Solubility amphipathic molecules

You will recognize that this is an amphipathic molecule, one with geographically distinct regions of hydrophilic and hydrophobic character, specifically a soap, with clearly separated hydrophobic, CH3—(CH2)n—, and hydrophilic parts, —COO . As a consequence of their amphipathic character, these molecules may be soluble in water and can form interesting and unusual structures once dissolved. 

The break in curve 3 in Figure 7.14 is characteristic of this type of plot for soluble amphipathic molecules. Note that it appears in the experimental curves of Figure 7.15 also. The break is understood to indicate the threshold of micelle formation (see Chapter 1, Section 1.3a), known as the critical micelle concentration (see Chapter 8). We do not discuss this phenomenon any further since the next chapter is devoted entirely to micelles and related structures. 

The digestion of triacylglycerols in adult nonruminant mammals has been described as initiated in the mouth by hngual lipase released in the sahva at the base of the tongue (52). Up to 6% of the fatty acids are hydrolyzed and initiate emulsion formation in the stomach. The digesta (called chyme at this location) is released from the stomach slowly into the duodenum to ensure complete mixing with the bile salts and emulsification. Lipolysis occurs by association of pancreatic lipase and co-lipase at the surface of the bile salt-stabihzed emulsion. Amphipathic molecules (fatty acids, sn-2 monoacylglycerols, and lysolecithins) are produced and associate with the bile salts to form water-soluble micelles from which absorption occurs. 

Bile salts act as "detergents" in nature to maintain insoluble (fat-soluble) compounds in water solution. The bile salts form mixed micelles that consist of amphipathic bile salt molecules surrounding the lipophilic (fal-soJublc) molecules. The hydrophilic ends of the amphipathic molecules face outward, forming a lipophilic environment in the interior of the micelle. Bile salt molecules contain acid groups, such as carboxyl and sulfonyl groups, that usually are ionized under physiological conditions. 

Amphipathic Molecules Water-Soluble and Fat-Soluble Nutrients... 

Modification of the surface properties by introducing additional hydrophobic residues via mixed carboxylic acid anhydrides of fatty acids and oxa derivatives [43] or amphipathic molecules like polyethylene glycol [44] has been generally used to effect the stability or reaction rates of enzymes in non-polar organic media. In some cases the enzyme became insoluble in aqueous systems [43] or soluble and active in organic solvents [45,46]. 

Detergents are amphipathic molecules which have enhanced solubility and biodegradability properties compared to soaps. Instead of having a sodium salt in the polar portion of the molecule, other ionic and polar groups are used giving rise to what are called "cationic", "anionic" and "nonionic" detergents. 

Cholesterol, a common steroid, is found in the membranes of most animal cells. It is an amphipathic molecule and is readily soluble in the hydrophobic re-... 

Figure 2.2 portrays the reason why amphipathic molecules are such effective steric stabilizers. It would be expected that the polymer that was nominally insoluble in the dispersion medium would attach itself to the particle. This attachment to a colloidal particle could occur by physical adsorption on to a preformed colloidal particle, or by physical and/or chemical adsorption on to (or even incorporation into) a growing particle. Whatever the mechanism of attachment, the nominally insoluble polymer serves to anchor the soluble moieties to the colloidal particles. Such polymer is accordingly referred to as the anchor polymer. The role of the soluble polymer is to impart steric stabilization and, for this reason, such chains are termed the stabilizing moieties. 

The soluble polyester synthase converts into an amphipathic molecule upon availability of substrate and subsequent hydrophobic polyester chain elongation [15]. This leads to self-assembly of so-called PHA granules with the hydrophobic PHA in the core and the active polyester synthases at the surface, representing the water-polyester interface. Analysis of the granule-associated polyester synthase from R. eutropha showed about 40-fold increased enzyme activity, as compared with the soluble enzyme [46]. This data suggests that interfacial activation occurred and a lid-like structure as found in lipases and also found in the R. eutropha polyester synthase model may also play a role in polyester synthases [37]. 

The formation of particles in polymer colloids ordinarily is accomplished by the free radical polymerization of an organic monomer in a liquid which is a non-solvent (diluent) for the polymer. A surface active material, such as a soap or other amphipathic molecule, is usually added to stabilize the colloidal particles as they are formed. The particle size distribution varies from very narrow to extremely broad depending primarily upon the solubility of the monomer in the diluent, the stabilizer concentration and the ionic strength. 

The carboxylate (COO ) is normally water soluble but the long hydrocarbon chain repels water (hydrophobic) rendering the entire molecule water insoluble. Because fatty acids have both hydro-phobic and hydrophilic regions they are referred to as amphipathic molecules (amphi = both). Resultant from the amphipathic property, fatty acids in a polar environment organize themselves into micelles in which the hydrocarbon chains are directed towards the interior of the structure with the carboxylate groups on the outside in contact with the polar solvent. The structure is held together by weak non-covalent attractive forces called van der Waals forces between the hydrocarbon chains. 

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