Micelle structural features

The practical development of plant sterol drugs as cholesterol-lowering agents will depend both on structural features of the sterols themselves and on the form of the administered agent. For example, the unsaturated sterol sitosterol is poorly absorbed in the human intestine, whereas sitostanol, the saturated analog, is almost totally unabsorbable. In addition, there is evidence that plant sterols administered in a soluble, micellar form (see page 261 for a description of micelles) are more effective in blocking cholesterol absorption than plant sterols administered in a solid, crystalline form. 

The overall objective of this chapter is to review the fundamental issues involved in the transport of macromolecules in hydrophilic media made of synthetic or naturally occurring uncharged polymers with nanometer-scale pore structure when an electric field is applied. The physical and chemical properties and structural features of hydrophilic polymeric materials will be considered first. Although the emphasis will be on classical polymeric gels, discussion of polymeric solutions and nonclassical gels made of, for example, un-cross-linked macromolecular units such as linear polymers and micelles will also be considered in light of recent interest in these materials for a number of applications... 

In some polysaccharides, the reducing terminal is linked, through a phosphoric diester linkage, to O-1 of a 2,3-di-6 -acylglycerol. This structural feature has been demonstrated for some capsular polysaccharides from E. coli and Neisseria species, - but is probably more common than that. Non-covalent linkage between the lipid part and the cell membrane may explain why extracellular polysaccharides often occur as capsules, and the high (apparent) molecular weight observed for these polysaccharides may be due to micelle formation in aqueous solution. 

Figure 4 Structural features and diversity of bacteriocins. Solution NMR structures are shown for (a) the lantibiotic actagardine (PDB code 1AJ1) the pedocin-like bacteriocins (b) curvacin A (PDB code 2A2B) and (c) leucocin A (PDB code 1CW6) (d) the A and B chain of the two-chain bacteriocin lactococcin G in DPC micelles (PDB codes 2JPJ and 2JPK) and the cyclic bacteriocins (e) subtilosin (PDB code 1AJ1) and (f) AS-48 (PDB code 1E68). Disulphide bonds and lanthionine linkages are shown in ball-and-stick. Chain termini are labelled N and C and the cyclisation points on the cyclic peptides are labelled with residue numbers.

The highest resolution cryo-EM data has thus far been obtained for proteins present in 2-D crystals (a technique known as electron crystallography). This technique measures the structural features of membrane proteins reconstituted into 2-D crystals in the presence of lipid bilayers. In contrast to the 3-D crystals used in XRC, in which proteins are solubilized in detergent micelles that can disrupt crystal formation and reduce crystal quality, formation of 2-D crystals forces the membrane proteins to pack within a lipid bilayer, thus restoring their native environment. Another advantage of 2-D crystallization is that it requires very small amounts of protein (as opposed to NMR, which requires milligram quantities). 

A mechanistic model has been proposed for PPIase catalysis in which a twisted peptide bond, a structure involving substrate strain, is stabilized by noncovalent interaction with the enzyme [156], However, catalytic antibodies generated to transition state analogs containing twisted carbonyl moieties do not show a PPIase-like catalytic efficiency [157,158], Consequently, small detergent micelles and phosphatidylcholine membranes are able to catalyze CTI of typical PPIase substrates in a manner reminiscent of that observed for catalytic antibodies [159]. Apparently, sequestration of hydrophobic substrates within the enzyme may account for both a small portion of the catalytic power of FKBP and the acceleration of CTI by catalytic antibodies. Despite overall amino acid sequence dissimilarity the structural features making up the active sites of prototypic enzymes such as Cypl8 and ParlO proved to be similar (Fig. 10.6). 

Physicochemical studies of the synthetic peptide spanning residues 106-126 of human PrP led to similar conclusions. PrP106-126 consists of an N-terminal polar head (KTNMKHM-) followed by a long hydrophobic tail (-AGAAAAGAWGGLG) and its structural features are markedly influenced by solvent composition, ionic strength, and pH. CD spectroscopy showed that the peptide adopts a random coil conformation in deionized water, a combination of random coil and p sheet in phosphate buffer pH 7.0, a predominantly P-sheet structure in phosphate buffer pH 5.0, and an a-helical structure in trifluoroethanol or in the presence of micelles formed by a 5% SDS solution. Notably, the P-sheet conformation is extremely stable, because it is not affected by trifluoroethanol if the peptide was previously suspended in phosphate buffer pH 5.0 (De Gioia et al., 1994). 

The above studies have demonstrated that various photosensitizers, in conjunction with many available experimental techniques, can be used to probe different regions of the colloidal model membranes systems. Careful choice of sensitizers is important in determining different regions of the micelles, reverse micelles, or liposomes, and their different dynamic and structural features. 

We saw that a modification of the shell-forming segments in the block copolymers affects the internalization of the micelles inside the cells. On the other hand, modification of the core-forming segments of the block copolymers induces a change in the structural features of the micelles. Because a stable core is a prerequisite for the micelles maintaining of their structure, the micelle structure can be dissociated if... 

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