Reactions of copper(II)-phenol systems with O₂: Models for TPQ biosynthesis in copper amine oxidases

Copper(II) complexes supported by a series of phenol-containing bis(pyridin-2-ylmethyl)amine N₃ ligands (denoted as L^oH, L^mH, and L^pH) have been synthesized, and their O ₂ reactivity has been examined in detail to gain mechanistic insights into the biosynthesis of the TPQ cofactor (2,4,5- trihydroxyphenylalaninequinone, TOPA quinone) in copper-containing amine oxidases. The copper(II) complex of L^oH (ortho-phenol derivative) involves a direct phenolate to copper(II) coordination and exhibits almost no reactivity toward O₂ at 60 °C in CH₃OH. On the other hand, the copper(II) complex of L^mH (meta-phenol derivative), which does not involve direct coordinative interaction between the phenol moiety and the copper(II) ion, reacts with O₂ in the presence of triethylamine as a base to give a methoxy-substituted para-quinone derivative under the same conditions. The product structure has been established by detailed nuclear magnetic resonance (NMR), infrared (IR) spectroscopy, and electrospray ionization-mass spectroscopy (ESI-MS) (including ¹⁸O-labeling experiment) analyses. Density functional theory predicts that the reaction involves (i) intramolecular electron transfer from the deprotonated phenol (phenolate) to copper(II) to generate a copper(I)-phenoxyl radical; (ii) the addition of O₂ to this intermediate, resulting in an end-on copper(II) superoxide; (iii) electrophilic substitution of the phenolic radical to give a copper(II)-alkylperoxo intermediate; (iv) O-O bond cleavage concomitant with a proton migration, giving a para-quinone derivative; and (v) Michael addition of methoxide from copper(II) to the para-quinone ring and subsequent O₂ oxidation. This reaction sequence is similar to that proposed for the biosynthetic pathway leading to the TPQ cofactor in the enzymatic system. The generated para-quinone derivative can act as a turnover catalyst for aerobic oxidation of benzylamine to N-benzylidene benzylamine. Another type of copper(II)-phenol complex with an L^pH ligand (para-phenol derivative) also reacts with O₂ under the same experimental conditions. However, the product of this reaction is a keto-alcohol derivative, the structure of which is qualitatively different from that of the cofactor. These results unambiguously demonstrate that the steric relationship between the phenol moiety and the supported copper(II) ion is decisive in the conversion of active-site tyrosine residues to the TPQ cofactor.