Concerted hypothesis

It would have been possible to rescue the concerted hypothesis at this point by claiming that the transition state was structurally similar to the diolate, that is, that the alkene extrusions involved little C O bond cleavage prior to the transition state. However, the measurement of a significant secondary deuterium kinetic isotope effect (kHJkw = 1.25 0.05 at 100°C) gave evidence of significant C—O bond cleavage, whatever the mechanism. 

The concerted hypothesis advanced by Monod, Wyman and Changeux is based on the assumption that one enzyme molecule could contain only one type of subunit - R or T. 

As a result, there would be more R enzyme and less T enzyme in the solution. Since R is the high affinity form, the overall enzyme/substrate affinity is increased. The result is positive substrate cooperativity, that is, the binding of one substrate increases the ability of the enzyme to bind other substrates. Therefore, the concerted hypothesis cannot account for negative cooperativity. 

This outcome was consistent with a hypothesis that structural deterioration could have been a byproduct of microorganism activity. The higher lipid content in the poorly preserved tissue suggests that those lipids are primarily extrinsic, that is, that they were produced by bacteria and/or fungi. As the food source for such microorganisms, the protein within the bone may have been substantially altered in concert with the microstructure deterioration. The quantification of the changes to the organic fraction became our next focus of research. 

Nickel(O) reacts with the olefin to form a nickel(0)-olefin complex, which can also coordinate the alkyl aluminum compound via a multicenter bond between the nickel, the aluminum and the a carbon atom of the trialkylaluminum. In a concerted reaction the aluminum and the hydride are transferred to the olefin. In this mechanistic hypothesis the nickel thus mostly serves as a template to bring the olefin and the aluminum compound into close proximity. No free Al-H or Ni-H species is ever formed in the course of the reaction. The adduct of an amine-stabihzed dimethylaluminum hydride and (cyclododecatriene)nickel, whose structure was determined by X-ray crystallography, was considered to serve as a model for this type of mechanism since it shows the hydride bridging the aluminum and alkene-coordinated nickel center [31]. 

The observation that the transition state volumes in many Diels-Alder reactions are product-like, has been regarded as an indication of a concerted mechanism. In order to test this hypothesis and to gain further insight into the often more complex mechanism of Diels-Alder reactions, the effect of pressure on competing [4 + 2] and [2 + 2] or [4 + 4] cycloadditions has been investigated. In competitive reactions the difference between the activation volumes, and hence the transition state volumes, is derived directly from the pressure dependence of the product ratio, [4 + 2]/[2 + 2]p = [4 + 2]/[2 + 2]p=i exp —< AF (p — 1)/RT. All [2 + 2] or [4 + 4] cycloadditions listed in Tables 3 and 4 doubtlessly occur in two steps via diradical intermediates and can therefore be used as internal standards of activation volumes expected for stepwise processes. Thus, a relatively simple measurement of the pressure dependence of the product ratio can give important information about the mechanism of Diels-Alder reactions. 

STOCs arise from the concerted activation of up to 100 Ca2+-activated K+ channels (KCa) in the sarcolemma as a consequence of Ca2+ release from the SR. This release may take the form either of non-propagating focal events such as sparks or puffs , or of more regenerative Ca2+ waves. Since a temporal correlation between them exists, STOCs have been attributed to focal nonpropagating Ca2+ release events. However, this view requires that each STOC is a spatially restricted membrane current that occurs at selected areas of membrane closely apposed to the SR. While essential to the prevalent hypothesis for STOCs,... 

One sees that both proponents of the idea that substitutions at sulfinyl sulfur involve a concerted mechanism (154a) and those who favor a stepwise process (154b) where an intermediate is present on the reaction coordinate can find experimental results to support their point of view. In Section 9 in discussing substitutions at sulfonyl sulfur we will present a tentative hypothesis which we believe can rationalize the seemingly conflicting experimental evidence. 

Just as was true for substitutions at S=0, and as will also be true for substitutions at S02 (Section 9), the experimental results do not clearly resolve the question of whether substitutions at sulfenyl sulfur are best represented as concerted (173a) or stepwise (173b). Proponents of either pathway can find certain results that seem to support their viewpoint and others that are difficult to reconcile with it. In Section 9, in connection with discussion of the same problem vis-a-vis substitutions at sulfonyl sulfur, we shall present a tentative hypothesis that we think may provide a means of reconciling the seemingly contradictory experimental results within a single conceptual framework and that is applicable to substitutions at sulfur in all of the different oxidation states. 

Skell s hypothesis proved to be extremely useful in carbene chemistry even though it was frequently opposed. The principal significance of these rules is represented in the scheme below. The singlet reaction occurs in a concerted step, the cis-addition product being formed in a stereospeciiic manner. In the triplet addition, which is a two-step reaction, rotation is thought to be faster than intersystem crossing (spin inversion) and ring closure, i.e., which would... 

The experimentally observed substituent effect on the triplet and singlet quantum yields in the complete series of methyl-substituted dioxetanes, as well as the predicted C—C and 0—0 bond strength for the four-membered peroxidic rings , have led to the hypothesis that a more concerted, almost synchronized, decomposition mechanism should lead to high excitation quantum yields (as in the case of tetramethyl-l,2-dioxetane), whereas the biradical pathway presumably leads to low quantum yields (as in the case of the unsubstituted 1,2-dioxetane)" . However, it appears that this criterion of concertedness is difficult to apply generally to structurally dissimilar dioxetane derivatives. 

There is extensive clinical evidence that NE plays a role in hiunan anxiety. Well-designed psychophysiological studies have been conducted that have documented heightened autonomic or sympathetic nervous system arousal in combat veterans with chronic PTSD. Because central noradrenergic and peripheral sympathetic systems function in concert (Aston-Jones et al. 1991), the data from these psychophysiology investigations are consistent with the hypothesis that noradrenergic hyperreactivity in patients with PTSD may be associated with the conditioned or sensitized responses to specific traumatic stimuli. 

The hypothesis of a continuous transition of the elimination mechanism from the extreme El through concerted E2 to the other extreme ElcB with the change of reactant structure and catalyst nature, described in Sect. 2.1, can be easily adopted for dehalogenation also. The data summarised in Sect. 2.4.3 show some inconsistencies but the over-all picture is clear. This can be demonstrated for some selected examples. 

In the case of 2-(trimethylsilyl)thiazole (83TL2901) the acylation of nitrogen and desilylation at C-2 are likely to be concerted giving rise directly to a A-acylthiazolium 2-ylide which then evolves to the 2-acylthiazole by a multi-step process (Scheme 41). This hypothesis is supported by a mechanistic study of the reaction of the same silylated thiazole with aldehydes (93JOC3196). 

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