Stability mechanisms, multiple

Two patients stabilized on a phenytoin regimen suffered a loss of seizure control after taking shankhapushpi, an Ayurvedic antiepileptic medicine, three times a day. There was also a significant decrease in serum phenytoin concentration from 9.6 to 5.1mg/L. To investigate the possible mechanisms, multiple doses of shankhapushpi were administered to rats and resulted in decreased plasma phenytoin concentrations, whereas single-dose administration was reported to interfere with the antiplatelet effect of phenytoin, thereby implying both a pharmacokinetic and pharmacodynamic basis for the interaction (73). 

Polyisocynanurate Polymer hydrolytic stability, mechanical properties, and cost advantages. A polymer containing multiple isocyanate-to-isocyanate bonds. A material consisting of molecules characterized by the repetition of one or more types of chemical units (poly = many, mer from monomer, mono = one). 

Hydrogen bonds are also invaluable as mechanisms by which secondary and tertiary structure can be imparted to proteins. The most well-known examples are the formation of a-helices and p-shccls. In the former, linear sequences of amino acids form a spiral that is stabilized through multiple interactions between aligned amine hydrogen atoms and carbonyl oxygen atoms. In the latter the same interactions are found between amino acid sequences that are aligned in the same plane. 

A process with potential practical applicability is the hydrogenation of edible oils. Reduction of multiply unsaturated triglycerides with hydrogen over Ni-based catalysts is frequently used to gain autoxidative stability of edible oils. According to the Polanyi-Horiuti mechanism, multiple 1,2 or 1,4 diadsorption of the fatty acid tail with exclusively c/s-configuration around the double bonds causes cis-trans isomerisation, whilst the number of double bonds is being reduced. The trans-fatty acid chains have adverse effects on the human metabolism and must be minimized. 

Here we report preliminary results on the multiple fluorescence emission of 1 and 2. From structure-property relationships, solvent effect and temperature effect studies, we are able to show that the multiple emission is from the emission of free squaraine in solution, the emission of the solute-solvent complex and the emission of a twisted relaxed excited state. Further solvent effect study using 2 as a model shows that squaraine forms strong solute-solvent complexes with alcoholic solvent molecules. Analogous complex-ation process is also detected between 1 and the hydroxy groups on the macromolecular chains of poly(vinyl formal). The Important role of this complexation process on the stabilization mechanism of particles of 1 in polymer solution is discussed. 

Tyrosine hydroxylase, the rate-hmiting enzyme, is a substrate for PKA, PKC, and CaM kinase phosphorylation may increase hydroxylase activity, an important acute mechanism whereby NE and Epi, acting at autoreceptors, enhance catecholamine synthesis in response to elevated nerve stimulation. In addition, there is a delayed increase in tyrosine hydroxylase gene expression after nerve stimulation, occurring at the levels of transcription, RNA processing, regulation of RNA stability, translation, and enzyme stability. Thus, multiple mechanisms maintain the content of catecholamines in response to increased transmitter release. In addition, tyrosine hydroxylase is subject to allosteric feedback inhibition by catecholamines. 

The dyes that alter their emissions on binding to nucleic acids have high utility in molecular and cell biology. The mechanism of binding is intercalation of the planar aromatic dye structure into double-helical DNA, which is favored and stabilized by multiple electrostatic and hydrophobic interactions between the two matching chemical structures (see Fig. 4). Such an interaction has significant impact on the electronic structure of the dye,... 

Zuend, S. J. Jacobsen, E. N. Mechanism of Amido-Thiourea Catalyzed Enantioselective Imine Hydrocyanation Transition State Stabilization via Multiple Non-Covalent Interactions. ]. Am. Chem. Soc. 2009,131,15358-15374. 

The most recent class of light stabilizer is the Hindered Amine Light Stabilizer (HALS). These materials have been shown to function as radical traps, thus interrupting the radical chain degradation mechanism. The cyclic stabilization mechanism proposed for HALS involves multiple regeneration of the active nitroxyl stabilizer. The surprising performance of HALS at relatively low concentrations supports this non-sacrificial mechanism. 

The electrostatic stabilization mechanism is well documented for simple OfW emulsions (Myers, 1998a). Multiple emulsion droplets are much larger in size, and therefore the repulsive electrostatic forces are less pronounced (Figure 5.5). 

Biological and volcanic activities also have roles in the natural mobilization of elements. Plants can play multiple roles in this process. Root growth breaks down rocks mechanically to expose new surfaces to chenaical weathering, while chemical interactions between plants and the soil solution affect solution pFF and the concentration of salts, in turn affecting the solution-mineral interactions. Plants also aid in decreasing the rate of mechanical erosion by increasing land stability. These factors are discussed more fully in Chapters 6 and 7. 

Reactivity factors in additions to carbon-hetero multiple bonds are similar to those for the tetrahedral mechanism of nucleophilic substitution. If A and/or B are electron-donating groups, rates are decreased. Electron-attracting substituents increase rates. This means that aldehydes are more reactive than ketones. Aryl groups are somewhat deactivating compared to alkyl, because of resonance that stabilizes the substrate molecule but is lost on going to the intermediate ... 

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