Stability in aqueous solution

Other families of microporous solids, such as the aluminophosphates, are much more reactive in water, even in vapour form. Aluminium in these structures readily coordinates water to adopt five- or six-fold coordination, and exposure of many calcined AlPOs to moisture results in hydrolysis of framework bonds and loss of crystallinity. One way to avoid this, and thereby to enable handling, is to adsorb volatile organic compounds upon cooling after calcination. This protects the most reactive parts of the framework against 

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The -t-4 stale is stabilized in aqueous solution by fluoride ions. Cm02 and Cmp4 are formed by strong oxidation or the action of fluorine. 

GB is unstable in the presence of water. Maximum stability in aqueous solutions occurs from pH 4.0—6.5 with the hydrolysis rate increasing as the pH increases. The half-life in distilled water at 25°C is ca 36 h, but hydrolysis is accelerated in the presence of acids or bases. Because bases are far more effective in this respect than acids, caustic solutions are useful for decontamination. 

Many of the properties oj -hydroxypyridines are typical of phenols. It was long assumed that they existed exclusively in the hydroxy form, and early physical measurements seemed to confirm this. For example, the ultraviolet spectrum of a methanolic solution of 3-hydroxypyridine is very similar to that of the 3-methoxy analog, and the value of the dipole moment of 3-hydroxypyridine obtained in dioxane indicates little, if any, zwitterion formation. However, it has now become clear that the hydroxy form is greatly predominant only in solvents of low dielectric constant. Comparison of the pK values of 3-hydroxypyridine with those of the alternative methylated forms indicated that the two tautomeric forms are of comparable stability in aqueous solution (Table II), and this was confirmed using ultraviolet spectroscopy. The ratios calculated from the ultraviolet spectral data are in good agreement with those de-... 

In addition to the rather scattered data reported in the previous survey (76AHCSl,p. 510), a few new studies appeared on purine-6- and -2-thiones. Tire MO calculations of solvent effects (AMl-SMl and AM1-SM2) on the tautomerism of 6-thiopurine indicated that l//,9//-tautomer 7a is greatly stabilized in aqueous solution [94THE(309)137].Tlie same results were obtained experimentally from UV and and NMR studies (75JA3215, 75JA4627,75JA4636). 

The majority of metals and alloys available depend for their resistance to corrosion on the properties of an oxide film or corrosion product which is formed initially by the corrosion process. In many cases the protectiveness of the oxide film is determined by its stability in aqueous solutions in a specific pH range, either chemically dissolving to form aquocations at lower pH values or complex anions (aluminate, ferroate, plumbate, zincate, etc.) at higher pH values (Fig. 2.22). An important property of the chemical is therefore the pH value that it develops when dissolved in water. For many materials and many chemicals this is the overriding factor and in many cases... 

Situation and Interpretation A series of peptides was assessed for stability in aqueous solutions. The data in files ARRHENX.dat X = 1, 2, or 3) was found in a doctoral thesis. " Figure 4.27 shows one case where a temperature range of 50°C was covered. (See also Table 4.25.)... 

The chelate ring size principle can have structural effects as well as effects on thermodynamic stability in aqueous solution. An example is coordination of metal ions by sugars (44). The cyclic polyol cts-inositol can coordinate metal ions in two distinct ways (Fig. 14) (45). In ax-ax-ax bonding (Fig. 14), the metal ion is part of three fused six-membered chelate rings. Alternatively, in ax-eq-ax coordination, the metal ion is part of two fused five-membered and one six-membered chelate rings. Angyal has noted that metal ions of radius more than 0.8 A adopt the ax-eq-ax structure (44), whereas with an ionic radius... 

Higuchi and Lachman [122] pioneered the approach of improving drug stability by complexation. They showed that aromatic esters can be stabilized in aqueous solutions in the presence of xanthines such as caffeine. Thus, the half-lives of benzocaine, procaine hydrochloride, and tetracaine are increased by approximately two- to fivefold in the presence of 2.5% caffeine. This increase in stability is attributed to the formation of a less reactive complex between caffeine and the aromatic ester. Professor K. A. Connors has written a comprehensive textbook that describes methods for the measurement of binding constants for complex formation in solution—along with discussions of pertinent thermodynamics, modeling statistics,... 

The O-donor complexes of Tc(V) exhibit moderate and differential stability in aqueous solution. In the presence of reducing agents, such as stannous chloride, they are reduced to mainly undefined products of Tc in a lower oxidation state. However, at the low technetium concentration of "mTc that is used in nuclear medicine, the rate of the reduction process is very low. This makes it possible to prepare Tc(V) radiopharmaceuticals with O-donor ligands by the usual procedure, in which an excess of reducing agent over technetium is unavoidably used. The Tc(V) complexes also tend either to be easily oxidized or to disproportionate [23],... 

Chapman et al. [131] reported the synthesis of poly(ethylene oxide) (PEO) supported dendritic f-BOC-poly(a, c-L-lysines). These dendritic polymers termed as hydramphiphiles formed foams possessing good temporal stability in aqueous solution. Scrimin et al. [132] synthesized a three-directional polypeptide having uses in membrane permeability modulation. Decapeptide fragments were linked to TREN [tris(2-aminoethyl)amine] core. 

To overcome these issues, the water-soluble TCEP was synthesized and successfully used to cleave organic disulfides to sulfhydryls in water (Burns et al., 1991). The advantage of using this phosphine derivative in disulfide reduction as opposed to previous ones is its excellent stability in aqueous solution, its lack of reactivity with other common functionalities in biomolecules, and its freedom from odor. 

Figure 14.3 Small particles often are stabilized in aqueous solution by like charge repulsion, which prevents particle aggregation and precipitation.

A significant disadvantage of the 1st and 2nd generation ChR created on the basis of pyridine aldoximes is their own, relatively high toxicity (DL50 amounts to 146-250 mg/kg, i.m.) as well as low stability in aqueous solutions. 

Carpenter, J. F., Crowe, J. H., Arakawa, T. Comparison of solute - induced protein stabilization in aqueous solution and in the frozen and dried states. Developments in biological Standardization Vol. 74, p. 225-239. Acting Editors Joan C. May - F. Brown. S. Karger AG, CH-4009 Basel (Switzerland), 1992... 

Organic dyes like fluorescein and TRITC and the majority of NIR fluorophores suffer from poor photostability [77]. In addition, many NIR dyes, such as clinically approved indocyanine green (ICG) reveal poor thermal stability in aqueous solution [78]. Moreover, the presence of ozone can result in dye decomposition as observed for Cy5 [79]. In the last years, many organic dyes like the Alexa dyes have been... 

Potassium dichromate exhibits much greater stability in aqueous solution in comparison to potassium permanganate. Potassium dichromate possesses an inherent oranage colour that is not intense enough to serve its own end-point signal, specifically in the presence of the green Cr3+ ion, which is supposed to be present at the end-point. Hence, redox indicators are usually employed to locate the exact end-point e.g., barium diphenylamine sulphonate. 

The significance, reversibility, and mechanisms of nonenzymatic lactone hydration may be aptly illustrated with pilocarpine, an extensively investigated drug whose stability in aqueous solution is of great pharmaceutical relevance. Pilocarpine (7.76, Fig. 7.13), whose absolute configuration is... 

N. Mizuno, C. Shimizu, E. Morita, D. Shinkuma, Y. Yamanaka, Determination of Isosorbide Nitrates by HPLC and Their Stability in Aqueous Solution , J. Chromatogr. 1983, 264, 159-163. 

An oximine (i.e., a substituted oxime) analogue of alprenoxime was examined in an attempt to overcome the problem of low stability in aqueous solution. To this end, the methoxime analogue of alprenolol (11.74, R = Me) was prepared and evaluated [109]. Stability in solution was greatly improved at neutral pH. Topical administration to rabbits produced a decrease in intraocular pressure that had the same onset and intensity as that produced by alprenolol, but that lasted longer. Alprenolol was, indeed, formed in eye tissues as a metabolite, with the peak concentration reached 30 min after topical administration of the methoxime. 

Oxazolidines are five-membered cyclic ft-Mannich bases, some of which have, indeed, been examined as potential produgs of /l-amino alcohols of medicinal relevance such as ephedrines and /3-blockers. For example, 3,4-dime-thyl-5-phenyloxazolidine (11.106), the oxazolidine of ephedrine (11.107) undergoes hydrolysis to ephedrine and formaldehyde slowly at pH 1 and 12, but very rapidly in the neutral pH range (tm < 1 min at 37°) [135], Interestingly, the equilibrium reached between the reactants and products of hydrolysis was markedly pH- and concentration-dependent. However, despite its poor stability in aqueous solution, the oxazolidine was delivered through human skin significantly faster than ephedrine when applied as 1% aqueous solutions of pH 7 - 11. The lower basicity of the oxazolidine (pKa 5.5) compared to that of ephedrine (pKa 9.6) may explain the efficient skin permeation. 

In general, the formation of monolayers with silanization reagents requires a precise reaction scheme regarding humidity, the selection of solvents, temperature, and reaction time. Often, not enough consideration is given to these factors. As a result a number of authors concluded that the reproducibility of homogeneous silane-monolayers is insufficient, that their stability in aqueous solution is limited, or that they are prone to the formation of multilayers. 

The pX a (6.5-8.2) and nucleophilicity of MINA, 44, and of a series of aliphatic oximes derived from it, were found to be consistent with their ability to reactivate AChE inhibited by the nerve agents, sarin and VX. Yet, despite their ability to significantly reactivate AChE in the brains of sarin-intoxicated rats, these aliphatic oximes are not used as antidotes for treatment of OP poisoning in humans this is presumably due to their poor stability in aqueous solution and to their rapid clearance from the circulation. 

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