Hypothetical mechanistic pathways

A central hypothesis derived from this data is as follows Ambient air particles are complex mixtures with intrinsic toxicity. In concert with preexisting inflammation, particulate exposure results in stimulation of lung receptors and immune cells, release of reactive oxygen species (ROS), and induction of pro-inflammatoiy mediators that lead to local and systemic effects, which ultimately account for the epidemiological associations between adverse health effects and particulate air pollution. Hypothetical mechanistic pathways by which inhalation of ambient particles in urban air may lead to morbidity and mortality are outlined in Figure 1. 

Figure 1 Hypothetical mechanistic pathways by which inhalation of concentrated ambient particles may lead to morbidity and mortality.

There is a triple bond between the two nitrogen atoms, which is composed of one a bond and two orthogonal n bonds. Furthermore, there is a lone pair on each nitrogen that points away from the multiple bond. Each nitrogen has a full octet. Now form the hypothetical anion N2, by the addition of an electron. Write down the mechanistic pathway for the production of this intermediate. 

It is immediately possible to identify one mechanistic pathway that cannot be correct, namely the simple addition of the incoming nucleophile to give an adduct, which then subsequently disassociates to give the desired products. This cannot be a permissible pathway, because, in forming the adduct, ten electrons would reside on the carbon atom. If you are in any doubt about this, count the number of electrons around the central carbon atom in the resultant anion in the following hypothetical reaction. 

The great structural complexity encountered among sesquiterpenes implies that multiple cyclizations, shifts, and rearrangements of the parent cyclic intermediates may occur before stabilization to the final product, and in many instances several routes to the same sesquiterpene are feasible. Whether such reactions are concerted, take place sequentially on the enzyme surface, or involve discrete free intermediates is not yet generally known, and all three mechanistic alternatives, as well as multiple pathways, have been invoked to rationalize the same sesquiterpene structures. Because very few cell-free systems are available to examine these questions directly, and because of the difficulties associated with obtaining direct evidence of biosynthetic routes via in vivo tracer studies, hypothetic il pathways based mainly on chemical reasoning have been the rule (Roberts, 1972 Rucker, 1973 Anderson ef al., 1978). 

While the mechanistic scheme as outlined so far accounts for the majority of structural changes in ring A-dienone isomerizations, a few cases require modifications of this general pathway. The B-nor dienone (215) is transformed exclusively to the linear dienone (217) in dioxane solution. The preferential fission of the 5,10-bond in the hypothetical precursor (216) has... 

The purpose of the present study was to identify those volatile compounds which significantly contribute to the seasoning-like note of fenugreek using the approach of sensory directed chemical analysts. Gas chromatography in combination with olfactometry and mass spectrometry have been used as key steps of this approach (18,19). The formation of flavor impact compound(s) was studied in model systems using the quantification technique Isotope Dilution Assay (20, 21) The mechanistic study was based on a hypothetical pathway proposed for the formation of sotolone via thermally induced oxidative deamination of HIL (10). 

We propose two alternative mechanistic models for the temporal regulation of cellular responses to DNA damage. We describe them as the integrative surveillance (IS) and the autonomous pathway (AP) models (Fig. 3). These two hypothetical models represent two opposing philosophical views, but they may not be mutually exclusive. 

The recent literature about trifluoromethylation reactions with compounds 1 and 2 oftem conveys hypothetical or postulated mechanistic schemes not backed up by experiments. While sueh mechanistic suggestions may be realistic, they still need to be examined in a vtay taitieal manner. It is often assumed that the formation of a TEMPO-CF3 adduct demonstrates a radical pathway. However, it is often not clear whether the radical pathway is actually part of the trilluoro-methylation reaction of the substrate or whether it is actually induced by the presence of TEMPO, because TEMPO is a trifluoromethylation substrate itself in the absence of a stronger nucleophile, under a series of conditions. 

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