Polarity interior

The long reaction time needed for this apparendy simple neutralization is on account of the phase inversion that takes place, namely, upon dilution, the soap Hquid crystals are dispersed as micelles. Neutralization of the sodium ions with sulfuric acid then reverses the micelles. The reverse micelles have a polar interior and a hydrophobic exterior. They coalesce into oil droplets. 

A micelle is an assembly of amphiphilic molecules dispersed in water. Such molecules are made of two parts, a polar head group and a non-polar tail . The polar head is for example a carboxylic acid which can dissociate into ions (—COO- and H+) the non-polar tail is a saturated hydrocarbon chain. Since the non-polar parts are insoluble in a polar solvent, these molecules aggregate in water to form micelles which are microscopic droplets with a non-polar interior and polar groups at the water interface. This picture of micelles is probably an oversimplification, because water penetrates to some extent between the molecules it is however sufficient for an understanding of the special properties of micellar suspensions in photochemistry. 

Micellar Catalysis. Non-polar molecules such as aromatic hydrocarbons are practically insoluble in water. In a micellar suspension they concentrate in the non-polar interior of the micelles where they can reach relatively high concentrations, even though their overall concentration may be very low. The quantum yields of bimolecular reactions like photoadditions are therefore greatly increased in micellar suspensions. 

Orientational Effects. Solute molecules which have distinct polar and non-polar parts take up specific orientations in micelles, such that their non-polar end stays in the non-polar interior of the micelle, the polar group residing at the water interface. Photocycloaddition of these molecules will therefore lead preferentially to the head-to-head dimer, even if the head-to-... 

Figure 18-5 Schematic representation of (a) a membrane lipid, (b) a bilayer structure formed by lipid molecules in polar media the interior of the bilayer is nonpolar, and (c) a continuous bilayer structure (liposome) with polar interior and exterior...

Some 20 years ago it was observed that certain antibiotics could induce the movement of aqueous K+ ions into the mitochondria of cells, but not that of aqueous Na+ ions. These antibiotics, many of which are naturally occurring, are termed ionophores, i.e. neutral molecules which can mediate the transport of the essential groups IA and IIA cations across biological membranes.76 The essential features of an ionophore are a highly polar interior, a hydrophobic exterior and conformational flexibility. Many are cyclic peptides, the coordination properties of the cyclic molecules are considerably different to those of the linear peptides. These differences are outlined in Chapter 20.2. 

How does an ionophore transfer an ion across a membrane The ionophore binds the ion on one side of the membrane in its polar interior. It can then move across the membrane because its hydrophobic exterior interacts with the hydrophobic tails of the phospholipid. The ionophore then releases the ion on the other side of the membrane. This ion-transfer role is essential for normal cell function. This process is illustrated in Figure 3.8. 

Lipophilicity is a measure of a chemical s affinity for the lipid bilayer of biological membranes. The logarithm of the partition coefficient between water and 1-octanol (log Kow) is used as an indicator of a chemical s lipophilicity. The parabolic relationship between log P and effect can be used as evidence that there are limits to absorption for super-lipophilic compounds , and why these limits exist (20). Chemicals that have log P values greater than 6 tend to dissolve in the non-polar interior of a membrane inhibiting transport. 

In 1969, Wipf and Simon reported an outstanding potassium ionophore, valinomycin. Valinomycin is an example of a mobile ion carrier. It is a ring-shaped polypeptide that increases the permeability of a membrane to K . The ring has a hydrophobic exterior, made up of valine side chains, and a polar interior, where a single K can fit precisely (see Figure 8.30). In the electrode process, valinomycin transports potassium ions across the membrane by picking up on the solution side of the membrane and releasing it at the transducer surface. 

Hydrophobic interactions result because the association of nonpolar groups (e.g., methyl or phenyl) is energetically favored in aqueous or other polar solutions. In proteins, this association serves to bend and fold a molecule in a way that brings nonpolar R groups inside to the less polar interior polar R groups are oriented outside toward the more polar aqueous environment. 

A. 16.8 There is a large energy barrier to passing a polar headgroup through the non-polar interior of the lipid bilayer. 

Figure 8 Schematic cross-section of a chloroplast membrane showing an intrinsic protein spanning the membrane, with hydrophilic regions located at the membrane surfaces and a hydrophobic portion shaded) embedded within the non-polar interior of the lipid bilayer. Anderson 55 postulates that the chlorophyll molecules represented above with the hydrophobic portion of the chlorin ring shaded) are located as part of the boundary lipid of a chlorophyll-protein complex (Reproduced by permission from Nature, 1975, 253, 536)...

Expectedly, the outlook is quite different in case of non-ionic surfactants, like those belonging to poly(oxyethylene)alkyl or alkylphenyl ethers [61]. The hydrophilic part of these molecules can be in the form of chains longer than the corresponding hydrophobic part. An example is Triton X-100. As a result of the above, the structure of the polar interior of a reverse micelle of such amphiphiles does not resemble that of a reverse micelle of an ionic surfactant. The resemblance, indeed, is more with the interior of a normal micelle (of ionic surfactants). In reverse micelles of surfactants like Triton X-100, the polar interior can be invaded to an extent by a non-polar solvent like cyclohexane. 

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