Models for the active site in galactose oxidase: Structure, spectra and redox of copper(II) complexes of certain phenolate ligands

Galactose oxidase (GOase) is a fungal enzyme which is unusual among metalloenzymes in appearing to catalyse the two electron oxidation of primary alcohols to aldehydes and H₂O₂. The crystal structure of the enzyme reveals that the coordination geometry of mononuclear copper(II) ion is square pyramidal, with two histidine imidazoles, a tyrosinate, and either H₂O (pH 7.0) or acetate (from buffer,pH 4-5) in the equatorial sites and a tyrosinate ligand weakly bound in the axial position. This paper summarizes the results of our studies on the structure, spectral and redox properties of certain novel models for the active site of the inactive form of GOase. The monophenolato Cu(II) complexes of the type [Cu(L1)X][H(L1) = 2-(bis(pyrid-2-ylmethyl)aminomethyl)-4-nitrophenol and X⁻ = Cl⁻ 1, NCS⁻ 2, CH₃COO⁻ 3, ClO₄ ⁻ 4] reveal a distorted square pyramidal geometry around Cu(II) with an unusual axial coordination of phenolate moiety. The coordination geometry of 3 is reminiscent of the active site of GOase with an axial phenolate and equatorial CH₃COO⁻ ligands. All the present complexes exhibit several electronic and EPR spectral features which are also similar to the enzyme. Further, to establish the structural and spectroscopic consequences of the coordination of two tyrosinates in GOase enzyme, we studied the monomeric copper(II) complexes containing two phenolates and imidazole/pyridine donors as closer structural models for GOase. N,N-dimethylethylenediamine and N,N’-dimethylethylenediamine have been used as starting materials to obtain a variety of 2,4-disubstituted phenolate ligands. The X-ray crystal structures of the complexes [Cu(L5)(py)], (8) [H₂(L5) = N,N-dimethyl-N’,N’-bis(2-hydroxy-4-nitrobenzyl) ethylenediamine, py = pyridine] and [Cu(L8)(H₂O)] (11), [H₂(L8) = N,N’-dimethyl-N,N’-bis(2-hydroxy-4-nitrobenzyl)ethylenediamine] reveal distorted square pyramidal geometries around Cu(II) with the axial tertiary amine nitrogen and water coordination respectively. Interestingly, for the latter complex there are two different molecules present in the same unit cell containing the methyl groups of the ethylenediamine fragmentcis to each other in one molecule andtrans to each other in the other. The ligand field and EPR spectra of the model complexes reveal square-based geometries even in solution. The electrochemical and chemical means of generating novel radical species of the model complexes, analogous to the active form of the enzyme is presently under investigation.     

Bioinspired polymer functionalized with a diiron carbonyl model complex and its assembly onto the surface of a gold electrode via “click” chemistry

A complex pendant with two ethynyl groups, [Fe₂(μ‐SCH₂CCH)₂(CO)₆] ( 2 ), as a model of the diiron subunit of [FeFe]‐hydrogenase was polymerized and the {Fe₂(CO)₆} core was successfully incorporated into the polymer matrix. The polymer was characterized by a variety of spectroscopic techniques, TGA, FTIR, SEM, TEM, and NMR. The resultant polymer was immobilized via “click” chemistry using its terminal C?CH bond onto the surface of a gold electrode, which was premodified with azidothiol by self‐assembled monolayer (SAM). The assembled electrode showed electrochemical responses. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2410–2417, 2010     

Redox Potential and Crystal Chemistry of Hexanuclear Cluster Compounds

Most of TM₆-cluster compounds (TM = transition metal) are soluble in polar solvents, in which the cluster units commonly remain intact, preserving the same atomic arrangement as in solids. Consequently, the redox potential is often used to characterize structural and electronic features of respective solids. Although a high lability and variety of ligands allow for tuning of redox potential and of the related spectroscopic properties in wide ranges, the mechanism of this tuning is still unclear. Crystal chemistry approach was applied for the first time to clarify this mechanism. It was shown that there are two factors affecting redox potential of a given metal couple: Lever’s electrochemical parameters of the ligands and the effective ionic charge of TM, which in cluster compounds differs effectively from the formal value due to the bond strains around TM atoms. Calculations of the effective ionic charge of TMs were performed in the framework of bond valence model, which relates the valence of a bond to its length by simple Pauling relationship. It was also shown that due to the bond strains the charge depends mainly on the atomic size of the inner ligands.     

Redox,ionic strength,and pH sensitive supramolecular polymer assemblies

Formation of micelle‐type assembly from supramolecular complexation of a surfactant and an oppositely charged homopolymer is demonstrated. The lower CAC observed for these assemblies suggest that the electrostatic interaction provides an amphiphilic homopolymer‐like structure. The stimulus‐induced disassembly of these supramolecular structures has been accomplished with variations in redox characteristics, ionic strength, and pH of the medium. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1052–1060, 2009     

Palladium(0) pre‐catalysts with heteroditopic NHC–amine ligands by transmetallation from their silver(I) complexes

The synthesis of an air‐stable series of Pd⁰ complexes with dissymmetric bidentate N‐heterocyclic carbene–amine ligands has been performed. The key step is an unprecedented carbene transfer from Ag^I to obtain electron‐rich zero‐valent palladium precatalysts. The coordination behavior of the ligands was determined with X‐ray crystallography. Surprising results were obtained in the catalysis of transfer semi‐hydrogenation, where the addition of base appeared not to be necessary to obtain the desired product stereoselectively. Copyright © 2009 John Wiley & Sons, Ltd.     

Chemical Synthesis,Structure Characterization,and Optical Properties of Hollow PbSx–Solid Au Heterodimer Nanostructures

Heterodimer nanostructures have attracted extensive attention, owing to an increasing degree of complexity, functionality, and then importance. So far, all the reported ones are built from solid nanoparticles. Herein, nearly monodisperse heterodimer nanostructures are constructed by hollow PbS_x and solid Au domains simultaneously through a mild reaction between PbS nanocrystals and the gold species in the presence of dodecylamine. Control experiments clearly reveal the underlying formation mechanism of the hollow PbS_x–solid Au heterodimers. The Au^III species in the solution, lead to the etching of PbS nanocrystals and the Au^I species facilitate the control of the number of gold domains per nanoparticle. Dodecylamine molecules not only work as a stabilizer in the reaction, but also act as a reducing agent that could greatly affect the morphology of the product. The optical properties of the heterodimers are investigated based on UV/Vis absorption spectroscopy and Raman spectroscopy. This novel heterodimer nanostructure pushes the development of complex nanocrystal‐based architectures forward, and also provides many opportunities for potential applications.