Spectroscopic and Catalytic Characterization of a Functional Fe^IIIFe^II Biomimetic for the Active Site of Uteroferrin and Protein Cleavage

Abstract

A mixed-valence complex, [Fe^IIIFe^II<b>L1</b>(μ-OAc)₂]­BF₄·H₂O, where the ligand H₂<b>L1</b> = 2-{[[3-[((bis­(pyridin-2-ylmethyl)­amino)­methyl)-2-hydroxy-5-methylbenzyl]­(pyridin-2-ylmethyl)­amino]­methyl]­phenol}, has been studied with a range of techniques, and, where possible, its properties have been compared to those of the corresponding enzyme system purple acid phosphatase. The Fe^IIIFe^II and Fe^III₂ oxidized species were studied spectroelectrochemically. The temperature-dependent population of the <i>S</i> = ³/₂ spin states of the heterovalent system, observed using magnetic circular dichroism, confirmed that the dinuclear center is weakly antiferromagnetically coupled (<i>H</i> = −2<i>JS</i>₁·<i>S</i>₂, where <i>J</i> = −5.6 cm^–1) in a frozen solution. The ligand-to-metal charge-transfer transitions are correlated with density functional theory calculations. The Fe^IIIFe^II complex is electron paramagnetic resonance (EPR)-silent, except at very low temperatures (<2 K), because of the broadening caused by the exchange coupling and zero-field-splitting parameters being of comparable magnitude and rapid spin–lattice relaxation. However, a phosphate-bound Fe^III₂ complex showed an EPR spectrum due to population of the <i>S</i>_tot = 3 state (<i>J</i>= −3.5 cm^–1). The phosphatase activity of the Fe^IIIFe^II complex in hydrolysis of bis­(2,4-dinitrophenyl)­phosphate (<i>k</i>_cat. = 1.88 × 10^–3 s^–1; <i>K</i>_m = 4.63 × 10^–3 mol L^–1) is similar to that of other bimetallic heterovalent complexes with the same ligand. Analysis of the kinetic data supports a mechanism where the initiating nucleophile in the phosphatase reaction is a hydroxide, terminally bound to Fe^III. It is interesting to note that aqueous solutions of [Fe^IIIFe^II<b>L1</b>(μ-OAc)₂]⁺ are also capable of protein cleavage, at mild temperature and pH conditions, thus further expanding the scope of this complex’s catalytic promiscuity