Bioinspired Cu(II) complexes with tunable axial donors: unravelling structure-function correlations in phenoxazinone...

Phenoxazinone synthase (PHS) is a multi-copper oxidase that catalyzes the oxidative coupling of o-aminophenol (OAP) to form phenoxazinone chromophores, a crucial step in the biosynthesis of the anticancer agent, Actinomycin D. Inspired by the structural and functional diversity of the copper sites in native PHS, three mononuclear copper(II) complexes (1, 2, and 3) were designed bearing tunable axial donor atoms (N, O, and S respectively). These complexes emulate the diversity in the coordination motifs of the different types of copper centers present in PHS and were fully characterized by multiple spectroscopic techniques. Substitution of the axial donor atom induced systematic changes in geometry, redox potential, and catalytic efficiency toward OAP oxidation. The Jahn-Teller distortion was most pronounced in the N4 and N3S systems, while complex 2 (N3O) exhibited a nearly centrosymmetric geometry and superior substrate binding affinity. All complexes catalyzed the aerial oxidation of OAP to 2-amino-3H-phenoxazin-3-one (APX) under physiological conditions without external oxidants, with the catalytic efficiency following the order 2 > 3 > 1. Density functional theory (DFT) and time-dependent DFT (TDDFT) studies corroborated the experimental observations. The optimized geometries closely matched the crystal structures, and the computed spin densities indicated greater Cu-ligand covalency in the N3O complex compared to the N4 and N3S analogues. The combined experimental and computational results highlight how subtle modifications in the primary coordination sphere influence the electronic structure, redox behaviour, and catalytic performance of the bio-inspired copper systems, thus providing a mechanistic rationale for phenoxazinone synthase activity.