A Mononuclear Scenario for the Copper-Catalyzed Monooxygenation of Phenolic Substrates

Area of Science:

• Bioinorganic Chemistry
• Computational Chemistry
• Enzyme Mechanisms

Background:

• Tyrosinase enzymes, a type 3 copper enzyme, utilize a dinuclear pathway for phenol monooxygenation.
• Small-molecule copper complexes offer simplified models to study reaction mechanisms.
• Understanding these mechanisms is crucial for bioinorganic chemistry and enzyme catalysis.

Purpose of the Study:

• To investigate the feasibility of a mononuclear pathway for phenol monooxygenation in small-molecule copper systems.
• To elucidate the intermediates and transition states involved in this mononuclear mechanism.
• To compare the identified mechanism with known biological processes, such as topaquinone (TPQ) cofactor biosynthesis.

Main Methods:

• Density Functional Theory (DFT) calculations were employed to model five different copper complexes.
• Detailed analysis of intermediates and transition states was performed using DFT.
• Mass spectrometry/Mass spectrometry (MS/MS) experiments with ¹⁸O₂ isotope labeling were used for mechanistic confirmation.

Main Results:

• DFT calculations confirmed the energetic feasibility of a mononuclear pathway for phenol monooxygenation.
• Key intermediates and transition states of the mononuclear mechanism were identified.
• A copper-geminal diolate species was observed, confirming the hydroxylation step and providing insights into TPQ biosynthesis.

Conclusions:

• A mononuclear pathway is a viable mechanism for phenol monooxygenation in small-molecule copper systems.
• The identified mechanism shares similarities with the non-catalytic biosynthesis of the TPQ cofactor in amine oxidase (AO).
• The study provides a comprehensive understanding of copper-mediated hydroxylation reactions and their biological relevance.