Unusual kinetics

Gazaryan LA. Lagrimini L.M. (1996) Purification and unusual kinetic properties of a tobacco anionic peroxidases // Phytochemistry. V. 41. P. 1029-1034. 

The analogous quantitative oxidation of methanolic allyl alcohol to 2,3-di-bromopropan-l-ol shows unusual kinetics viz. 

Y(03SCF3)3 to afford a monoaminoalkylation product in good yield in aqueous media.40 Zinc tetrafluoroborate is also highly effective for such couplings in aqueous THF.41 Kobayashi also reported a Mannich-type reaction of imines with silyl enolates catalyzed by neutral salts such as sodium triflate in water as a suspension medium. Unusual kinetic behavior indicates that the presence of the Mannich adduct facilitates the rate of its formation.42... 

It is important to note that, even when the coordination geometry prescribed by the macrocyclic cavity is ideal for the metal ion involved, unusual kinetic and thermodynamic properties may also be observed (relative to the corresponding open-chain ligand complex). For example, very often the macrocyclic complex will exhibit both enhanced thermodynamic and kinetic stabilities (kinetic stability occurs when there is a reluctance for the ligand to dissociate from its metal ion). These increased stabilities are a manifestation of what has been termed the macrocyclic effect - the multi-faceted origins of which will be discussed in detail in subsequent chapters. 

An additional surprise is, that the reaction rates of these spontaneous back-reactions 153 - 152 (X = Br, Cl) are temperature-independent between 8.5 and 25 K. In our opinion this unusual kinetic behavior might be the outcome of a double two-state reaction involving a surface crossing with the triplet state of... 

Since the discovery of the exceptional basicity of l,8-bis(dimethylamino)-naphthalene (Alder et al., 1968) and its unusual kinetic behaviour (Hibbert, 1973, 1974, 1975) there has been considerable interest in the acid-base properties of hindered diaminonaphthalenes. 1,8-Bis(dimethylamino)naph-thalene (pK — 12.1 at 25°C and ionic strength 0.1 mol dm-3) (Alder et al., 1968 Hibbert, 1974 Chiang et al., 1980) is more basic than most aliphatic amines and the pAT-values of the partially methylated diamines [52] illustrate the dramatic effect of introducing the fourth methyl group (Alder et al., 1968). Reaction of protonated l,8-bis(dimethylamino)naphthalene with... 

A possible mechanism which explains the unusual kinetic behaviour is given in Scheme 9. According to this two routes from reactant to product are... 

The latter reaction frequently has negligible activation energy and an anomalously low pre-exponential factor (Adamic et al., 1971) such unusual kinetic behaviour may lead to dramatic changes in the relative concentrations... 

An unusual kinetic result has been reported51 when phenylhydroxylamine reacted anaerobically with bisulphite anion. The product distribution was as expected, i.e. both 2- and 4-aminophenol and the 2- and 4-aminobenzenesulphonates were formed. Kinetic measurements showed a first-order dependence upon [bisulphite], in contrast to the earlier work with Cl- and later with N3 -. The authors propose a mechanism involving direct attack by the nucleophile at the 2- and 4-positions as the rate-limiting step, followed by proton transfers and solvent attack to form the sulphonate products. 

Slow transitions produced by enzyme isomerizations. This behavior can lead to a type of cooperativity that is generally associated with ligand-induced conformational changes . A number of enzymes are also known to undergo slow oligomerization reactions, and these enzymes may display unusual kinetic properties. If this is observed, it is advisable to determine the time course of enzyme activation or inactivation following enzyme dilution. See Cooperativity Bifurcation Theory Lag Time... 

The unusual kinetics observed for proton transfer to a hydride ligand in the cluster [W3S4(PH3)6H3]+ has been interpreted by a mechanism that includes two... 

Solvent effects—highly unusual kinetic effects have been found to accompany solvent change, such as bell-shaped kinetic profiles in DMSO-H2O mixtures . 

C. Many allosteric enzymes have multiple subunits whose interaction accounts for their unusual kinetic properties. 

In the late 1960s, two groups reported results that indicated the possibility of more than one form of MAO. In 1967, Maitre suggested that tissue (brain vs liver)-dependent potencies of inhibitors could be explained by the presence of different forms of MAO [4]. In 1968, Johnston published an analysis of unusual kinetics observed with the new MAO inhibitors that were being developed. He concluded that the most reasonable hypothesis to explain such results as double sigmoid curves was the existence of at least two forms of the enzyme [5]. By the mid-1980s, differences of substrate and inhibitor specificity of two MAOs, now called MAO A and B, were well understood [6]. 

Only a few trihydroxo-bridged chromium(III) species are known. The (tacd)Cr(OH)3Cr(tacd)3+ and the (metacn)Cr(OH)3Cr(metacn)3+ ions are unusually kinetically and thermodynamically stable in acidic solution, i.e., they do not undergo hydroxo-bridge cleavage in 1 M... 

The first example of a free-radical chain reaction successfully conducted in sc C02, which demonstrated the potential of this solvent for preparative scale chemistry, was a report from the McHugh group (Suppes et al., 1989) dealing with the oxidation of cumene (eq. 4.4). The propagation steps for this reaction are depicted in Scheme 4.11. Pressure (and thus viscosity) had little effect on the initiation, propagation, or termination rate constants. No unusual kinetic behavior was observed near the critical point. 

The number of intermediates involved and the number of possible ratedetermining stages together make nitrosation a reaction of unusual kinetic complexity, but, for many nitrosation reactions, the contributions of the separate reaction paths have been clearly established and the corresponding rate-determining stages have been identified. 

The analogous considerations are valid for polymer systems as well. Indeed, amphiphilic monomer units also tend to occupy interfacial areas of macromolecular associates as it is normal for low molecular weight surfactants to adsorb at polymer-poor solvent boundaries. And, if such interfacial groups of the polymer associate catalyze chemical transformation of a compound which tends to adsorb at the associate interfaces, this can result in unusual kinetics effects. Okhapkin et al. [18] studied the influence of temperature-induced aggregation on the catalytic activity of thermosensitive... 

Most P450 oxidations and drug interactions can be predicted from inhibition studies, since most P450 inhibitors show competitive Michaelis-Menten kinetics. However, there are examples of unusual kinetics, and most of these are associated with CYP3A oxidations. In this chapter, both Michaelis-Menten kinetics and more complex kinetics will be discussed. General experimental protocols that can be used to obtain and analyze kinetic data will be presented, and the implications of the results when predicting drug interactions will be discussed. 

If enzyme activation and the other unusual kinetic characteristics result from multiple substrates in the active site, kinetic parameters will be difficult to characterize and drug interactions will be more difficult to predict, since they are a function of the enzyme and of both the substrates. In addition, there are some indications that non-Michaelis-Menten kinetics can be seen in vivo (27-29). 

Uglem, G. L., Dupre, R. K. Harley, J. P. (1983). Allosteric control of pyrimidine transport in Hymenolepis diminuta an unusual kinetic isotope effect. Parasitology, 87 289-93. 

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