Amphipathic structure

Effects of Surfactants on Solutions. A surfactant changes the properties of a solvent in 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 interfaces, orientation of the adsorbed surfactant ions or molecules, micelle formation in the bulk of the solution, and orientation of the surfactant ions or molecules in the micelles, which are caused by the amphipathic structure of a surfactant molecule. The magnitude of these effects depends to a large extent on the solubility balance of the molecule. An efficient surfactant is usually relatively insoluble as individual ions or molecules in the bulk of a solution, eg, 10-2 to 10-4 mol/L. 

As the o-helix has 3.6 residues per turn, hydrophobic side chains spaced at combinations of three and four residues apart are required to make an amphipathic structure. For helices in globular proteins, a variety of combinations of three-plus-four spacings of hydrophobic residues are observed (Chothia et al., 1981). This leads to a range of helix-helix packing angles and arrangements. However, to form more persistent fibrous coiled-coil... 

Diferent proteins variation in the degree of phosphorylation, glycosylation, hydrophobicity and amphipathic structures. 

Nearly all AMPs described in the literature are membrane active. They can be grouped into two main groups, the cyclic peptides due to the presence of one or more disulfide bridges, and the linear peptides with a potential to form amphipathic structures. 

A surface-active agent (or surfactant) is a substance that lowers the surface or interfacial tension of the medium in which it is dissolved. Surfactants have a characteristic molecular structure consisting of hydrophobic and hydrophilic groups. This is known as an amphipathic structure, and causes not only concentration of the surfactant at the surface and reduction of the surface tension of the solvent, but also orientation of the molecule at the surface with its hydrophilic group in the aqueous phase and its hydrophobic group oriented away... 

Figure 18.1 Models for different modes of peptide-lipid interaction of membrane-active peptides. The peptide remains unstructured in solution and acquires an amphipathic structure in the presence of a membrane. The hydrophobic face of the amphipathic peptide binds to the membrane, as represented by the grayscale. At low concentration, the peptide lies on the surface. At higher peptide concentrations the membrane becomes disrupted, either by the formation of transmembrane pores or by destabilization via the "carpet mechanism." In the "barrel-stave pore" the pore consists of peptides alone, whereas in the "toroidal wormhole pore" negatively charged lipids also line the pore, counteracting the electrostatic repulsion between the positively charged peptides. The peptide may also act as a detergent and break up the membrane to form small aggregates. Peptides can also induce inverted micelle structures in the membrane.

Membrane-active antimicrobial peptides (AMPs), or host-defense peptides, kill microorganisms by permeabilizing their membrane. They often form amphipathic structures upon binding to lipid membranes. At low peptide concentrations they are normally in a monomeric surface-bound S-state, and at higher concentrations they may self-assemble and insert into the bilayer in a functionally active T- or I-state (see Figures 18.1 and 18.3). In our previous 19F NMR investigations we have compared three such AMPs, which are described below. 

The classification of surfactants into denaturing and nondenaturing poses the question as to the origin of the distinction between the two types of behavior. Since all surfactants have an amphipathic structure, why do synthetic ionic surfactants denature proteins while natural ionics and synthetic nonionics do not The answer must clearly... 

Proteins are biopolymers that are encountered in many applications, such as food emulsions, hair conditioners, photographic emulsions, and various medical diagnostic products. Many of these applications are frequently based on the unique surface activity of the proteins, which is reflected in functional properties such as foaming, emulsification, and gelling. The proteins are composed of polymeric chains containing many hydrophobic and hydrophilic domains, often giving the molecules an amphipathic structure somewhat similar to that of polymeric surfactants. 

Fig. 4. Helical wheel presentation of the presumed structure of segment 167-184 of apolipoprotein A-I. In the Apo A-I-I ano mutant, Arg is substituted with cysteine. This substitution leads to the disappearance of an ion pair, Glu -Arg with a resulting loss in amphipathic structure and lipid binding capacity [11]...

Surfactants are the ultimate example of an amphipathic structure. They combine a long-chain alkyl group, which is hydrophobic, with an ionic group (sometimes polar group). 

A vast number of reports show that small molecules or amphiphilic peptides may assemble into nanofibers, vesicles, and gels. Organic foldamers which are able to do so are very attractive for their potential applications in materials science. Motivated by the observation of the liquid crystalhnity of globally amphiphilic )8-peptides, Gellman and coworkers designed j8-peptide iso-A which possesses a globally amphipathic structure when folded, and -peptide A which does not. It was anticipated that iso-A but not A would form a liquid-crystalline (LC) phase... 

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