Enhanced stability

Xie Q, Perez-Cordero E and Echegoyen L 1992 Electrochemical detection of and enhanced stability of fullerides in solution J. Am. Chem. Soc. 114 3978-80... 

Commercial polymers are made so that they possess terminal end groups for enhanced stability. Differences in the nature of these end groups have been claimed to be the main reason for the higher hot water and alkali resistance of the coplymers over the homopolymers (see section 19.3.3). 

Because the five-membered ring is a substituted cyclopentadienide anion in some dipolar resonance stmctures, it might be expected that exocyclic groups that could strongly stabilize a positive charge would lead to a larger contribution from dipolar stmctures and enhanced stability. The stmctures 13 and 14 are cases in which a dipolar contribution would be feasible. 

Most organic free radicals have very short lifetimes, but certain structural features enhance stability. Radicals without special stabilization rapidly dimerize or disproportionate. The usual disproportionation process for alkyl radicals involves transfer of a hydrogen from the carbon P to the radical site, leading to formation of an alkane and an alkene ... 

The 4-chloropyrimidin-2-yl dichlorophosphate ester has been isolated in 58% yield as an intermediate in the chlorination of 6-(trifluoromethyl)uracil with POClj followed by PC15 (83JHC219). The enhanced stability of this compound was believed to be due, in part, to electron withdrawal by the CF3 group. Tri-n-propylamine was found to be the most effective in a comparison of the influence of bases on chlorination with POCl3 (87JHC1243). 

At temperatures above Tm, chemical and enzymatic degradation of microbial exopolysaccharides is enhanced. The apparent enhanced stability of microbial exopolysaccharides in their ordered confirmation is thought to be due to the glycosidic bonds in the backbone of the polymer which raises the activation energy. This restricted movement would also restrict access of enzymes and chemicals to the backbone. 

There is a substantial literature on the thermal and photochemical degradation of PS and it is well established that polymer properties are sensitive to the manner in which a particular sample of PS is prepared. For example, it has been reported that PS prepared by anionic polymerization shows enhanced stability with respect to that prepared by a radical mechanism.2 10 This has often been attributed to the presence of "weak links" in the latter polymers. However, the precise nature of the "weak links" remains the subject of some controversy. T he situation is further confused by all PS prepared by radical mechanisms often being considered as a class without reference to the particular polymerization conditions employed in their preparation. In many cases the polymers are "commercial samples" with details of the method of preparation incomplete or unstated. 

Co-administration of ofloxacin and chitosan in eyedrops increased the bioavailabUity of the antibiotic [290]. Trimethyl chitosan was more effective because of its solubility (plain chitosan precipitates at the pH of the tear fluid). On the other hand, N-carboxymethyl chitosan did not enhance the corneal permeability nevertheless it mediated zero-order ofloxacin absorption, leading to a time-constant effective antibiotic concentration [291]. Also W,0-carboxymethyl chitosan is suitable as an excipient in ophthalmic formulations to improve the retention and the bioavailability of drugs such as pilocarpine, timolol maleate, neomycin sulfate, and ephedrine. Most of the drugs are sensitive to pH, and the composition should have an acidic pH, to enhance stability of the drug. The delivery should be made through an anion exchange resin that adjusts the pH at around 7 [292]. Chitosan solutions do not lend themselves to thermal sterilization. A chitosan suspension, however. 

High yields and stable products result when R = i-Bu and R = Me2N, i-BuO or i-Bu or if R and R are (Me3Si)2CH If substituents with lower electron-releasing capacity are used, the method fails because of disproportionation into AIR 3 + Al. Addition of y-picoline (y-pic) to Al2(i-Bu)4 gives " Al2(i-Bu)4(y-pic)2 with enhanced stability of the metal-metal bond. The action of B2H(j on Al2(i-Bu)4 produces Al2(BH2)4 in which both Al—Al and Al — B bonds occur ". ... 

With the enhanced stability as compared to that of 63 e, 76 should be an ideal starting material for the construction of larger organometallic molecular objects of dendrimeric, cyclic or polymeric nature. 

Liposomes reveal an enhanced stability against mechanical and thermal challenges An enhanced long-term stability is observed in reconstitution experiments... 

This study relates to a continuous process for the preparation of perfluoroalkyl iodides over nanosized metal catalysts in gas phase. The water-alcohol method provided more dispersed catalysts than the impregnation method. The Cu particles of about 20 nm showed enhanced stability and higher activity than the particles larger than 40 nm. This was correlated with the distribution of copper particle sizes shown by XRD and TEM. Compared with silver and zinc, copper is better active and stable metal. 

Diaz AF, Logan JA(1980)Electroactive polyanihne films. JElectroanalChem 111 111-114 Noufi R, Nozik AJ, White J, Warren LF (1982) Enhanced stability of photoelectrodes with electrogenerated polyanUine films. J Electrochem Soc 129 2261-2265 Noufi R, Tench D, Warren LE (1981) Protection of semiconductor photoanodes with photoelectrochemicaUy generated polypyrrole films. J Electrochem Soc 128 2596-2599 Jaeger CD, Fan FRF, Bard AJ (1980) Semiconductor electrodes. 26. Spectral sensitization of semiconductors with phthalocyanine. J Am Chem Soc 102 2592-2598 Gerischer H (1977) On the stability of semiconductor electrodes against photodecomposition. J Electroanal Chem 82 133-143... 

Moreover, it was found that incorporation of nanoparticles about 8 nm in diameter in a-Si H led to improved properties, the most important one being enhanced stability against light soaking and thermal annealing [387]. A later study revealed a typical crystallite size of 2-3 nm. with a hexagonal close-packed structure [388]. Diamond structures can also be observed [389]. Hence the name polymorphous silicon is justified. 

An examination of mutant PKA proteins was undertaken. Phosphorylation of Thr-197 is required to activate PKA and phosphorylation of Ser-338 enhances stability of the protein. Replacement of Thr-197 and/or Ser-338 by Ala was examined to determine any conformational changes in the protein. Both single substitution mutants were expressed in Escherichia coli in similar levels to wild-type protein. However, both mutants were found to be less stable, as had been previously described. The double mutant... 

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