title: A novel asymmetric di-Ni(II) system as a highly efficient functional model for phosphodiesterase: synthesis, structures, physicochemical properties and catalytic kinetics.
abstract: A novel asymmetric phenol-based 'end-off' dinucleating ligand 2-{[(2-piperidylmethyl)amino]methyl}-4-bromo-6-[(1-methylhomopiperazine-4-yl)methyl]phenol (HL) and three dinuclear nickel(II) complexes, [Ni₂L(μ-OH)] (ClO₄)₂ (1), [Ni₂L(DNBA)₂(CH₃CN)₂]BPh₄ (2) and [Ni₂L(BPP)₂(CH₃CN)₂]BPh₄ (3) have been synthesized and characterized by a variety of techniques including: NMR, infrared and UV-vis spectroscopies, mass spectrometry, elemental analysis, molar conductivity, thermal analysis, magnetochemistry and single-crystal X-ray diffractometry. The UV-vis spectrum of complex 1 exhibits a strong peak at 510 nm, a characteristic absorption of a d-d transition of the square-planar four-coordinated Ni(II) center. Utilizing this feature, the stepwise formation of mono- and dinickel centers in solution can be monitored. Phosphodiesterase activity of a dinuclear Ni(II) system (complex 1), formed in situ by a 2 : 1 mixture of Ni(2+) ions and the ligand HL, was investigated using bis(4-nitrophenyl)phosphate (BNPP) as the substrate. The pH dependence of the BNPP cleavage in water-ethanol (1 : 1, v/v) reveals a bell-shaped pH-k(obs) profile with an optimum at about pH 8.3 which is parallel to the formation of the dinuclear species [Ni₂L(μ-OH)](2+), according to the increase of the peak at 510 nm in the UV-vis absorption spectrum . These studies reveal that the di-Ni(II) system shows the highest catalytic activity reported so far, with an acceleration rate 1.28 × 10⁷ times faster than the uncatalyzed reaction. The bridging hydroxyl group in [Ni₂L(μ-OH)](2+) is responsible for the hydrolysis reaction. The possible mechanism for the BNPP cleavage promoted by di-Ni(II) system is proposed on the basis of kinetic and spectral analyses. This study provides a less common example of the asymmetric phosphodiesterase model, which is like the active sites of most native metallohydrolases.
authors.affiliation: School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, China.
authors.forename: Yan-wei
authors.initials: YW
authors.lastname: Ren
authors.forename: Jia-xian
authors.initials: JX
authors.lastname: Lu
authors.forename: Bo-wei
authors.initials: BW
authors.lastname: Cai
authors.forename: Da-bin
authors.initials: DB
authors.lastname: Shi
authors.forename: Huan-feng
authors.initials: HF
authors.lastname: Jiang
authors.forename: Jun
authors.initials: J
authors.lastname: Chen
authors.forename: De
authors.initials: D
authors.lastname: Zheng
authors.forename: Bin
authors.initials: B
authors.lastname: Liu
mesh_terms: D002384:Catalysis; D056831:Coordination Complexes / Q000138:chemical synthesis / Q000737:chemistry* / Q000378:metabolism; D018360:Crystallography, X-Ray; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D007700:Kinetics; D008280:Magnetics; D008958:Models, Molecular; D008968:Molecular Conformation; D009532:Nickel / Q000737:chemistry*; D009596:Nitrophenols / Q000737:chemistry / Q000378:metabolism; D010727:Phosphoric Diester Hydrolases / Q000378:metabolism; D013056:Spectrophotometry, Ultraviolet
chemical_list: D056831:Coordination Complexes; D009596:Nitrophenols; C002887:bis(4-nitrophenyl)phosphate; D009532:Nickel; D010727:Phosphoric Diester Hydrolases