Hydrophobic exterior

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. 

Fig. 20. A view looking parallel to the sheet formed by the hydrogen bonding in Et2Si(OH)2, showing the hydrophilic interior of the sheet and the hydrophobic exterior, with hydrogen atoms omitted for clarity. Drawn using coordinates taken from the Cambridge Crystallographic Database.

Recently reported synthetic macrocyclic molecules with hydrophilic cavities containing multiple binding atoms and with hydrophobic exteriors make possible controlled studies of selective cation complexa-tion. Certain of these cation-ligand systems appear to mimic biological systems which have remarkable element specificities. Such cation-ligand systems may be considered as models for the study of this unusual property of living systems. 

This generalized structure was mimicked by a very simplified artificial molecule 1. The hydrophilic core part 2 was substituted simply by an oligoether carboxylate anion. The carboxylate may act as the polar ionic head group outside the membrane and the ether part of the molecule may be located in the interior part of the membrane to make an ion-conducting pathway. The molecular lengths were adjusted to fit the lipid monolayer in an extended or a helical conformation, with n being 2 or 3 in 1. The hydrophobic exterior was substituted by dioctadecyldimethyl-ammonium cation, which was ion-paired with the carboxylate. 

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. 

Synthetic peptides such as (Leu-Ser-Leu-Glu)12 act as channels in artificial membranes, and reproduce the hydrophobic exterior (from leucine side chains) and hydrophilic interior (from serine hydroxyl groups) characteristic of natural channel formers. 

These stoichiometries allow the 8-membered pseudo-chelate rings (B in Figure 6) to be preserved and provide the complex unit with a very hydrophobic exterior. 

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. 

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. 

The N-bonded silanetriol (2,4,6-Me3C6H2)N(SiMe3)Si(OH)3 [26] organizes itself in a tubular form which is made up of four linear columns. Further, these columns are displaced with respect to each other by a 90° rotational relationship. As a result of this arrangement there is an interesting formation of silanetriol tubes in the crystal which contain a hydrophilic interior and a hydrophobic exterior. 

Structure of valinomycin (a) and its complex with K" " (b). Valinomycin, which consists of three identical fragments of D-valyl-L-lactyl-L-valyl-D-a-hydroxyisovaleric acid (D-Val-L-Lac-L-Val-D-Hyi), is a mobile or channel-forming ionophore and an uncoupler of oxidative phosphorylation. Note the hydrophobic exterior and the hydrophilic interior of the complex. [Structure (b) is reproduced, with permission, from B. C. Pressman Biological application of ionophores. Amu. Rev. Biochem. 45,501 (1976). 1976 by Annual Reviews Inc.]... 

As families containing the double displacement glycohydrolases can also contain transglycosylases, so CE 1, which contains double-displacement carboxyl esterases, also contains transacylases. Trehalose [a-D-Glcp-(l<->l)-a-D-Glcp] esterified on 06 with mycolic acid plays a role in maintaining the hydrophobic exteriors of the tubercle bacterium Mycobacterium tuberculosis, and the crystal structure of three related mycoloyl transesterases are... 

Various lipophilic natural product antibiotics that have a hydrophobic exterior and a shielded hydrophilic interior capable of binding and transporting ions are known as ionophores. The three ionophores that are most commonly used as probes of ion transport across mitochondrial membranes are the Streptomyces cyclic dodecadepsipeptide valinomycin (57), gramicidin pentadecapeptides (e.g., valine-gramicidin A,... 

Depressants The purpose of depressants is to inhibit or retard the floatation of a desired type of particle. Its mode of action is to either inhibit the collector molecule from adsorbing onto the particle surface or to mask the adsorbed collector species from the bulk solution so that the particle does not display a hydrophobic exterior surface in contact with bulk solution. Examples of depressants include starch, tannin, silicates, aluminum salts, and dextrin. 

Carrier-facilitated transport - Valinomycin, an antibiotic, is an example of a carrier-facilitated transport system. It contains a hydrophobic exterior for interacting with the hydrophobic portion of the membrane s lipid bilayer and an interior designed specifically to accommodate a potassium ion. It transports by the mechanism depicted in Figure 10.20b. 

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