Molecules amphipathic

Such solubilized protein-detergent complexes are the starting material for purification and crystallization. For some proteins, the addition of small amphipathic molecules to the detergent-solubilized protein promotes crystallization, probably by facilitating proper packing interactions between the molecules in all three dimensions in a crystal (Figure 12.2b). Therefore, many different amphipathic molecules are added in separate crystallization experiments until, by trial and error, the correct one is found. 

Further addition of fatty acid eventually results in the formation of micelles. Micelles formed from an amphipathic lipid in water position the hydrophobic tails in the center of the lipid aggregation with the polar head groups facing outward. Amphipathic molecules that form micelles are characterized by a unique critical micelle concentration, or CMC. Below the CMC, individual lipid molecules predominate. Nearly all the lipid added above the CMC, however, spontaneously forms micelles. Micelles are the preferred form of aggregation in water for detergents and soaps. Some typical CMC values are listed in Figure 9.3. 

CTC, used extensively to monitor calcium release in both whole cells and isolated organelles (28-33), is an amphipathic molecule that easily passes through cell membranes (see Figure 1). The fluorescence of this probe is enhanced more than fiftyfold by binding of calcium when the dye is intercalated into biological membranes. 

If amphipathic molecules are mixed with water, three different types of lipid structure are possible the type of aggregate formed depends on the physicochemical conditions and the lipid species involved. The thermodynamic parameter involved is the hydrophobic interaction. 

Although the notion of monomolecular surface layers is of fundamental importance to all phases of surface science, surfactant monolayers at the aqueous surface are so unique as virtually to constitute a special state of matter. For the many types of amphipathic molecules that meet the simple requirements for monolayer formation it is possible, using quite simple but elegant techniques over a century old, to obtain quantitative information on intermolecular forces and, furthermore, to manipulate them at will. The special driving force for self-assembly of surfactant molecules as monolayers, micelles, vesicles, or cell membranes (Fendler, 1982) when brought into contact with water is the hydrophobic effect. 

Cells are bounded by proteins arrayed in lipid bilayers 21 Amphipathic molecules can form bilayered lamellar structures spontaneously if they have an appropriate geometry 22... 

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... 

Commercially, the most important non-ionic surfactants (APEOs and AEOs) are amphipathic molecules consisting of a hydrophilic (ethylene oxide chains of various length) and a hydrophobic (alkyl phenols, fatty acids, long chain linear alcohols, etc.) part. The polyethoxylated... 

There are a number of structurally distinct classes of amphipathic molecules, which have various kinds of structures. The simplest amphipathic molecules have a straight chain of carbon atoms, usually 8-18 in number, to which is attached a polar group that may be anionic, cationic, zwitterionic or nonionic. 

At the water-air interface hydrophilic groups are oriented toward the water, hydro-phobic groups are oriented toward air. At solid-water interfaces, the orientation depends on the relative affinities for water and for the solid surface. The hydrophilic groups of amphipathic molecules may - if the hydrophobic tendency is relatively small - interact coordinatively with the functional groups of the solid surface (Ulrich et al., 1988) (see Fig. 4.10). 

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. 

Molecules with sharply demarcated regions of hydrophilic and hydrophobic character are known as amphipathic molecules. Soaps provide an example. These form a variety of interesting structures. Such molecules may be thought of as schizophrenic, simultaneously struggling to satisfy two opposing natures. In water, amphipathic molecules will act so as to expose their hydrophilic structures to the aqueous environment while trying to find ways to hide their hydrophobic structures from it. One possibility among several is to form bimolecular layers or, more simply, bilayers. 

We have encountered examples of simple lipid bilayers earlier. These bilayers are composed largely of amphipathic molecules. In water, they have their hydrophobic parts occupying the center of the bilayer and their hydrophilic parts occupying the bilayer surface. Such bilayers form a continuous and essential structural feature of virtually all biological membranes. We need to distinguish between that layer which faces out from the cell and is in contact with the external environment, the exoplasmic leaflet, and that which faces in and is in contact with the cellular contents, the cytoplasmic leaflet. As we shall see, these two aspects of the lipid bilayer are quite distinct. 

The separation of oil and water (B) can be prevented by adding a strongly amphipathic substance. During shaking, a more or less stable emulsion then forms, in which the surface of the oil drops is occupied by amphipathic molecules that provide it with polar properties externally. The emulsification of fats in food by bile acids and phospholipids is a vital precondition for the digestion of fats (see p.314). 

Membrane lipids are strongly amphipathic molecules with a polar hydrophilic head group and an apolar hydrophobic tail. in membranes, they are primarily held together by the hydrophobic effect (see p. 28) and weak Van der Waals forces, and are therefore mobile relative to each other. This gives membranes a more or less fluid quality. 

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