Molecular structure and catechol oxidase activity of a new copper(I) complex with sterically crowded monodentate N-donor ligand Ádám Kupán a , József Kaizer a , Gábor Speier a, * , Michel Giorgi b , Marius Réglier c , Ferenc Pollreisz d a Department of Chemistry, University of Pannonia, H-8200 Veszprém, Hungary b Laboratoire de Cristallochimie et Laboratoire de Bioinorganique Structurale Université Paul Cézanne Aix-Marseille III F.S.T. Saint-Jérôme, Service 432, Avenue Escadrille Normandie-Niemen, 13397 Marseille cedex 20, France c Biosciences, Université Paul Cézanne Aix-Marseille III F.S.T. Saint-Jérôme, Service 432 Avenue Escadrille Normandie-Niemen, 13397 Marseille cedex 20, France d Institute of Structural Chemistry, Chemical Research Center of the Hungarian Academy of Sciences, Pusztaszeri út 59-67, H-1025 Budapest, Hungary article info Article history: Received 2 June 2008 Received in revised form 25 November 2008 Accepted 25 November 2008 Available online 6 December 2008 Keywords: Catechol Catechol oxidase Biomimetic oxidation Copper complexes N-ligand abstract The attempted alkylation of 1,3-bis(2 0 -pyridylimino)isoindoline (indH) by the use of n-BuLi and subse- quent alkyl halides led to quaternization of the pyridine nitrogens and the zwitterionic monodentate N-ligand (Me 2 ind)I was formed. By the use of the ligand the copper(I) complex ½Cu I ðMe 2 indÞI 2  was pre- pared and its structure determined. It was found to be good catalyst for the oxidation of 3,5-di-tert-butyl- catechol (DTBCH 2 ) to 3,5-di-tert-butyl-1,2-benzoquinone (DTBQ) and H 2 O 2 by dioxygen. Detailed kinetic studies revealed first-order dependence on the catalyst and dioxygen concentration and saturation type behavior with respect to the substrate. Ó 2008 Published by Elsevier Inc. 1. Introduction The degradation of aromatic compounds proceeds via succes- sive hydroxylations leading to catechols which are catabolized fur- ther through oxidative ring cleavage reactions [1–4]. The oxygenation/oxidation of catechols seems to be the crucial step in the degradation of aromatics (Scheme 1). Catechol dioxygenases have iron [5], copper [6] manganese [7– 9], or magnesium [10] ions at their active sites. Copper-containing dioxygenase enzymes [6] and model systems [11–16] mediate only intradiol scission of catechols. To the contrary catechol oxidases contain only dicopper centers with type 3 copper ions [17–19]. Well known representatives of type 3 copper proteins are hemocy- anin [20–26], the dioxygen carrier for arthropods, molluscs, and tyrosinase [27]. Catechol oxidase belongs, like tyrosinase, to the polyphenol oxidases, which oxidize phenolic compounds in the presence of dioxygen to the corresponding quinone [28,29]. Whereas tyrosinase catalyzes the hydroxylation of tyrosine to dopa (cresolase activity) and the oxidation of dopa to dopaquinone (catecholase activity) with electron transfer to dioxygen, but catechol oxidase exclusively catalyzes the oxidation of catechols to quinones without acting on tyrosine [30]. This reaction is of great importance in medical diagnosis for the determination of the hormonally active catecholamines adrenaline, noradrenaline, and dopa [31]. Secondary reactions (melanin formation) follow the oxidation of the substrate in the presence of polyphenol oxi- dase, which causes the brown color of the injured plants. The structures of the oxidized and reduced forms of the catechol oxidase, extracted from sweet potato, were determined by X-ray crystallography [17,32,33]. Both consist of a binuclear copper cen- ter that is coordinated by three histidine nitrogen donor atoms. In the oxidized form of the enzyme, the two copper(II) centers are bridged by a hydroxide group being the fourth ligand of the four- coordinated trigonal pyramid. Furthermore, the Cu(II)–Cu(II) dis- tance was found to be 2.9 Å. In the reduced form the Cu(I)–Cu(I) length increases to 4.4 Å. A water molecule coordinates to one of the copper with a distorted trigonal pyramidal geometry. The other copper possesses a square planar geometry with one empty coor- dination site. Notable advances in the understanding of the structural and chemical properties of this protein have been achieved by model studies of synthetic analogs. Furthermore, it would be interesting to know what factors influence selectivities between oxidase vs. dioxygenase actions of copper containing proteins or model com- pounds. They can serve as structural or functional mimics for catechol oxidases, or provide new biomimetic catalysts for oxida- tion reactions [34–58]. Studies concentrated mainly on substrate 0162-0134/$ - see front matter Ó 2008 Published by Elsevier Inc. doi:10.1016/j.jinorgbio.2008.11.015 * Corresponding author. Tel.: +36 88 624 657; fax: +36 88 624 469. E-mail address: speier.gabor@almos.uni-pannon.hu (G. Speier). Journal of Inorganic Biochemistry 103 (2009) 389–395 Contents lists available at ScienceDirect Journal of Inorganic Biochemistry journal homepage: www.elsevier.com/locate/jinorgbio