Oxidative Half Cycle

Wilmot et al. (1999) have reported high resolution crystal structures of three species relevant to understanding the chemistry of the oxidative half cycle of amine oxidases, (i) anaerobic substrate-reduced ECAO, (ii) anaerobic substrate-reduced ECAO with bound nitric oxide (an oxygen mimic) and (iii) ECAO reacted aerobically with substrate to reach an equilibrium turnover state, then cryo-trapped. In all these species, product aldehyde remains bound at the back of the substrate binding pocket and this seems to be crucial in allowing build up of intermediates. 

In the nitric oxide complex, electron density from a diatomic molecule is seen at the position previously occupied by the axial water (Wa) and bridging to 02 of the aminoquinol form of TPQ. The distance of the nitric oxide to copper is long (2.4), suggesting a rather weak interaction. Thus it is possible that during the oxidative half cycle, dioxygen binds close to the aminoquinol 02 and is reduced there to superoxide as suggested by Su and Klinman (1998). 

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Fig. 13.11. Poly(vinyiferrocene) break-in first oxidation half cycle, first step (coupled elec-tron/ion transfer). In this and subsequent figures, the darker arrow(s) represent(s) the step(s) under discussion and the lighter arrow(s) represent(s) the preceding step(s). Also, shaded circles represent species that either have been or are presently being accessed.

Fig. 13.12. Poly(vinyiferrocene) break-in first oxidation half cycle, second step (reconfiguration). Reconfiguration is assumed faster than solvation in the upper cube.

Fig. 13.14. Poly(vinylferrocene) break-in completion of first oxidation half cycle, by a second reconfiguration followed by a second solvation step, to form the most stable reduction product (doubly solvated, doubly reconfigured O ).

Electrodes. The first observations of SERS were made from species on silver electrodes. An essential prerequisite for a strong SERS signal from molecules adsorbed on the electrode, surface roughness, is obtained by running electrochemical oxidation-reduction cycles (ORC) (Chang and Laube, 1984 Koglin and Sequaris, 1986 Roth et al., 1993). During the oxidation half cycle a metal salt, usually a halide, is formed at the electrode surface ... 

The oxidative half cycle has been little studied but it is likely that dioxygen binds to Cu+ and electron transfer occurs from both Cu+ and from Tyr 272 to generate hydrogen peroxide and restore the activated form of the enzyme (Cu and the Tyr 272 cation radical). The source of the two hydrogens in the product peroxide is unknown (cfd amine oxidases, section 3.2 where the source of the hydrogens can be assigned with reasonable confidence). 

The catalytic mechanism of amine oxidases can be fonnally divided into reductive and oxidative half cycles... 

Typical cyclic voltammograms for a silver electrode in 0.1 M KC1 and in 0.1 M K2S04 supporting electrolytes are shown in Fig. 10. These are produced under ORC conditions typically used for SERS activation. During the oxidation half-cycle, the Ag° electrode surface is oxidized to Ag+, which forms an insoluble layer of AgCl or Ag2S04 on the surface. The amount of... 

We can of course suggest other pathways for the RJOp interconversion, depending on whether the electron transfer occurs first, second, or third in the reaction sequence. The possibilities are outlined in Table 1.2. For the oxidation half-cycle, there are six possible pathways. There are another six pathways for the reverse Op— R ... 

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