Spontaneous aggregation

Binding proteins such as antibodies, receptors, and lectins may spontaneously aggregate in presence of bi- or multivalent specific ligands. This process, however, often results in assemblies that lack well-defined morphological and structural elements. 

A biomolecular system of glycoproteins derived from bacterial cell envelopes that spontaneously aggregates to form crystalline arrays in the mesoscopic range is reviewed in Chapter 9. The structure and features of these S-layers that can be applied in biotechnology, membrane biomimetics, sensors, and vaccine development are discussed. 

It has been recently demonstrated that the simplest of the cobalt porphyrins (Co porphine) adsorbed on a pyrolytic graphite electrode is also an efficient electrocatalyst for reduction of 02 into 1120.376 The catalytic activity was attributed to the spontaneous aggregation of the complex on the electrode surface to produce a structure in which the cobalt-cobalt separation is small enough to bridge and activate 02 molecules. The stability of the catalyst is quite poor and largely improved by using porphyrin rings with mew-substitu-tion.377-380 Flowever, as the size of the mew-substituents increases the four-electron reduction efficiency decreases. 

Protein molecules contain both polar and apolar groups. For proteins dissolved in water, these apolar groups tend to be buried in the interior of the globular structure, as a result of expulsion by the surrounding water. However, other interactions, as well as geometrical constraints, interfere with the hydrophobic effect, so that a minor fraction of the water-accessible surface of the protein molecule may be apolar. Protein molecules that do not spontaneously aggregate in water do not have pronounced apolar patches at their surfaces. 

Photoinduced, spontaneous aggregation processes have been shown to occur when indolinobenzopyrans are irradiated in aliphatic solvents. The aggregates which are globular in appearance, consist of submicron cores of crystalline materials with an amorphous exterior and are termed "quasicrystals" (L-3). Spectroscopic studies by Krongauz and coworkers (1) indicate that the composition of the cores are AnB (n=2,3) and the amorphous exteriors AB. The most stable quasicrystals have been derived from 1-(/ -metha-crolyloxyethyl)-3,3 dimethyl-61-nitrospiro- (indoline-2,2 —[2H-1 ] benzopyran (SP-A) and its associated merocyanine form (SP-B). 

The phosphate end of the phospholipids is relatively polar, making it water soluble or hydrophilic. In comparison, the fatty acid ends are relatively insoluble or hydrophobic. Molecules with hydrophilic heads and hydrophobic tails are termed amphiphiles. When their aqueous concentrations reach a critical level, they spontaneously aggregate to isolate their hydrophobic ends from water. 

Protein misfolding is most commonly caused by a gene mutation, which produces an altered protein. An example of this is the amyloid protein that spontaneously aggregates in many degenerative diseases, for example, Alzheimer disease. 

A micelle is an aggregate of surfactants. In water, the hydrophobic tails form clusters that are, in effect, little oil drops insulated from the aqueous phase by the ionic head groups. At low concentration, surfactant molecules do not form micelles. When their concentration exceeds the critical micelle concentration, spontaneous aggregation into micelles occurs.8 Isolated surfactant molecules exist in equilibrium with micelles. Nonpolar organic solutes... 

Spontaneous aggregation of lipids — double chain amphiphiles — in aqueous medium (smectic phase). Hydrophobic core of long alkyl chains (lamellar orientation) is covered by hydrophilic headgroups (see Figs. 2, 15). 

One characteristic property of surfactants is that they spontaneously aggregate in water and form well-defined structures such as spherical micelles, cylinders, bilayers, etc. (review Ref. [524]). These structures are sometimes called association colloids. The simplest and best understood of these is the micelle. To illustrate this we take one example, sodium dode-cylsulfate (SDS), and see what happens when more and more SDS is added to water. At low concentration the anionic dodecylsulfate molecules are dissolved as individual ions. Due to their hydrocarbon chains they tend to adsorb at the air-water interface, with their hydrocarbon chains oriented towards the vapor phase. The surface tension decreases strongly with increasing concentration (Fig. 3.7). At a certain concentration, the critical micelle concentration or... 

Spheroids are formed through spontaneous aggregation of cells, and their use allows the analysis of the effects of three-dimensional distribution of cells to test compounds, without the disadvantages presented by organ culture. Parameters such as penetration barriers in non-vascular areas and metabolic and proliferation gradients can be studied by use of this model. 

An alternative candidate for a cell membrane is one composed of mostly hydrophobic molecules that would spontaneously aggregate if suspended in water. It is important that the molecule is not entirely hydrophobic, like hexane or benzene, as some part of it must be solvated if it is to aggregate with others to form a structure capable of encapsulating other species. Candidates for this type of molecule are the lipids and related compounds. The formation of protocells by lipids has been discussed previously as part of the Lipid World hypothesis. In the context of compartmentalization it is important to consider how such an aggregate could evolve. 

SAM, conventional LB films are also useful for some applications in the nanotechnology field [11]. In this way, self-assembling can be defined as not only simple spontaneous aggregation but also functionalized organization. 

A surfactant molecule is an amphiphile, which means it has a hydrophilic (water-soluble) moiety and a hydrophobic (water-insoluble) moiety separable by a mathematical surface. The hydrophobic tails of the most common surfactants are hydrocarbons. Fluorocarbon and perfluorocarbon tails are, however, not unusual. Because of the hydrophobic tail, a surfactant resists forming a molecular solution in water. The molecules will tend to migrate to any water-vapor interface available or, at sufficiently high concentration, the surfactant molecules will spontaneously aggregate into association colloids, i.e., into micelles or liquid crystals. Because of the hydrophilic head, a surfactant (with a hydrocarbon tail) will behave similarly when placed in oil or when put in solution with oil and water mixtures. Some common surfactants are sodium or potassium salts of long-chained fatty acids (soaps), sodium ethyl sulfates and sulfonates (detergents), alkyl polyethoxy alcohols, alkyl ammonium halides, and lecithins or phospholipids. 

Sensitivity in measurements of spontaneous aggregation is higher than in routine light transmittance. 

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