Catalytic wave

Only three steps of the proposed mechanism (Fig. 18.20) could not be carried out individually under stoichiometric conditions. At pH 7 and the potential-dependent part of the catalytic wave (>150 mV vs. NHE), the —30 mV/pH dependence of the turnover frequency was observed for both Ee/Cu and Cu-free (Fe-only) forms of catalysts 2, and therefore it requires two reversible electron transfer steps prior to the turnover-determining step (TDS) and one proton transfer step either prior to the TDS or as the TDS. Under these conditions, the resting state of the catalyst was determined to be ferric-aqua/Cu which was in a rapid equilibrium with the fully reduced ferrous-aqua/Cu form (the Fe - and potentials were measured to be within < 20 mV of each other, as they are in cytochrome c oxidase, resulting in a two-electron redox equilibrium). This first redox equilibrium is biased toward the catalytically inactive fully oxidized state at potentials >0.1 V, and therefore it controls the molar fraction of the catalytically active metalloporphyrin. The fully reduced ferrous-aqua/Cu form is also in a rapid equilibrium with the catalytically active 5-coordinate ferrous porphyrin. As a result of these two equilibria, at 150 mV (vs. NHE), only <0.1%... 

Hidalgo et al. [509] reported a method for the determination of molybdenum (VI) in natural waters based on differential pulse polarography. The catalytic wave caused by molybdenum (VI) in nitrate medium following preconcentration by coflotation on ferric hydroxide was measured. For seawater samples, hexadecyltrimethylammomum bromide with octadecylamine was used as the surfactant. The method was applied to molybdenum in the range 0.7-5.7 Xg/l. 

Fe 2S], a [4Fe-4S] and a [3Fe-4S] center. The enzyme catalyzes the reversible redox conversion of succinate to fumarate. Voltammetry of the enzyme on PGE electrodes in the presence of fumarate shows a catalytic wave for the reduction of fumarate to succinate (much more current than could be accounted for by the stoichiometric reduction of the protein active sites). Typical catalytic waves have a sigmoidal shape at a rotating disk electrode, but in the case of succinate dehydrogenase the catalytic wave shows a definite peak. This window of optimal potential for electrocatalysis seems to be a consequence of having multiple redox sites within the enzyme. Similar results were obtained with DMSO reductase, which contains a Mo-bis(pterin) active site and four [4Fe 4S] centers. 

FIGURE 5.1. Ping-pong mechanism, normalized catalytic wave. From left to right log c — oo,2, 1, 0, —oo. Adapted from Figure 3 of reference 9, with permission from Elsevier. 

It is also worth examining how the entire catalytic wave depends on kinetic control by the substrate and/or cosubstrate in terms of shape and location on the potential axis. From Figure 5.1 we see that there is a small positive shift of the wave as kinetic control passes from reaction (1) to reaction (2). The shape of the wave also changes, going from... 

FIGURE 5.10. Kinetic control by the enzymatic reaction. Normalized catalytic waves. From right to left log [feC l/fc.z + l/fci,2 + 1/fciCj)] = — oo,0,1,2,3. Adapted from Figure 2 of reference 20a, with permission from Elsevier. 

A is not reduced at the electrode at the potential where the catalytic wave occurs). 

The component waves in the voltammograms (a lower potential catalytic wave and a higher potential switch wave associated with the activation/inactivation of the enzyme) can be seen in Fig. 5.13. The positions of their inflection points were obtained as local extrema in the first derivative with respect to potential (Fig. 5.13, inset), switch corresponds to the potential of the reductive reactivation process. Figure 5.13 shows that as pH is increased Eswitch and both decrease. Furthermore, the pH dependence of Eswitch could be fitted to a 1H+ le stoichiometry with an apparent pK value of 7.7 and a potential at the alkaline limit of -166 mV. 

Figure 31 compares the currents of the catalytic waves obtained in identical experimental conditions with 2-P2Wi5Mo2Cu and 0 2-P2Wi7Cu respectively. The electrocatalytic process is less favorable with the latter complex. 

The classical dc polarography of vitamins A, B, B2, B6, BI2, and C, nicotinamide, tocopherols, and naphthoquinones has been reviewed [55]. Other studies have examined in detail the cyclic voltammetry of vitamin B12 employing rapid-scan voltammetry at the DME [90] and the HMDE [91]. Vitamin B12 is complexed with trivalent cobalt ion at the heterocyclic nitrogen atoms. As a result of the complexation, a catalytic hydrogen wave is formed for the compound. In addition to the catalytic wave, a wave corresponding to the reduction of the trivalent cobalt to the monovalent state is observed. 

Further evidence that cobalt metal is not responsible for the catalytic hydrogen waves comes from the observation that the higher the stability of the complex formed between the sulfur-containing ligand and Co2+, the more pronounced is the catalytic wave.378 This is confirmed since... 

The generally accepted,376 although not fully proven, mechanism for the catalytic wave is as shown in equations (107)-(109) and involves reduction to a zerovalent complex which is the catalytically active species. Whether the catalysis involves two or one electron steps (Co2+ or Co+) intermediates is not known but it is probable that the neutral zerovalent complex is adsorbed on the electrode surface.380,396... 

For those curves which do not fulfil this condition (e.g. catalytic waves), a calibration curve must be constructed. 

For catalytic waves of hydrogen evolution in ammoniacal cobalt solutions, it has been observed (132) that ery/Aro-phenylcysteine gives a higher catalytic wave than the threo form (Fig. 28). These differences can be explained partly by differences in acid dissociation constants, and partly by variations in the stability constants of the cobalt-phenyl-cysteine complexes. 

Fig. 28. Steric effects on catalytic waves of phenyl cysteins. To 0.001 M cobaltous chloride, 0.1 M ammonia, 0.1 M ammonium chloride added 3 X 10-5iMT erylhro-and ttra>-phenylcystein. Curves starting at —0.8 V S.C.E., 200 mV/absc., full scale sensitivity 28 p.A...

As seen in Figure 3b-d, the catalytic wave shows a small but distinct decrease upon addition of the nucleophile. 

Sometimes more waves are visible on the polarograms than can be realized by macroelectrolysis. Some may be catalytic waves, and sometimes it is found that the product from the first reduction does not give these waves. In these cases a tautomeric change similar to that described later in the reduction of some cyclic azines may be operating so that it is the primarily formed species which is responsible for the observed waves, whereas the more stable tautomeric form is reduced by another route. 

As the polarographic curve, furthermore, can be influenced by certain compounds or inhibitors, adsorption phenomena can complicate the interpretation of the curves, and catalytic waves may suggest further reductions than those found by macroelectrolysis, a certain caution must be exercised in evaluating the voltammetric data. In most cases, however, no complications arise, and with a little experience the differences mentioned above are not serious drawbacks, but are of value as the combination of polarography and macroelectrolysis then throws light on one or more of the steps in the reaction. 

A sharp and intense cathodic peak was observed at 0.2V in the CVs of Co(4-TCPyP) modihed glassy carbon electrodes [Fig. 42(a)] in O2 saturated aqueous solution, in contrast with the bare electrode. Such a catalytic wave is much more intense than those associated with the Ru(III)Ru(III)Ru(III)/Ru(III)Ru (III)Ru(II) and Co(III/II)P redox pairs and exhibits a diffusion controlled pattern, showing the predominant contribution of the dioxygen reduction process. The... 

Anodic catalytic waves for hydroquinone were observed at a rotating gold elec-tode, confirming the regeneration mechanism with a rate constant for Phenidone regeneration of 1.3 x 10 L mol s [57]. 

The heat of oxidation (heat evolution) of trans-3-t-butylthietane 1-oxide by perlauric acid is 1.5 . 9 kcal/mole, greater than that for the cis isomer as expected on the basis of the puckered ring. In N sulfuric acid, no catalytic wave corresponding to hydrogen evolution was observed in the electrolysis of thietane 1-oxide, although some acyclic sulfoxides did give one. ... 

---