EC-type mechanism

The thin layer cyclovoltammogram of 35a showed two independent oxidation processes (i) an irreversible wave at Epa = 1034 mV (vs. Fc/Fc+) (fcpi = -232 mV (vs. Fc/ Fc+)) and (ii) a reversible wave (E1//2 (het-ox), which corresponds to the formation of the radical cation of the dibenzodioxin subunit (Figure 28). The irreversible wave represents a two-step process involving a one-electron oxidation of the DHA subunit followed by a chemical step (EC-type mechanism) leading to a significant change in the molecular structure. Since polyenic radical cations have a preference for dimerization, 34 it is reasonable to speculate on the formation of the dimeric dication species as shown in structure 41. The chemical reversibility of this EC-type process was confirmed by multisweep thin layer experiments. 

A well-defined irreversible anodic peak was noted in the CV at 1.44 V vs. Ag/AgCl, for naproxen in 0.1 M LiC104 containing CH3CN on BDD. The electrochemical process is diffusion-controlled, and the number of electrons involved in the rate-determining step is equal to one. A slope of 30.8 mV per decade was obtained fi"om the plot of log Ep vs. log sweep rate, which indicates that the rate control is first order following electron transfer. From this, the electrochemical reaction can he expressed by a EC type mechanism, where one-electron oxidation of naproxen proceeds with the formation of a cation radical intermediate in the aromatic rings, followed hy the rapid protonation from the side chain. 

In 1973 two papers appeared almost simultaneously (73T101, 73CPB2026) describing the formation, as a minor product, of 3,4,5-trimethoxycarbonyl-l-phenylpyrazole (346) in the reaction between benzaldehyde phenylhydrazone and DMAD (EC=CE). To account for the formation of (346) George et al. (73T101) proposed a tentative mechanism (Scheme 29) involving a Diels-Alder reaction of type (a Figure 25), followed by a retro-Diels-Alder elimination of methyl phenylpropiolate (347). 

Diabetic patients have reduced antioxidant defences and suffer from an increased risk of free radical-mediated diseases such as coronary heart disease. EC has a pronounced insulin-like effect on erythrocyte membrane-bound acetylcholinesterase in type II diabetic patients (Rizvi and Zaid, 2001). Tea polyphenols were shown to possess anti-diabetic activity and to be effective both in the prevention and treatment of diabetes (Choi et al, 1998 Yang et al, 1999). The main mechanism by which tea polyphenols appear to lower serum glucose levels is via the inhibition of the activity of the starch digesting enzyme, amylase. Tea inhibits both salivary and intestinal amylase, so that starch is broken down more slowly and the rise in serum glucose is thus reduced. In addition, tea may affect the intestinal absorption of glucose. 

Although the absolute configurations of the products are opposite to that of antiinflammatory active compounds, and the substrate specificity is rather restricted as to the steric bulkiness around the reaction center, the enzyme system of A. bronchisepticus was proved to have a unique reactivity. Thus, detailed studies on the isolated enzyme were expected to elucidate some new interesting mechanism of the new type of decarboxylation. Thus, the enzyme was purified. (The enzyme is now registered as EC 4.1.1.76.) The molecular mass was about 24kDa. The enzyme was named as arylmalonate decarboxylase (AMDase), as the rate of the decarboxylation of phenylmalonic acid was faster than that of the a-methyl derivative. ... 

Electrode reactions coupled to adsorption eqtrilibria and chemical reactions are among the most complex mechanisms treated in the theory of SWV. In the literature published so far, four types of adsorption coupled EC mechanisms have been considered [86,128-130]. In all cases, the follow-up chemical reaction is an irreversible process (Ci). The simplest case is an ECi mechanism with adsorption of the reactant only [86] ... 

Assuming, as in Sect. 2.1.2, that the current perturbation is purely faradaic, the surface concentrations given for the mechanisms with only one charge transfer reaction (CE, EC, catalytic) contain two types of function... 

A number of lyases are known which, unlike the aldolases, require thiamine diphosphate (TDP) as a cofactor in the transfer of acyl anion equivalents [389-391], but proceed via enolate-type intermediates by a mechanism that resembles the classical benzoin addition. The most important representative is the transketolase (EC 2,2.1.1) [392] which stems from the oxidative pentose... 

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