Catechol oxidase model compounds based on aminocarbohydrates: new structure types and investigations on the catalytic reaction

Mannich base Cu(II) complexes as biomimetic oxidative catalyst

Galactose Oxidase (GOase) and catechol oxidase (COase) are the metalloenzymes of copper having monomeric and dimeric sites of coordination, respectively. This paper summarizes the results of our studies on the structural, spectral and catalytic properties of new mononuclear copper (II) complexes [CuL(OAc)] (1), and [CuL₂] (2), (HL = 2,4‑dichloro‑6‑{[(2'‑dimethyl‑aminoethyl)methylamino]methyl}‑phenol) which can mimic the functionalities of the metalloenzymes GOase and COase. The structure of the compounds has been elucidated by X-ray crystallography and the mimicked Cu(II) catalysts were further characterized by EPR. These mimicked models were used for GOase and COase catalysis. The GOase catalytic results were identified by GC-MS and, analyzed by HPLC at room temperature. The conversion of benzyl alcohol to benzaldehyde were significant in presence of a strong base, Bu₄NOMe in comparison to the neutral medium. Apart from that, despite of being monomeric in nature, both the homogeneous catalysts are very prone to participate in COase mimicking oxidation reaction. Nevertheless, during COase catalysis, complex 1 was found to convert 3,5‑ditertarybutyl catechol (3,5-DTBC) to 3,5‑ditertarybutyl quinone (3,5-DTBQ) having greater rate constant, k_cat or turn over number (TON) value over complex 2. The generation of reactive intermediates during COase catalysis were accounted by electrospray ionization mass spectrometry (ESI-MS). Through mechanistic approach, we found that H₂O₂ is the byproduct for both the GOase and COase catalysis, thus, confirming the generation of reactive oxygen species during catalysis. Notably, complex 1 having mono-ligand coordinating atmosphere has superior catalytic activity for both cases in comparison to complex 2, that is having di-ligand environment.

Hydroxide-bridged dicopper complexes: the influence of secondary coordination sphere on structure and catecholase activity

This work illustrates the syntheses, structures and catecholase activities of dicopper(ii) complexes having a Cu(μ-OH)Cu core encased within a secondary coordination sphere intricately created by appended heterocyclic rings.

Catecholase activity, DNA binding and cytotoxicity studies of a Cu(ii) complex of a pyridoxal schiff base: synthesis, X-ray crystal structure, spectroscopic, electrochemical and theoretical studies

A Cu(ii) complex, [Cu(L¹H^py)Cl]₂(ClO₄)₂, of a tridentate pyridoxal Schiff base shows exceptionally high catalytic efficiency (k_cat = 3.46 × 10⁵ h⁻¹) for catecholase activity in MeOH solution, the highest value reported in literature so far.

The ODN probes conjugating the Cu(II) complex enhance the luminol chemiluminescence by assembling on the DNA template

Potent peroxidase-like activity of the β-ketoenamine (1)-dicopper (II) complex (2) for the chemiluminescence (CL) of luminol either in the presence or absence of H(2)O(2) has been previously demonstrated by our group. In this study, the β-ketoenamine (1) as the ligand unit for copper(II) was incorporated into the oligonucleotide (ODN) probes. It has been shown that the catalytic activity of the ODN probes conjugating the ligand-Cu(II) complex is activated by hybridization with the target DNA with the complementary sequence. Thus, this study has successfully demonstrated the basic concept for the sensitive detection of nucleic acids by CL based on the template-inductive activation of the catalytic unit for CL.

Prospects of metal complexes peripherally substituted with sugars in biomedicinal applications

Metal complexes possess unique tunable properties, such as radioactivity, cytotoxicity or photophysical features, enabling them to act as diagnostic tracers or therapeutic agents. In applying them in biological systems, it is often necessary to enhance their solubility and biocompatibility. To achieve such goals, like the targeting of binding domains, transport systems and enzyme activities, the attachment of carbohydrate moieties appears to be suitable. Sugar-substitution in the periphery of metal complexes has therefore become a strongly growing field of research. Outlined herein is a selection of recent examples.

Trinuclear Copper(II) Complexes Derived from Schiff-Base Ligands Based on a 6-Amino-6-deoxyglucopyranoside: Structural and Magnetic Characterization

The trinuclear copper(II) complexes ([CuL1)(mu-ac)Cu(mu-ac)CuL1) (1) and ([CuL2)(mu-ac)Cu(mu-ac)CuL2) (2) of the tridentate aminosaccharide-derived Schiff-base ligands H2L1 [6-N-(salicylidene)amino-6-deoxy-1,2,3-tri-O-methyl-alpha-D-glucopyranoside] and H2L2 [6-N-(3,5-di-tert-butylsalicylidene)amino-6-deoxy-1,2,3-tri-O-methyl-alpha-D-glucopyranoside] were synthesized and structurally characterized. The trinuclear complex units can be described as two terminal copper-ligand moieties bridged by a central copper acetate moiety, with the Cu centers arranged in a triangular fashion. IR and UV/vis spectroscopic studies strongly indicate that the trinuclear structure is maintained in a methanolic solution. The temperature dependence of the magnetic susceptibility of both complexes shows a moderate antiferromagnetic coupling and can be well interpreted by applying a symmetric Cua-Cub-Cua' model with linear spin topology. The fit of the magnetic data affords coupling constants J of -34 and -24 cm(-1) for 1 and 2, respectively [H = -J(SaSb + SbSa')]. For mu-alkoxo-mu-acetato-bridged copper(II) complexes with a large dihedral angle between the adjacent coordination planes, as found in 1 and 2, such an antiferromagnetic coupling is unusual. However, density functional theory calculations of 2 using BP86, B3LYP*, and B3LYP density functionals confirmed a symmetric doublet ground state.

Binuclear copper(II) complexes of 5-N-(β-ketoen)amino-5-deoxy-1,2-O-isopropylidene-α-d-glucofuranoses: synthesis, structure, and catecholoxidase activity

The synthesis of 5-amino-5-deoxy-1,2-O-isopropylidene-alpha-D-glucofuranose (8) was carried out via 5-azido-5-deoxy-1,2:3,4-O-diisopropylidene-alpha-D-glucofuranose (6), its reduction with Raney-Nickel and deprotection. 5-N-(beta-Ketoen)amino-5-deoxy-1,2-O-isopropylidene-alpha-D-glucofuranoses (8a-f) were synthesized from 5-amino-5-deoxy-1,2-O-isopropylidene-alpha-D-glucofuranose and beta-ketoenolethers leading to ligands with symmetrically substituted double bonds (8a, 8b) and e/z isomeric mixtures with unsymmetrical substitution (8c-f). Reaction of the ligands with Cu(II) ions leads to binuclear complexes of the general formula Cu2L2. In contrast to copper(II) complexes which are not derived from amino carbohydrates the metal centers in the compounds saturate their coordination sphere by complexation of additional solvent molecules, interaction with neighboring complex molecules, or free hydroxyl groups of the own ligand. Residues of the ketoen moiety, R1 and R2, also influence the electronic properties of the metal centers. The combination of factors leads to different catalytic properties of the complexes in catecholoxidase-like reactions.