Amphipathic solutes

FIG. 7.14 Three types of variation of 7 with c for aqueous solutions (1) simple organic solutes, (2) simple electrolytes, and (3) amphipathic solutes. 

Curve 3 in Figure 7.14 was presented as a typical illustration of the way surface tension y varies with concentration for an amphipathic solute in aqueous solution. In discussing that... 

FIGURE 2.22 Schematic representation of the three types of surface energy variation with solute concentration. Curve 1 is for typical organic solutes of small molecules curve 2 is for simple electrolytes and curve 3 is for amphipathic solutes. 

Naturally hydrophobic minerals such as graphite and talc are common gangue minerals found in sulfide ores and are difficult to separate due to their tendency to float together with valuable sulfide minerals. Despite the relatively successful use of the polysaccharide group of chemicals (e.g., dextrin, guar gum, and carboxymethyl cellulose) as flotation depressants for these naturally hydrophobic minerals, the nature of the adsorption processes remains in debate. Consequently, the adsorption of amphipathic solutes at naturally hydrophobic minerals such as coal and graphite, talc, and sulfur is of interest to many researchers, and a substantial amount of research has been discussed. 

The typical soap film consists of two surfaces of amphipathic solute ions plus a distribution of solute ions in the water between the surfaces (Fig. 2.4(a)). It will be recalled that these amphipathic ions consist of two dissimilar parts. One part is hydrophilic with an affinity for water and tends to become surrounded by water molecules. This is the polar carboxyl head . The other part... 

Effects of Surfactants on Solutions. A surfactant changes the properties of a solvent ia which it is dissolved to a much greater extent than is expected from its concentration effects. This marked effect is the result of adsorption at the solution s iaterfaces, orientation of the adsorbed surfactant ions or molecules, micelle formation ia the bulk of the solution, and orientation of the surfactant ions or molecules ia the micelles, which are caused by the amphipathic stmcture of a surfactant molecule. The magnitude of these effects depends to a large extent on the solubiUty balance of the molecule. An efficient surfactant is usually relatively iasoluble as iadividual ions or molecules ia the bulk of a solution, eg, 10 to mol/L. 

Amphipathic lipids spontaneously form a variety of structures when added to aqueous solution. All these structures form in ways that minimize contact between the hydrophobic lipid chains and the aqueous milieu. For example, when small amounts of a fatty acid are added to an aqueous solution, a mono-layer is formed at the air-water interface, with the polar head groups in contact with the water surface and the hydrophobic tails in contact with the air (Figure 9.2). Few lipid molecules are found as monomers in solution. 

Interestingly, certain other pore-forming toxins possess helix-bundle motifs that may participate in channel formation, in a manner similar to that proposed for colicin la. For example, the S-endotoxui produced by Bacillus thuringiensis is toxic to Coleoptera insects (beetles) and is composed of three domains, including a seven-helix bundle, a three-sheet domain, and a /3-sandwich. In the seven-helix bundle, helix 5 is highly hydrophobic, and the other six helices are amphipathic. In solution (Figure 10.32), the six amphipathic... 

Phospholipids are the most important of these liposomal constituents. Being the major component of cell membranes, phospholipids are composed of a hydrophobic, fatty acid tail, and a hydrophilic head group. The amphipathic nature of these molecules is the primary force that drives the spontaneous formation of bilayers in aqueous solution and holds the vesicles together. 

Meso- and (+ )-azobis[6-(6-cyanododecanoic acid)] were synthesized by Porter et al. (1983) as an amphipathic free radical initiator that could deliver the radical center to a bilayer structure controllably for the study of free radical processes in membranes. The decomposition pathways of the diazenes are illustrated in Fig. 36. When the initiator was decomposed in a DPPC multilamellar vesicle matrix, the diazenes showed stereo-retention yielding unprecedented diastereomeric excesses, as high as 70%, in the recombination of the radicals to form meso- and (+ )-succinodinitriles (Brittain et al., 1984). When the methyl esters of the diazene surfactants were decomposed in a chlorobenzene solution, poor diastereoselectivity was observed, diastereomeric excesses of 2.6% and 7.4% for meso- and ( )-isomers respectively, which is typical of free radical processes in isotropic media (Greene et al, 1970). 

Transmembrane proteins are adapted to an environment comprised of two distinct aqueous media and the highly complex membrane phase [1], Handling them in aqueous solution requires their complexation by amphipathic molecules that screen their hydrophobic transmembrane surface from contact with water. Traditionally, this role is fulfilled by detergents. Detergents are small surfactants that cooperatively assemble at the transmembrane surface of the protein at concentrations close to their critical micellar... 

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... 

The cahnodulin-binding peptides assume random coil structures in solution, but in the presence of calmodulin they form amphipathic or amphiphilic (containing both polar and nonpolar residues) helices. All of these peptides have nanomolar (very high) affinities for calmodulin. Table 6.9 shows the primary amino acid sequence of some of the cahnoduUn-binding peptides, and it is informative to compare them as they are discussed in the following material. 

It follows from study of the kinetics of transfer of ions across the phase boundary between two immiscible electrolyte solutions (see chapter 9) that ion-exchanger ions, where the ion is as nearly as possible symmetrically surrounded by hydrophobic groups on all sides, are especially suitable. Amphiphilic (amphipathic) substances, in whose molecules the hydrophobic part is separated from the hydrophilic part, are less suitable because they have a tendency to become adsorbed on the membrane/water phase boundary, thus retarding ion transfer across this boundary. 

Although we started out this chapter by discussing insoluble monolayers, it is evident that we have slipped into examples for which soluble amphipathics are being considered. In the next section we examine the thermodynamics of adsorption from solution. 

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