Mechanisms alternative

The formation and decomposition of benzenediazoazide and phenylpentazole can be described by a mechanism alternative to the one discussed in Section III, A, 1 [Eq. (3)]. In contrast to the tacitly assumed independent formation and decomposition of phenylpentazole, e.g. one-step four-centered processes as described by 21 and 22,... 

The overall reaction includes allylic transposition of a double bond, migration of the allylic hydrogen and formation of a bond between ene and enophile. Experimental findings suggest a concerted mechanism. Alternatively a diradical species 4 might be formed as intermediate however such a species should also give rise to formation of a cyclobutane derivative 5 as a side-product. If such a by-product is not observed, one might exclude the diradical pathway ... 

It has been seen that iron has an adverse effect because it forms a second phase (insoluble) material in the alloy which acts as an effective local cathode. Sequestering is the technique of adding an alloying addition that will cause an alternative intermetallic compound with iron to form. This compound might form a dross to be removed mechanically. Alternatively the new intermetallic compound could be a less effective cathode in which case removal would not be necessary. 

Excess acidity correlations have been used to show that some aromatic sulfonic acid desulfonations have an A-SE2 mechanism.188,189 This mechanism (alternative terminologies are Ad-E2 and A(E) +A(N))190 has also been found to apply to the hydration of acetylene itself,191 to ynamines192 and to many other alkynes,193-195 as well as to many different alkenes196-199 and vinyl ethers.200-203 The excess acidity method has been used to evaluate aA values for several alkene hydrations.204 205... 

The cyanide ion was one of the first adsorbed ions which was found to be surface enhanced (8,9) on silver electrodes. In this first stage of SERS investigation of the cyanide ion, it was assumed to be linearly bonded to Ag, through the carbon atom, perpendicular to the metal surface (9). However faced with the pressure of explaining the SERS mechanism, alternative structures were proposed. With the conception of the ad-atom model for SERS, Otto conjectured that the main SERS band was due... 

Sensors are usually attached chemically or physically to other materials here referred as the carrier, like polymers, antibodies, and optical fibers in order to facilitate the sensing process. These carriers generally affect the luminescent characteristics of the sensor molecules. The modification of the luminescent characteristics of the sensor is caused by the creation of more than one microphase or microenvironment for the sensor. Each molecule in its particular microenvironment may return to the ground state following a different set of processes or mechanisms. Alternatively, the nonra-diative decay rate of each microphase may be different for each sensor molecule. Depending on the characteristics of the carrier and the sensor, the number of microphases may be one, two, three, or an infinite number. 

If the mutual interaction of adsorbed particles is strong enough the surface diffusion can become organized and leads to a formation of islets of adsorbed molecules in the sense of the Volmer-Weber drop mechanism, or to a layer association, parallel to the crystalline surface, according to the Frank-van der Merve s mechanism. Alternatively, it can form a tridimensional organized region (Stranski-Krastanow s mechanism) similar to that of adsorption mixed crystals74). 

The data seem to support an alternative mechanism compared with the so-called Lyons mechanism in which O2 is activated forming a metal-oxo (M=0) species.1831 Via reaction of a second metalloporphyrin with a primarily formed superoxo (Me-OO0) or peroxo species, a metal-oxo is formed, reacting eventually with an alkane according to the oxygen rebound mechanism. Alternatively, radicals present in solution, upon reaction with dioxygen, may form alkyl hydroperoxides that are decomposed by metalloporphyrins.[83]... 

Toxicity there are many mechanisms by which a compound can be toxic. Toxicity issues may arise from PK or ADME or selectivity issues, depending on the mechanism. Alternatively a compound may simply react harmfully with tissues or organs in a direct manner. 

In ancient cases like the construction of the Egyptian pyramids, muscle power did not have a competing mechanical alternative. 

Cycloadditions with electronically excited carbonyl groups (or homologs) usually follow biradical mechanisms. Alternatively, PET-pathways can be observed either for energetically favorable donor-acceptor situations or in highly polar solvents. For... 

The biogenesis of many cyclic terpenoids requires a cts-double bond in the acyclic precursor. A cis-double bond is frequently incorporated into the terminal isoprenoid unit via an allylic rearrangement mechanism. Alternatively, a cis unit could be incorporated directly (as rubber). Two results this year highlight some features of this problem. Both geraniol (22) and nerol (23) are derived from all-trans units (i.e. retention of the 4i -proton of mevalonate) so that the cis-double bond of nerol is derived by isomerization. The sesquiterpenoid gossypol (34) is derived from c/s,cis-farnesyl pyrophosphate. Hence in its biosynthesis either... 

Seyferth and Wiseman reacted as-obtained silazanes with catalytic amounts of potassium hydride, KH. They observed dehydrocoupUng of Si-H with N-H units and proposed a mechanism involving silylene-imine Si=N motifs as key species which rapidly add Si-H intermolecularly in hydrosilylation-type reactions. There has been, until now, no experimental proof for the proposed mechanism. Alternatively, a mechanism excluding silylene-imine formation was suggested... 

Another plausible mechanism fa- the Sml2-mediated Barbier reaction involves coupling of ketyl and alkyl radictds in a diradical coupling mechanism. Alternatively, addition of an alkyl radical to a Sm -activated ketone carbonyl may be invoked. ... 

The studies of mast cell cytokine production described above have shown that maximal induction of cytokine synthesis and release usually occurs in response to IgE-dependent activation. In common with many cell types, there is evidence that FccRI on mast cells is coupled to the phospholipase C effector system that controls two distinct signal transduction pathways, one regulated by Ca " ions and the other by protein kinase C (PKC). Exocytotic degranulation is associated with an increased cytoplasmic level of Ca ions, and activation of mast cells can be therefore achieved by the use of calcium iono-phores which raise intracellular calcium concentrations through a receptor-independent mechanism. Alternative mast cell stimuli include phorbol-12-myristate-13-acetate (PMA) which activates PKC and induces mediator secretion from basophils and rodent mast cells but not from human mast cells, and concanavalin A (Con A), a lectin which can stimulate mast cells by cross-linking of cell-bound IgE and/or cell surface glycoproteins. 

Several reaction mechanisms, alternative to the retro-aldol cleavage, have been proposed on the basis of kinetic, stereochemical, and reaction specificity studies and exhaustively reviewed. However, only the relatively recent resolution of crystal structures of the enzyme from several sources " and characterization of site-specific mutants have resolved some of the crucial questions concerning the catalytic mechanism of SHMT. 

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