Insoluble nuclei

The two extremes of insoluble nuclei are nuclei which are easily wetted and those which are not. Nuclei which are easily wetted rapidly take on the appearance of a droplet and subsequently behave as one. To predict droplet growth or evaporation, these particles with easily wettable surfaces can be considered to be pure drop nuclei, and the Kelvin equation can be used directly (but with a lower limit on nucleus size). 

In cases where the particle surfaces are not wettable, condensation proceeds with much more difficulty. This is because the condensing liquid tends to pull into small spheres on the particle surface, and only when the entire surface is covered with these spheres is a liquid coating formed. Fletcher (1958a, b) has treated this problem by considering the contact angle between an embryo sphere formed on the particle and the particle surface. His results correspond to what has been observed experimentally—it is very difficult to get condensation to take place on nonwettable particles unless high supersaturations are used. The role of insoluble nuclei in the condensation process is still in question and remains another problem for future investigators to solve. 

Utilization also depends on the chemical composition of the nuclei. There are two general classes of condensation nuclei to be considered soluble nuclei and insoluble nuclei. With soluble nuclei the condensing vapor dissolves the nucleus, changing the properties of the embryo drop from that of a pure liquid. With insoluble nuclei, surface characteristics are important, since once the nucleus is coated with liquid, it... 

The formation of a water-soluble complex of poly(sodium acrylate) with 5,6 ionen bromide of 3 1 composition has been observed108. Its mass and size are independent of the complex origin and are equal to 660 x 103 and 33 nm respectively. The soluble complex presumably consists of the insoluble nucleus of the equimolar polyelectrolyte complex and adsorbed excess of poly(sodium acrylate) on its surface. The soluble polyelectrolyte complex dissociates at higher ionic strengths (0.05-0.1) whereas the insoluble equimolar complex in the core is still stable and precipitates from the solution. 

Structure of colloids of iron hydroxide. Colloidal solutions of iron hydroxide usually are obtained experimentally by hydrolysis of a ferric chloride solution. The particles that are formed consist of molecules of Fe(OH)3, which constitute the insoluble nucleus—the main mass of the micelle. The number of molecules in the nucleus is not constant and may range from tens to 500. 

In order to obtain a homogenous and stable latex compound, it is necessary that insoluble additives be reduced in particle size to an optimum of ca 5 )Tm and dispersed or emulsified in water. Larger-size chemical particles form a nucleus for agglomeration of smaller particles and cause localized dispersion instabiHty particles <3 fim tend to cluster with similar effect, and over-milled zinc oxide dispersions are particularly prone to this. Water-soluble ingredients, including some accelerators, can be added directly to the latex but should be made at dilute strength and at similar pH value to that of the latex concentrate. 

Cyclosporine A (CsA) is a water-insoluble cyclic peptide from a fungus composed of 11 amino acids. CsA binds to its cytosolic receptor cyclophilin. The CsA/cyclophilin complex reduces the activity of the protein phosphatase calcineurin. Inhibition of this enzyme activity interrupts antigen receptor-induced activation and translocation of the transcription factor NEAT to the nucleus which is essential for the induction of cytokine synthesis in T-lymphocytes. 

It seems that deep-seated cleavage of the dioxin nucleus must accompany dechlorination in methanol. When pure dibenzo-p-dioxin (II) was irradiated in cyclohexane solution in a quartz cuvette, it darkened in color, and a precipitate of intractable dark brown material was collected and was insoluble in the common solvents except for methanol. 

In many tissues cholesterol and other sterols exist as a mixture of the free alchohol and its long chain fatty acid ester (esterified at position 3 of the steroid nucleus). The determination of the cholesterol content of a sample may involve the measurement of either of these two fractions individually or the total cholesterol. It is possible to precipitate free cholesterol by adding an equal volume of digitonin (1 gl-1 in 95% ethanol), a naturally occurring glu-coside, to form a complex that is insoluble in most solvents, including water. 

Nicotinamide Nucleotides.—A number of dehydrogenases have been purified by affinity chromatography, using NAD+ linked to insoluble supports either through the 6-11)12 or the 8-positions13 of the adenine nucleus. The reverse process, the use of immobilized dehydrogenases to purify NAD+, has also been described recently.14... 

The tetrapyrrole nucleus frequently holds a co-coordinated metal atom, but it has been found that only diamagnetic metals such as Zn, Pd, In, Sn, and Lu allow the tetrapyrrole to retain its photosensitizing ability, while paramagnetic metals such as Fe, Cu, and Gd do not (Rosenthal et al., 1986). Many of these compounds are lipophilic and some are even insoluble in water. These compounds must either be delivered in an emulsion or else incorporated in liposomes. 

Similarly, a series of hydroxy-terminated poly(ether) dendrimers, 35, with a single carborane nucleus at their core, were observed to have water solubilities comparable to that of chloroacetic acid, or D,L-valine. Again this is in direct contrast to the starting carborane nucleus which is insoluble in aqueous solutions and permitted the use of 35 in neutron capture therapy. Similar effects have also been observed with dendrimers containing calixarenes [66] and porphyrins [67] as the central units. 

The Mannich reaction is a particularly good method of introducing a reactive functional group into a sensitive aromatic nucleus. The reaction has been very useful in ferrocene chemistry. Treatment of chromium acetylacetonate under Mannich conditions yielded a tris-V,N-dimethylaminomethyl chelate (XXXIII). This remarkable substance was very difficult to purify because of its extreme solubility in all solvents ranging from n-heptane to water. The trisamino chelate (XXXIII) is a deep purple, hydroscopic oil and behaves like a typical organic amine. Reaction of this amine with methyl iodide afforded a trisquater-nary ammonium salt (XXXIV), soluble in water but insoluble in organic solvents. When this salt (XXXIV) was treated with cyanide ion, trimethylamine was lost and the cyanomethyl chelate (XXXV) was formed. 

A number of benzimidazoles exist as prodrugs their anthelminthic activity is due to the fact that they are metabolized in the animal body to the biologically active benzimidazole carbamate nucleus. Due to their relatively slower excretion rates, the newer insoluble benzimidazoles have fairly long withdrawal periods for edible tissues and milk in contrast to the less effective and more rapidly excreted thiabendazole analogues. Strict compliance with withdrawal periods is always necessary because of the potentially toxic and teratogenic effects of some of the benzimidazoles and their metabolites. 

Finally, it must be remembered that the hydration is also influenced by the structure of the ion. In view of the fact that the positive pole of a water molecule attaches itself to the negative ion, the molecule will be repelled by the positive nucleus of the positive ion. This means that the ion of a strong acid has a small hydration energy and will therefore have a smaller heat of solution and hence the tendency to form insoluble compounds. This effect is shown when CIOT combines with a large ion, and must be taken into account in explaining the solubility behaviour of caesium salts, which... 

Water-insoluble hormones (steroid, retinoid, and thyroid hormones) readily pass through the plasma membrane of their target cells to reach their receptor proteins in the nucleus (Fig. 23-4). With this class of hormones, the hormone-receptor complex itself carries the message it interacts with DNA to alter the expression of specific genes, changing the enzyme complement of the cell and thereby changing cellular metabolism (see Fig. 12 10). 

Salts of the metals are thus present in equilibrium with the violet tellurium compounds, and in the case of the dibisulphate the balance is controlled by the proportion of wrater present. This is rendered visible in the action of iron upon phenoxtellurine dibisulphate, for the violet mixture becomes red when dried on clay in a desiccator, but the violet colour is restored on exposure to moist air. The method of examining a coloured complex consists in treating it with water, when the acceptor 55 is set free and can be extracted by ether the residual salt can then be reduced by adding potassium bisulphite, and the nucleus of the donor again extracted by ether. The products are insoluble in water, so that fresh equilibria cannot presumably be set up. 

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