Copper(II)-Bioinspired Models for Copper Amine Oxidases: Oxidative Half-Reaction in Water

摘要

abstract_textpThe quinone cofactor is generated at the oxidative half-reaction of copper amine oxidases (CAO) activity, which is key biogenesis step in biology. The copper(II) complexes Cu(L1)SO3 CF3SO3CF3, 1; Cu(L1)ClO4ClO4, 2 L1 = 1,4-bis(pyridin-2-yl-methyl)-1,4-diazepane; Cu(L-2)SO3CF3SO3CF3, 3 and Cu(L2)-ClO4 ClO4, 4 L2= 1, 4-bis2-(pyridin-2-yl)ethyl-1,4-diazepane were synthesized as models for copper amine oxidases. The molecular structure of complexes was determined by single crystal X-ray studies and shows distorted square pyramidal geometry (tau, 0.064 - 0.285). The average Cu-N(2.0 angstrom) bond distances of model complexes are similar to the Cu-N-His bond distances in native CAOs enzyme (Cu-NHis-456, 2.0-2.1 angstrom, Cu-NHis-458, 1.9-2.2 angstrom and Cu-NHis-624,1.9 - 2.1 angstrom). Only one Cu(II)/Cu(I) redox couple (-0.296 to -0.343 V) was noted in acetonitrile for 1-4 but a well-defined redox couple around -0.349 to -0.404 V with an additional anodic peak around -0.056 to -0.128 V appeared in the water. The electronic spectra of 1-4 at pH - 6.0 in water, shows ligand-based absorption band around 260 - 264 nm with a shoulder around 290 nm and the d-d transition appeared around 593-640 nm. Also, an unusual low-energy transition is centered at 965 nm due to axial field splitting of the e(g) orbital sets. Interestingly, on raising of the pH solution to similar to 7 - 10 instigates a transition of square pyramidal coordination geometry into trigonal bipyramidal for 1-4. This geometrical interconversion is further supported by appearance of two Cu(II)/Cu(I) redox couples around -0.313 to -0.390 V and -0.056 to -0.309 V at pH ranges of similar to 7 - 10. All the complexes exhibits almost similar solution EPR parameter (g(), 2.21 - 2.45; A(), 174 - 193 3 10(-4) cm(-1)) to CAO (g(), 2.32; A(), 153x10(-4) cm(-1)) 5 at 70 K. The model complexes convert substrate 2-aminophenol into o-quinone simultaneously using molecular oxygen via oxidative deamination pathway and this formation accomplished by new absorption band at 430 nm with rate of 0.53 - 11.2 x 10(-3) s(-1) in water. The asymmetric Cu-N-py bonds and higher distortion in the square pyramidal geometry of 1 and 2 presumed to facilitate geometrical change via decoordination of one the pyridine arms and leads faster dioxygenation reaction than 3 and 4. This decoordinated pyridine arm act as like an acid-base catalyst to accept and donate protons as in CAO catalysis by amino acid residues at secondary coordination sphere./p/abstract_text