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

Abstract

AbstractThe 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)SO3CF3]SO3CF3, 1; [Cu(L1)ClO4]ClO4, 2 [L1=1,4‐bis[(pyridin‐2‐yl‐methyl)]‐1,4‐diazepane]; [Cu(L2)SO3CF3]SO3CF3, 3 and [Cu(L2)ClO4]ClO4, 4 [L2=1, 4‐bis[2‐(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 (τ, 0.064 ‐ 0.285). The average Cu‐N(2.0 Å) bond distances of model complexes are similar to the Cu‐NHis bond distances in native CAOs enzyme (Cu‐NHis‐456, 2.0‐2.1 Å, Cu‐NHis‐458, 1.9‐2.2 Å and Cu‐NHis‐624,1.9 −2.1 Å).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 eg orbital sets. Interestingly, on raising of the pH solution 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 ∼7 ‐ 10. All the complexes exhibits almost similar solution EPR parameter (g||, 2.21 ‐ 2.45; A||, 174 ‐ 193 × 10−4 cm−1) to CAO (g||, 2.32; A||, 153×10−4cm−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 ×10−3 s−1 in water. The asymmetric Cu‐Npy 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.