Commonly proposed mechanism

Hopanoids (the most common organic natural product on earth) must have been involved in the evolution of the biomembrane itself. All known membranes contain terpene derivatives, such as cholesterol or carotenoids, which belong to, or can be derived from, hopanoids. However, we still do not know their biological function. Their most commonly proposed mechanism is to regulate membrane fluidity. Another obvious effect is their influence on the lipid bilayer (or monolayer in the case of archaebacteria) curvature. The different types of hopanoids occurring will certainly favour the relative stability of either the planar or of the intrinsically curved membrane conformation. The ether lipids of archaebacteria, which are hopanoid derivatives, forming curved bilayers as discussed above, therefore provide evidence for cubosomes as the first organised form of life. 

Scheme 26 (a) The Kharasch reaction, (b) The commonly proposed mechanism for... 

The Visual Transduction Process. Picosecond absorption spectroscopy which utilizes OMCDs also has provided important mechanistic information that previously was not available by means of other techniques. Detailed pathways of a number of reactions which are important from a physical, chemical, and/or biological viewpoint have been elucidated by means of this technique. A recent picosecond spectroscopic study by Spalink et. al. (30) has demonstrated that an experimental criterion, which has been used to support the hypothesis that cis-trans isomerization (31) is the primary event in the visual transduction process, is not true. This criterion is based on the commonly occurring statement that both the naturally occurring 11-cis-rhodopsin and the synthetic 9-cis-rhodopsin lead to the same primary photochemical product, bathorhodopsin. Of course, the existence of a common intermediate generated from either 11-cis- or 9-cis-rhodopsin would support the commonly proposed mechanism of cis-trans isomerization as the primary event in the visual transduction process. However, the data obtained by Spalink et. al. (30) indicate that a common intermediate is not generated from both rhodopsins. 

The commonly proposed mechanism for the reaction is that hydrogen and oxygen recombine on the catalyst surface to yield a peroxy intermediate, which subsequently reacts with propene to yield PO and water.21... 

Reaction mechanism. The commonly proposed mechanism of the reaction [81-85] is based primarily on the early kinetic investigations by Steinberg [81], The initial step of this mechanism involves deprotonation of dialkyl H-phosphonate (RO)2P(0)H by a base B to give the dialkyl phosphite anion, (ROljPO". This anion then reacts as a nucleophile toward carbon tetrachloride, resulting in the sequence of reactions shown on the next scheme. In this scheme, step 1 leads to the formation of the active species (RO)2PO , while steps 2 and 3 in the scheme represent the cycle of reactions that lead to the formation of the products. 

A commonly proposed mechanism of the standard NHK protocol involves reduction of Ni(II) to Ni(0) by CrC, then oxidative addition of Ni(0) to the alkenyl iodide to generate an alkenyl Ni(II) species (Scheme 3-69). Transmetallation to CrCls (from the initial nickel reduction step) produces a Cr(III) alkenyl species, which undergoes direct addition to the aldehyde to generate product. Alternatively, alkenyl transfer from a vinyl nickel(II) species to CrCb may instead occur with release of the Cr(III) alkenyl species and a Ni(I) species. 

Formally, the MBH reaction involves a sequence of Michael addition, aldol reaction and P-elimination. The commonly proposed mechanism consists in a reversible conjugate addition of the nncleophile to the starting enone 237, generating an intermediate enolate 238. This enolate reacts with the electrophilic aldehyde in an aldol-type process, in which two stereogenic centers are formed, to give 239, which suffers an intramolecnlar acid-base eqnilibrinm to give another enolate 240. From this intermediate, the p-elimination of the nncleophile provides the MBH product... 

The SgAr reactions involve more than 20 distinctly different types of substitutions, yet these transformations have similar overall mechanisms. The commonly proposed mechanism involves interaction of an electrophilic species with the x-system of an arene (Scheme 1.2) [3]. The electrophile (E+) itself is often a cationic species vide infra), but S Ar reactions may also be initiated by dipolar... 

Several papers consider the promotional effects of copper on ceria-based catalysts," " " which results in very active soot combustion with a temperature for soot oxidation around 350 °C. The commonly proposed mechanism for the reaction is correlated with the presence of CuO particles and their capability of being reduced to Cu and then re-oxidized to Cu. The reduction of supported copper species by carbon during soot combustion was confirmed by EPR analysis by Lamonier et Liang and co-workers consid-... 

The most commonly proposed mechanism is shown in Figure 9. This mechanism involves the same basic steps (oxidative addition, transmetallation, isomerization, and reductive elimination) as discussed previously. It is important to note that the mechanism shown in Figure 6 is more consistent with the recent developments of the Stille reaction. The exact mechanism followed may vary depending on the exact substrates and reaction conditions. 

A hindered phenol is one of the most common antioxidants used in urethanes. This antioxidant traps radicals, which can degrade the polymer chain. The proposed mechanism is as follows [87] ... 

The metal complexes most commonly used in these photoredox systems are manganese and rhenium carbonyls. The proposed mechanism of the photoredox... 

Evidence against the covalent mechanism has been summarized by Mock, who has also proposed alternative general-base-catalyzed mechanisms for ther-molysin and carboxypeptidase A.143 He suggests that His-231 is the general base for thermolysin and the carboxy-terminal carboxylate for carboxypeptidase A. The one common feature of all the proposed mechanisms is the Zn2+ functioning as a Lewis acid to polarize the substrate. 

It should be noted that this expression is a general one that can be used for any photochemical reaction that can be quenched. It is commonly called the Stern-Volmer equation. This equation predicts that if the proposed mechanism is correct, the data, when plotted as 4>a0/4>a vs. [Q], should be linear with an intercept equal to unity and a slope equal to kqr. Linear plots were indeed observed out to large d>°/d> values. Assuming a value of 5 x 10 M 1 sec-1 for the quenching rate constant,(7) the data presented in Table 4.1 were obtained. 

However, as pointed out above, the commonly proposed free radical mechanism is not entirely consistent with the observed behavior of H-donor solvents and coal. Further, a thermally promoted C-C or C-0 bond-scission is inconsistent with our observations in the -PrOH work at 335°C. As also mentioned, a major fraction of the coal was converted in this system to a product with a number-average molecular weight of less than 500. If we consider that the rate constant for the unimolecular scission of the central bond in bibenzyl is expressed (5) as... 

It was proposed that mono-protonation of [CuL]2+ to yield [Cu(HL)]2+ occurs initially at the axial nitrogen atom since this Cu-N bond is expected to be weaker as a result of Jahn-Teller distortion. With respect to this, it should be noted that species such as [Cu(HL)]3+ are commonly observed during potentiometric studies of the formation of Cu(n) polyamine complexes. From the proposed mechanism, the following rate law can be deduced ... 

Fig. 18. (a) Representation of the tumor hypoxic state (diagram adapted from Ref. (83a). Arrow direction indicates decrease in pC>2 (< 1 mmHg), achieved for tumor depths larger than 100 pm (b) proposed mechanism for redox-mediated retention of [Cu(ATSM)] in hypoxic cells (101-105). Note Contrary to common belief cell membrane crossing solely by direct diffusion is unlikely for compounds of this family is unlikely, as indicated by fluorescence imaging work on aromatic Zn(II) analogs (vide infra). Endocytosis is the more likely uptake mechanism (112-113). 

Despite of the common reaction mechanism, peroxo complexes exhibit very different reactivities - as shown by the calculated activation energies -depending on the particular structure (nature of the metal center, peroxo or hydroperoxo functionalities, type and number of ligands). We proposed a model [72, 80] that is able to qualitatively rationalize differences in the epoxidation activities of a series of structurally similar TM peroxo compounds CH3Re(02)20-L with various Lewis base ligands L. In this model the calculated activation barriers of direct oxygen transfer from a peroxo group... 

Proposed aquaculture applications of degradable polymers include seaweed culture nets, fishing nets and lines, and temporary structures used for restoration of wet lands, beaches or other marsh areas [11-14], Weathering and hydrolysis are the most common degradation mechanisms encountered in aquatic applications. Continually submerged articles pose special challenges since temperatures are low and photodegradation and oxidation effects are limited. 

The proposed mechanism of the above cycloisomerizations are depicted in Scheme 11.30. The oxidative coupling of a metal to an enyne yields a bicyclic metaUacyclopentene, which is a common intermediate. The reductive elimination and subsequent retro-[2+2] cycloaddition gave vinylcyclopentene derivatives, while the two patterns of P-elimination and subsequent reductive eUmination gave cychc 1,3- and 1,4-dienes, respectively. The existence of a carbene complex intermediate might explain the isomerization of the olefinic moiety. 

While the Uterature is rich in scientific information on glucosylases, recent interest has focused on the hypothesis that all these enzymes share a common catal3iic mechanism, despite differences in their product specificity (57). Indeed, it has been proposed that all glycosylases share the same basic chemical mechanism (58). Tlie a-amylases have been the focus of much of this attention, as the primary protein sequence (59), tertiary protein structure (54,55) and catalytic mechanism (57) have been recently delineated. 

The authors propose a working model relying on the commonly accepted mechanism for BV reactions (Fig. 13, a). Thus the sense of asymmetric induction is determined by the conformation of the Criegee intermediate, which is dictated by the chiral environment created by the catalyst. However, an alternative noncova-lent, bifunctional mechanism may be considered (Fig. 13, b) [80], This work... 

Schemes, like figures and tables, area form ofgraphics. They are often used to depict proposed reaction mechanisms. A proposed mechanism, by its very nature, is interpretative hence, schemes are commonly found in Discussion sections.

Schemes are used to depict a series of steps that progress in time. (Note that schemes differ from charts, which list groups of compounds or structures that do not change in time.) Most commonly, schemes are used to illustrate chemical reactions. In such cases, schemes often include arrows (e.g., to denote a forward reaction, resonance, equilibrium, and/or electron movement), intermediates, transition states, reactants, and products. Schemes are numbered in order of appearance in the text (Scheme 1, Scheme 2, etc.). As with tables and figures, the scheme is mentioned in the text before the scheme is encountered. Schemes are perhaps most common in Discussion sections of journal articles (e.g., to illustrate proposed mechanisms) but can appear most anywhere in journal articles, posters, and proposals.

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