Reductive Half Cycle

Reductive half cycle Eox + RCH2NH2 — E d + RCHO Oxidative half cycle E,.d + O2 + H2O — Eox + H2O2 + NH3 

Stopped flow kinetic studies by Hartmann and Klinman (1991) on bovine serum amine oxidase and by Olsson et al. (1976) on pig plasma amine oxidase showed that the substrate Schiff-base has an absorbance maximum at 350nm. 

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The above considered reactions model the reductive half cycle of GO where a primary alcohol is oxidized to an aldehyde with concomitant reduction of a (phe-noxyl)copper(II) complex to the reduced (phenol)copper(I) species. In the first two cases, reoxidation of the reduced catalyst was achieved by an external oxidant such as tris(4-bromophenyl)aminium or an electrode but not dioxygen. 

In the artificial system Figure 4b, a polymerized surfactant vesicle is substituted for the thylakoid membrane. Energy is harvested by semiconductors, rather than by PSI and PSII. Electron transfer is rather simple. Water (rather than C02) is reduced in the reduction half cycle to hydrogen, at the expense of benzyl alcohol. In spite of these differences, the basic principles in plant and mimetic photosyntheses are similar. Components of both are compartmentalized. The sequence of events is identical in both systems energy harvesting, vectorial charge separation, and reduction. 

The shape of the current-voltage curve for the first reduction half cycle depends on the time scale imposed on the system by the scan rate used in the cyclic voltammogram. We have not experimentally explored the scan... 

Figure 13.17 represents the expected path for intermediate time scales. During the first reduction half cycle, the electron/ion transfer process would produce R , which would reconfigure to Rb and de-solvate to Rb. 

Assuming a ping-pong mechanism, the different steps in the reductive half cycle (Figure 5) can be described as follows. 

It will be seen from Figure 13 that copper is not invoked for the catalytic steps of the reductive half cycle and an unresolved issue is whether the copper-free form of amine oxidase can catalyse the reactions leading to the aminoquinol form of the TPQ cofactor and release of product aldehyde. For the lentil seedling amine oxidase (Rinaldi et al., 1989), it has been reported that the copper-free form can catalyse the reactions of the reductive half cycle but that the presence of copper is required for the TPQ semiquinone species to be observed (Bellelli et al., 1985). Further work is needed to establish fully the catalytic properties of apo-forms and metal substi-tuted-forms of amine oxidases. 

During the reduction half cycle, a roughened silver surface is reformed by Ag" -I- e Ag°. Scanning electron microscopy (SEM) of electrode surfaces after this oxidation-reduction procedure have revealed that the initially smooth surfaces have acquired bumps on the scale of 25 to 500 nm These bumps can be approximated as speres, hemispheres and prolate spheroids... 

In most cases, stability and oxidation potential issues mean that polythiophenes are prepared, studied, and used in non-aqueous media, and the influence of the solvent can be significant. The extent of solvent swelling is determined by polymer-solvent interactions, which can be manipulated both by choice of solvent and introduction of substituents, here in the 3- and 4-positions. Electropolymerization of alkyl-substituted thiophenes provides good examples of the effects one can observe. EQCM (with PBD) measurements of potentiodynamic deposition from acetonitrile benzonitrile (4 1) of poly(didodecylterthiophene) (with the substituents in the 3, 4 - or 3,3"-positions) showed that film deposition occurred primarily during the reductive half-cycle, in the form of short chain oligomers of the... 

Since we have two possibe initial states R and R, there are a total of four possible pathways by which the redox transformation can be accomplished in a half-cycle. Correspondingly for the reduction half-cycle, we can write... 

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