Amphipathic character

The amphipathic character of phospholipids suggests that the two regions of the molecule have incompatible solubihties however, in a solvent such as water, phos-phohpids organize themselves into a form that thermodynamically serves the solubihty requirements of both regions. A micelle (Figure 41 ) is such a structure the hydrophobic regions are shielded from water, while the hydrophilic polar groups are immersed in the aqueous environment. However, micelles are usually relatively small in size (eg, approximately 200 nm) and thus are hmited in their potential to form membranes. 

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. 

Subsequent conjugation of these acids with glycine or taurine forms the various bile salts, which have enhanced amphipathic character and are very effective detergents. 

The hydrophobic/hydrophilic nature of the different helical faces is clearly illustrated by this projection. The amphipathic character of a segment of a protein can be calculated by use of a Fourier series and calculation of a hydrophobic moment [18],... 

Structure/activity correlation, based on the above mentioned results, indicated that the terpenoid nucleus, the substitutions on the tetracyclic ring, and the aliphatic acidic side chain are important for the activity. In addition, the results of kinetic studies suggest that the mechanism of inhibition was competitive, and that binding to the catalytic site was due to the carboxylic group because when it was methylated, the activity disappeared. The amphipathic character of both triterpenes could explain the minor potency of masticadienoic acid in relation to schinol the ionised carboxylate group is probably at the surface whereas the tetracyclic nucleus must be in the hydrophobic zone of the bilayer [50]. 

Another example where the amphipathic feature of the molecules plays an important role is the case of some thermotropic ionic calamitic liquid crystals such as ditholium salts [64]. In spite of their rod-like shape, these compounds exhibit columnar mesophases with supramolecular organization similar to that of amphiphilic molecules. In this case, it is clear that the driving force for the supramolecular organization is the amphipathic character of the molecules... 

Cholesterol with a single hydroxy group possesses weak amphipathic character and leads to a specific orientation in the phospholipid structure. 

Due to the relatively weak adsorption of homopolymers at the L/L interface, and in some cases at the S/L interface, homopolymers are seldom used as emulsifiers or dispersants. For this purpose, the molecule is modified to include some specific units that have strong adsorption to the surface. A good example is partially hydrolysed poly (vinyl acetate), which is commercially referred to as poly(vinyl alcohol) (PVA). The polymer contains 4-12% acetate groups (i.e. 96-88% hydrolysed) and these groups impart an amphipathic character to the chain. The polymer becomes surface-active at the L/L interface and hence it can be used as an emulsifier. In addition, on a hydrophobic surface such as polystyrene, the acetate groups become preferentially adsorbed on the surface of the particles, thus leaving the PVA units dangling in solution as loops and tails . The latter provide the required steric stabilization. 

Finally Secondary structure calculations of spinach CF 6 (and of homologous sequences) predict an a-helix, starting at Val257. The helix and its strong amphipathic character, [8] and fig.l, is conserved in the 6-sequences of 12 species available (10 in table 2). 

Hollow structures can also be prepared by the self-assembly of stave or rod-like subunits into barrel or bundle-shaped frameworks. This is one of the most common strategies in nature for channel formation, where the rod-like molecules of the barrel-stave type are /S-sheets or a-helices of amphipathic character. The central cavity has hydrophilic properties, while the lipophilic area is oriented outward, in contact with the membrane of the cell. A natural example of this type of protein is a-hemolysin, a bacterium toxin formed by seven identical subunits that self-assemble when in contact with the cell membrane. This assembly gives rise to a mushroom-shaped structure, whose trunk is formed by a -barrel that is inserted into the cell membrane. The resulting channel has a diameter of 13 A at its narrowest point and can transport ions and other molecules. Other natural examples based on this model —but using an a-helix instead of f-sheets—include cholera toxin, potassium channels, porins, aquaporins, and the most powerful toxin known to date, botulinum neurotoxin (BoNT, known as Botox), broadly studied by Mental s group. ... 

Prior to investigating the interactions of the tridachiahydropyrones with the model membranes, the behaviour of the biomimetic precursors 44 and 43 in solution was studied to see whether their amphipathic character caused them to self aggregate... 

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