Ligand dependence in the copper-catalyzed oxidation of hydroquinones

doi:10.1016/j.abb.2004.11.025

Archives of Biochemistry and Biophysics

Volume 435, Issue 1, 1 March 2005, Pages 21-31

https://doi.org/10.1016/j.abb.2004.11.025 Get rights and content

Abstract

Transition metal-mediated oxidation of hydroquinones is an important physiologic reaction, and copper(II) effectively catalyzes the reaction in phosphate-buffered saline (PBS). Studies reported herein in phosphate buffer alone demonstrate that copper(II) is an ineffective catalyst in the absence of coordinating ligands, but that 1,10-phenanthroline and histamine facilitate the copper(II)-mediated oxidation of hydroquinone and its 2,5- and 2,6-di-tert-butyl analogs to the corresponding benzoquinones. The high concentration of chloride in PBS is the key element that allows copper(II) to work in this system. Although the bis-bathocuproine disulfonate complex of Cu(II), (BC)₂Cu(II), is a strong stoichiometric oxidant, stoichiometric amounts of copper(II) in the presence of ligands other than BC oxidize hydroquinones very slowly under anaerobic conditions. Thus, the rapid copper(II)-catalyzed reaction operating aerobically does not involve a simple ping-pong reduction of copper(II) to copper(I) by hydroquinone and reoxidation of copper(I) by O₂.

Section snippets

Reagents

Unless otherwise stated the solvents and reagents were of commercially available analytical grade quality. 1,4-Hydroquinone, 2,5-di-tert-butylhydroquinone (2,5-DTBHQ), 2,6-di-tert-butyl-1,4-benzoquinone, BC, 1,10-phenanthroline (phen), and diethylenetriaminepentaacetic acid (DTPA) were obtained from Aldrich Chemical (Milwaukee, WI). 2,6-Di-tert-butylhydroquinone (2,6-DTBHQ) was prepared freshly by reducing the corresponding quinone according to a published procedure [19].

General procedures

All reactions were

Effect of 1,10-phenanthroline on copper-catalyzed oxidation of HQ

The rate of Cu(II)-catalyzed oxidation of hydroquinones can theoretically be monitored either by the consumption of O₂ or by the production of the corresponding 1,4-benzoquinone. The latter could not be followed on account of spectral interference by the added phen, and thus we relied on O₂ consumption as a general rate indicator in this study. A complication attendant the use of −d[O₂]/dt is whether any of the products of O₂ reduction (e.g., $O_{2}^{-}$ or H₂O₂) can reform O₂ in the time period of the

Conclusions

The work described here finds that the presence of a suitable ligand can allow Cu(II) to act as an efficient aerobic redox catalyst for HQ oxidation. Phen and histamine are suitable, and so is chloride, when used at a sufficiently high concentration to insure that a substantial fraction of free Cu(II)_aq is converted to the CuCl⁺ (or perhaps CuCl₂) complex. Thus, the presence of Cl⁻ in PBS is not an innocent component of the buffer. The simplest mechanism for redox catalysis (ping-pong) is

Acknowledgments

This work was supported in part by NIH Grants GM 48812, HL 53315, and AG 14249.

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¹: These two authors contributed equally to this work.

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Ligand dependence in the copper-catalyzed oxidation of hydroquinones

Abstract

Section snippets

Reagents

General procedures

Effect of 1,10-phenanthroline on copper-catalyzed oxidation of HQ

Conclusions

Acknowledgments

Arch. Biochem. Biophys.

Chem-Biol. Interact.

Chem-Biol. Interact.

Inorg. Chim. Acta

Inorg. Chim. Acta

Inorg. Chim. Acta

Free Radic. Biol. Med.

Arch. Biochem. Biophys.

Biochim. Biophys. Acta

Mutat. Res.

Tetrahedron Lett.

Biochim. Biophys. Acta

J. Biochem. Biophys. Methods

Free Radic. Biol. Med.

J. Biol. Chem.

J. Biol. Chem.

Arch. Biochem. Biophys.

Biochem. Biophys. Res. Commun.

Free Radic. Biol. Med.

Biochim. Biophys. Acta

Biochem. Biophys. Res. Commun.

Chem.-Biol. Interact.

Curr. Opin. Chem. Biol.

Arch. Biochem. Biophys.

Carcinogenesis

Carcinogenesis

Mol. Pharmacol.

Bull. Chem. Soc. Jpn.

Chem. Pharm. Bull.