Micelle structural models

Home, D.S. (2006). Casein micelle structure models and muddles. Current Opinion in Colloid and Interface Science, 11, 148-153. 

In a class of reahstic lattice models, hydrocarbon chains are placed on a diamond lattice in order to imitate the zigzag structure of the carbon backbones and the trans and gauche bonds. Such models have been used early on to study micelle structures [104], monolayers [105], and bilayers [106]. Levine and coworkers have introduced an even more sophisticated model, which allows one to consider unsaturated C=C bonds and stiffer molecules such as cholesterol a monomer occupies several lattice sites on a cubic lattice, the saturated bonds between monomers are taken from a given set of allowed bonds with length /5, and torsional potentials are introduced to distinguish between trans and "gauche conformations [107,108]. 

According to the other kinetic model proposed for the soapless emulsion process, the growing macroradicals may also form micelle structures at earlier polymerization times since they have both a hydrophilic end coming from the initiator and a hydrophobic chain [74]. 

Neuman RD, Ibrahim TH (1999) Novel structural model of reversed micelles The open water-channel model. Langmuir 15 10-12... 

Bachmann, P. A., Luisi, P. L., and Lang, J. (1992). Autocatalytic self-replication of micelles as models for prebiotic structures. Nature, 357, 57-9. 

Micelle structure. Various models of casein micelle structure have been proposed and refined over the past 40 years. Progress has been reviewed regularly, including Schmidt (1982), McMahon and Brown (1984), Farrell (1988), Holt (1992, 1994), Rollema (1992) and Visser (1992). 

Gamier, J. 1973. Models of casein micelle structure. Neth. Milk Dairy J. 27, 240-248. 

Slattery, C. W. 1976. Review Casein micelle structure an examination of models. J. Dairy Sci. 59, 1547-1556. 

Two models for micelle structure were identiLed in their studies (Xing and Mattice, 1998). In analogy with the structural models for systems involving low molecular weight surfactants, two kinds of aggregates of spherical shape can be pictured, depending on how the solubilizates are located inside the block copolymer micelles. Solubilization takes places in two steps in the Xing and Mattice s simulations (1998). 

Another controversial and evolving idea concerning casein micelle structure is the concept of the submicelle. That there is some substructure to the micelle can hardly be denied, because all of the appropriate techniques have revealed some inhomogeneities over distances of 5-20 nm. Proponents of submicellar models of casein micelle structure interpret this evidence in terms of spherical particles of casein, the submicelles, joined together, possibly, by the calcium... 

Historically, ideas of casein micelle structure and stability have evolved in tandem. In the earlier literature, discussions of micellar stability drew on the classical ideas of the stability of hydrophobic colloids. More recently, the hairy micelle model has focused attention more on the hydrophilic nature of the micelle and steric stabilization mechanisms. According to the hairy micelle model, the C-terminal macropeptides of some of the K-casein project from the surface of the micelle to form a hydrophilic and negatively charged diffuse outer layer, which causes the micelles to repel one another on close approach. Aggregation of micelles can only occur when the hairs are removed enzymatically, e.g., by chymosin (EC 3.4.23.4) in the renneting of milk, or when the micelle structure is so disrupted that the hairy layer is destroyed, e.g., by heating or acidification, or when the dispersion medium becomes a poor solvent for the hairs, e.g., by addition of ethanol. 

This sequence of events has its parallel in the model of casein micelle structure proposed by Schmidt (1982) and Walstra (1990) in which small calcium phosphate particles link together discrete spherical protein subunits. 

Figure 21.8 Structural models for lamellar PEP-fr-PEO-b-PHMA block copolymer-aluminosilicate composite morphologies with a small PEP block. In the absence of the PEP block, the PEO (black) and PHMA (light grey) chains stretch into their respective domains while the aluminosilicate particles (white) partition into the hydrophilic PEO domain (a). Possible domain structures discussed in the text are illustrated as follows In the balls-in-lamellae structure the small PEP block (dark grey) forms round micellar domains (b). Dimple structure with PEP micelles at the PHMA/PEO-aluminosilicate interface (c). In the pillared-lamellae structure the PEP domains form pillars spanning across the PEO-aluminosilicate domain (d).37 (Reprinted with permission fiomG.E. S.Toombesetal., Chem. Mater. 2008,20,3278-3287. Copyright 2008 American Chemical Society.)...

Bachman PA, Luisi PL, Lang J (1992) Autocatalytic self-replicating micelles as models for prebiotic structures. Nature 357 57-59... 

Denisov, A. Y., Chen, G., Sprules, T, Moldoveanu, T, Beauparlant, P., Gehring, K. (2006) Structural model of the BCL-w-BID peptide complex and its interactions with phospholipids micelles. Biochemistry, 45, 2250-2256. 

The surfactant AOT forms reverse micelles in non-polar fluids without addition of a cosurfactant, and thus it is possible to study simple, water/AOT/oil, three component systems. To determine micelle structure and behavior in water/AOT/oil systems, investigators have studied a wide range of properties including conductivity (15), light (JL ), and neutron (12) scattering, as well as solution phase behavior (1 ). From information of this type one can begin to build both microscopic models and thermodynamic... 

Wasan et al. (27-28) explained the process of stratification on the basis of a micelle-latticing structure model. In Figure 8 a schematic of the latticing model for film thinning is provided. By fluctuations in the structure of the micellar lamellae (i.e. the individual rows of micelles in Figure 8), the film can change its thickness by stepwise transitions, each of which are equal to the micellar-lamellae thickness. According to this model the number of transitions will depend upon the micelle concentration. 

Crystalline polymers appear to be the most studied by ESR techniques. The model wiiich seems to emerge from these results is, in fact, a variant of a model proposed over twenty years ago by Cumberbirch and associates (Shirley Institute Memoirs) to explain the tenacity of wet raycm monofilaments. Briefly, Cumberbirch, et al propose a fringe-micelle structure in which the fringe r ons, swollen by water, are assumed to obey rubber elasticity theory. These fringe reglcms are, of course, the more accessble (to water), more disordered, r ons of the semicrystalline structure. 

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.

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