Molecules precipitation

The influences of herbicides on cell division fall into two classes, ie, dismption of the mitotic sequence and inhibition of mitotic entry from interphase (G, S, G2). If ceU-cycle analyses indicate increases in abnormal mitotic figures, combined with decreases in one or more of the normal mitotic stages, the effect is upon mitosis. Mitotic effects usually involve the microtubules of the spindle apparatus in the form of spindle depolymerization, blocked tubulin synthesis, or inhibited microtubule polymerization (163). Alkaloids such as colchicine [64-86-8J,viahla.stiae [865-21-4] and vincristine [57-22-7] dismpt microtubule function (164). Colchicine prevents microtubule formation and promotes disassembly of those already present. Vinblastine and vincristine also bind to free tubulin molecules, precipitating crystalline tubulin in the cytoplasm. The capacities of these dmgs to interfere with mitotic spindles, blocking cell division, makes them useful in cancer treatment. 

Many research compounds are poorly soluble in water. When very lipophilic molecules precipitate in the donor wells, it is possible to filter the donor solution before the PAMPA sandwich is prepared. On occasion, the filtered donor solution contains such small amounts of the compound that determination of concentrations by UV spectrophotometry becomes impractical. One strategy to overcome the precipitation of the sample molecules in the donor wells is to add a cosolvent to the solutions (Section 7.4.4). It is a strategy of compromise and practicality. Although the cosolvent may solubilize the lipophilic solute molecule, the effect on transport may be subtle and not easy to predict. At least three mechanisms may cause Pe and membrane retention (%R) values to alter as a result of the cosolvent addition. To a varying extent, all three mechanisms may simultaneously contribute to the observed transport ... 

Specifically, he developed relationships for the free energy of mixing of solvent and solute (polymer) molecules with different solute axial ratios (length/width) and solute interaction parameters. In the absence of interactions between solute molecules, the free energy of the system lowers when rodlike molecules precipitate out of solution and form a separate solid phase. This is due to the fact that small water molecules must order themselves around large rodlike macromolecules in solution and therefore the system is most stable when the water molecules and rodlike molecules are separated in space into different phases, such as liquid and solid phases. 

The solubility of a block copolymer in water decreases as the concentration of electrolyte increases. When the concentration of the polymer is larger than its solubility, the polymer molecules precipitate onto the surface or form aggregates that remain dispersed into the colloidal system or deposit on the wall of the vessel that contains the colloidal dispersion. Let us start from two parallel plates, and then calculate the stability ratio of the system for spheres using the Detjaguin approximation. 

The ratio of low molecular weight compounds to high molecular weight compounds is another factor in maintaining solubility of all compounds in petroleum. When this ratio is upset, large molecules precipitate (form asphaltene). This study shows that the precipitated... 

Fluorescence quenching studies showed that chlorophyll a molecules are solubilized by dimyristoylphosphatidylcholine (DMPC) only up to concentrations of around 1 mol %. Excess molecules precipitate almost exclusively in domains containing chlorophyll. Pheophytin a solubility in liquid DMPC amounts to about 10 mol %. 

Ammonium sulfate, (NH4)2S04, is the most commonly used compound for salting out of proteins because it is very soluble (706 g/L) and has four ionic charges per molecule. Precipitations are generally performed slowly with cold solutions to minimize protein denaturation due to the heat... 

The easiest (and most unreproducible) way is to elutriate the oligomer in an organic solvent, put a substrate in, and evaporate the organic solvent. Usually these films are not uniform in thickness, include remnants of solvent, and are often mechanically unstable. Similarly solvents with temperature dependent solubility can be used, from which the molecules precipitate at lower temperatures. On some substrates, the molecules adsorb spontaneously. This leads to more reproducible films and their structures are often thermodynamically controlled. 

For liquid dosage forms, altered bioavailability upon storage is generally manifest in precipitation of API or other formulation components. Precipitation can result from a number of factors. With small molecules, precipitation can be caused by shifts in the pH of the solution (suspension). Such shifts can be due to absorption of carbon dioxide, chemical degradation of a component that generates an acid or base or loss of a buffer component due, for example, to oxidation. Another factor with small molecules is precipitation due to an increase in the API particle size. This effect, called Ostwald ripening, is caused by the gradual dissolution of smaller... 

Ideally, in order to have the best opportunity to form a stable salt, there should be a difference of at least 3 units in the pKa values of the parent dmg and the proposed counterion. If the pKa values of the parent drug and counterion are closer than 3 units, the salt may be unstable or may not form at all. Also, if the parent dmg has a high molecular weight and is largely hydrophobic with several aromatic rings or extended alkyl chains it may form an unstable salt where the parent molecule precipitates rapidly after dissolution of the salt in aqueous media (including blood). The relative acidities and basicities of some functional groups commonly found in dmg stmctures are provided in Table 37.1. 

Figure 7.4 shows a schematic representation of the interface Pb/solution during formation of PbS04 crystals. When no expander is contained in NAM, the Pb surface is covered by a film of PbS04 crystals which dissolve partially, whereby Pb " " ions (or PbS04 molecules) precipitate onto the PbS04 nuclei or crystals and thus contribute to their growth (Fig. 7.4a). 

Precipitate protected peptidyi hydrazide by addition of water, or in the case of side-chain deproteOted molecules precipitate by addition of EtzO. 

Fractional precipitation is a useful tool for biomolecule purification because it can be very selective for specific classes of molecules. Precipitation of proteins may be initiated by pH manipulation high salt concentration, most notably with ammonium sulfate high concentration of organic or addition of certain chemicals such as perchloric acid [1,7]. Similarly, RNA can be specifically precipitated with lithium chloride [1]. The success of such techniques depends on the goals. If... 

Figure 22 A linear example of Aufbau crystal packing for triorganotin 2-[(ZJ)-2-(2-hydroxy-5-methylphenyl)-l-diazenyl]benzoates. Molecules precipitate from solution and adopt orientations to optimize intermolecular interactions, in this case secondary Sn- -O contacts. When the steric bulk of the tin-bound substituents allows for the close approach of molecules, as in the R = methyl example, a supramolecular chain is formed comprising trans-C3Sn02 tin atom geometries. When the steric bulk precludes close association, monomeric packing is found with d -Cs Sn02 tin atom geometries. The linear repeat distance between successive tin atoms is 4.89 A for the R = methyl structure compared to 5.17 A for the R = cyclohexyl derivative.

As was described in section 2 when the polymer molecules precipitate in the presence of colloidal particles the colloidal surfaces are harvesting the precipitating polymers. The proper choice of concentration of particles, polymers and speed of heterocoagulation allows to form smooth coverage of colloidal particles by precipitating polymers. The following idea might be to capture the precipitated polymers, which is now placed on surface of colloidal particles. The scheme is depicted on Fig. 4.1. 

Precipitation is one of the common methods for the separation of the target molecules. Precipitation of proteins is based on changing their solubility by the addition of organic solvents or a large amount of salts. However, this method is not very selective as it is limited to the difference in the surface... 

The rate per unit volume at which solute molecules precipitate by the mechanism of homogeneous nucleation (i.e., the molecular nucleation rate Wnci)... 

Figure 11 (a) The rates at which molecules precipitate during RESS processing are... 

Next, we calculate the initial rate per unit volume at which solute molecules precipitate by condensation (i.e., the molecular condensation rate ncnd,ini), which is obtained by multiplying the initial condensation rate /end [see Eq. (65b)] by the number density of particles, N2sm, that have been generated in the first step (i.e., nucleation) of the process (2SM refers to two-step model) ... 

It should be pointed out that thermo-reversible gels can be obtained from solutions of relatively low molecular weight species, and at concentrations well below , if these molecules precipitate as high-aspect ratio entities, such as fibrillar crystals or whiskers. The latter gels, however, invariably are brittle and not suitable for the applications discussed below. 

Passivation of the substrate by oxidising agents, inhibition by film formation of released molecules, precipitation of protective layers by interim partly soluble molecules or ions, enhanced barrier formation against corrosion stimulators and cathodic polarisation are the key models used to explain the protective properties of anticorrosive pigments. 

Addition of Nonsolvent Successive precipitation of polymer species from a solution by addition of a miscible nonsolvent. The larger molecules precipitate first. 

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