Effects of Metal Ions on the Electronic, Redox, and Catalytic Properties of Cofactor TTQ of Quinoprotein Amine Dehydrogenases

Abstract

Model compounds of novel organic cofactor TTQ (tryptophan tryptophylquinone) of quinoprotein methylamine and aromatic amine dehydrogenases have been shown to interact with a series of metal ions in anhydrous organic media. Spectroscopic analyses including UV−vis, NMR, and resonance Raman indicate that the metal ion binds to the TTQ model compounds at their o-quinone moiety, the binding mode of which is similar to that proposed for the interaction between cofactor TTQ and a cationic species in the native enzymes. The binding constants K ML for the metal ion complexes of TTQ model compounds have been determined from the spectral changes in UV−vis due to the complex formation. Remarkable enhancement of the oxidizing ability of the TTQ model compounds by the complexation with metal ions has been demonstrated as a large positive shift in the one-electron reduction potentials E 0 red of the complexes as compared to those of the TTQ model compounds in metal free form (e.g. ΔE 0 red = 1.17 V for the Mg2+ complex and ΔE 0 red = 1.16 V for the Sc3+ complex). The complexes can oxidize not only benzylamine but also aliphatic amines in anhydrous acetonitrile, whereas no reaction takes place in the absence of the metal ion under otherwise the same experimental conditions. Kinetic studies have revealed that the reaction proceeds via a transamination mechanism involving iminoquinone and product imine intermediates to yield the oxidized products and the reduced TTQ. The ESR spectra of the half-reduced species of TTQ model compounds, i.e. semiquinone radical anions, are detected successfully. The spin distribution derived from the hyperfine structures indicates that the spin is partially delocalized on the indole ring connected at the 4-position of the quinone skeleton. This indicates that the indole group plays an important role as a part of the electron-transfer pathway from the reduced TTQ to a blue copper protein in biological systems.

Oxidation of Unactivated Primary Aliphatic Amines Catalyzed by an Electrogenerated 3,4‐Azaquinone Species: A Small‐Molecule Mimic of Amine Oxidases

“…Substantial mechanistic information has been obtained through catalytic aerobic deamination of benzylamine mediated by synthetic models of natural quinones which confirm the occurrence of the transamination mechanism 7. However, these models failed to oxidize unactivated primary aliphatic amines under the same experimental conditions, with the exception of a recent study showing that the metal ion complex of the TTQ model compound was able to oxidize aliphatic amines in anhydrous organic media, whereas no reaction took place in the absence of the metal ion 8…”

Environmentally Friendly Chemoselective Oxidation of Primary Aliphatic Amines by Using a Biomimetic Electrocatalytic System

“…In contrast, no reaction took place in the absence of the metal ion. [9] A few years ago, we showed that electrogenerated o-azaquinone 1 ox (Scheme 1) acted as an effective biomimetic catalyst for the oxidation of benzylamine under metal-free conditions, through the pyridoxal-like transamination process reported for amine oxidase cofactors. The catalytic cycle produced the reduced catalyst 1 red and N-benzylidenebenzyl-A C H T U N G T R E N N U N G amine as the product of amine oxidation.…”

Catalytic control of electron-transfer processes

“…The Lewis acidity of metal ion has been shown to control not only the electron-transfer step, but also the subsequent chemical step in the overall redox reactions. There are more examples for the catalytic control of electron-transfer reactions of metal ions, other than those described in this article [30][31][32][33]. There still remains a wealth of important fundamen- tal questions with regard to catalytic control of electron-transfer reactions, which have been only partially explored so far, and which certainly deserve much more detailed attention in the future.…”