipathic substances. Thus, the chemical diffuses into the cell and is then pumped out. Substrates include anions conjugated with glutathione (GSH), glucuronic acid, and sulfate. 

The Alkylglycosides (AGs) and Sucrose esters of fatty acids (SEFAs) are families of nonionic glycosurfactants that have been used for their ability to gently extract membrane proteins with a minimal loss of functionality. These compounds can be synthesized and purified economically, with a worldwide production of thousands of tons per year. Chemically, AGs and SEFAs are a group of uncharged amphipathic compounds that consist of an aliphatic hydrocarbon chain attached to a sugar moiety. Certain AGs and SEFAs such as dodecanoyl sucrose have enjoyed widespread use as food-grade emulsifiers and in cosmetic preparations. 

Amphipathic peptides may stick to amphipathic surfaces, i.e., cells, media components, or chemical instruments, thus making their quantitative analysis rather difficult (35). 

Flexible macromolecules, such as proteins, and small molecules, such as surfactants, are amphipathic and may form a layer at the oil-water interface. These molecules may also partly stabilize emulsions by forming a physical barrier to close contact, thereby reducing the attractive van der Waals forces to ineffective levels (Dalgleish, 1989). Repulsion can arise in either of two ways and physico-chemical calculations are available for both mechanisms in oil-in-water systems. Either the approaching protein-coated particles will tend to compress or alternatively interpenetrate the adsorbed protein layer on adjacent particles. The optimum structure of the stabilizing protein will be dealt with in the section on protein as an ingredient. 

Figure 1 Physical and chemical stimuli affecting the gating of bacterial MS channels. (A) The structure of the pentameric MscL channel (left) and a channel monomer (right) from Mycobacterium tuberculosis according to the 3-D structural model of a closed channel (7). MscL is activated by membrane stretch, amphipaths (e.g., lysophopholipids, chlorpromazine, and trinitrophenol) and parabens. The channel activity is inhibited by Gd + and static magnetic fields (SMF) and is modulated by temperature and intracellular pH (3). (B) The structure of the MscS heptamer (left) and the channel monomer (right) from E. coli based on the 3-D structural model of MscS (8) most likely depicting an inactive or desensitized functional state of the channel (3). MscS is activated by membrane stretch, amphipaths, and parabens and is modulated by voltage. The activity of the channel is inhibited by Gd + and high hydrostatic pressure (HHP) (3). The arrows point at membrane structures (i.e., channel protein and/or lipid bilayer) affected by the specific stimuli.

Lipids are water-insoluble that are either hydrophobic (nonpolar) or amphipathic (polar and nonpolar regions). Lipids are in many ways the most diverse of the biological macromolecules, since they are something of a rag-tag bunch of leftovers. Lipids are pretty much everything in the cell that isn t very water soluble, and chemically they don t have a great deal in common with one another. The best known lipids are probably the fatty acids, so that is where we shall start. 

Liposomes are microscopic vesicles consisting of one or more phospholipid membranes surrounding a discrete water compartment. The lipid layer is composed of amphipathic phospholipids whose hydrophobic tails associate, while the polar hydrophihc heads align toward the bulk of the water phase. A variety of hposomes with unique physical and chemical structure can be manufactured by altering non-polar and polar groups. Excellent reviews have been recently pubhshed... 

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