Structural and Catalytic Characterization of a Heterovalent Mn(II)Mn(III) Complex That Mimics Purple Acid Phosphatases

Abstract

The binuclear heterovalent manganese model complex [Mn(II)Mn(III)(L1)(OAc) 2 ] ClO 4 3 H 2 O (H 2 L1 = 2-(((3-((bis-(pyridin-2-ylmethyl)amino)methyl)-2-hydroxy-5-methylbenzyl)(pyridin-2-ylmethyl)amino)-methyl)phenol) has been prepared and studied structurally, spectroscopically, and computationally. The magnetic and electronic properties of the complex have been related to its structure. The complex is weakly antiferromagnetically coupled (J ∼ -5 cm -1 , H = -2J S 1 3 S 2 ) and the electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD) spectra identify the Jahn-Teller distortion of the Mn(III) center as predominantly a tetragonal compression, with a significant rhombic component. Electronic structure calculations using density functional theory have confirmed the conclusions derived from the experimental investigations. In contrast to isostructural M(II)Fe(III) complexes (M = Fe, Mn, Zn, Ni), the Mn(II)Mn(III) system is bifunctional possessing both catalase and hydrolase activities, and only one catalytically relevant pK a (= 8.2) is detected. Mechanistic implications are discussed.

A New Mixed-Valence Mn(II)Mn(III) Compound With Catalase and Superoxide Dismutase Activities

“…Now, at least 10 lines are observed, which suggests the formation of a new chemical species. We tentatively assign this new intermediate as a Mn(II)Mn(III) species, since this compound type has been described as presenting a 12 line spectrum (Larson et al, 1992 ; Gelasco et al, 1997 ; Mitić et al, 2009 ; Smith et al, 2009 ).…”

“…Broad resonances at low field have been previously described for coupled Mn(II)Mn(III) systems, and were interpreted in terms of the presence of ferro- or antiferromagnetically coupled Mn(II)Mn(III) cores. A feature of this low field signal is that for an antiferromagnetically coupled system, the signal disappears when the temperature decreases (Smith et al, 2009 ). On the other hand, in ferromagnetically-coupled Mn(II)Mn(III) dimers, the signal grows at low temperatures (Gelasco et al, 1997 ).…”

“…This interpretation was further confirmed by magnetic measurements (see below). In addition, in Mn(II)Mn(III) systems with antiferromagnetic coupling, multiline features with as many as 36 lines can be observed around g = 2 due to the population of the S = state of the dinuclear manganese system (Smith et al, 2009 ; Sano et al, 2013 ; Jung and Rentschler, 2015 ; Magherusan et al, 2018 ). In contrast, for ferromagnetically-coupled Mn(II)Mn(III) complexes published EPR data vary, including compounds that only show a signal at low field (g >5), or only a signal at high field (g ~ 2), or a combination of both features (Schake et al, 1991 ; Gelasco et al, 1997 ; Rane et al, 2000 ).…”

Synthesis, Magnetic Properties, and Catalytic Properties of a Nickel(II)-Dependent Biomimetic of Metallohydrolases

“…Thus, although no suitable nucleophile (OH − ) is present in the original molecule the replacement of the two acetate bridges by water and/or substrate molecules (Figure 8 ) may not be rate-limiting. The complex is proposed to employ a similar mechanism as proposed for a series of analogous model systems for enzymes such as PAPs (Smith et al, 2009 ; Comba et al, 2012a , b ; Bernhardt et al, 2015 ; Roberts et al, 2015 ; Bosch et al, 2016 ). The majority of complexes listed in Table 3 attain optimal catalytic efficiency under alkaline conditions (>pH 9.0), somewhat higher than the pH optimum of the di-Ni(II) enzyme urease (pH 7.4).…”

Metallo‐β‐lactamases and Their Biomimetic Complexes

“…[12,13,33,35,92,93,95,99,100,[111][112][113] In addition, the study of dicobalt(II) and dicadmium(II) systems in our work have paralleled the use of these metal ions for the crystallographic, spectroscopic, and kinetic investigations of the MβLs. Like the many examples of metalloenzyme systems explored previously, such as the purple acid phosphatases [89,[123][124][125][126][127][128][129][130][131][132][133] and other metallohydrolases, [134][135][136][137] the biomimetic chemistry of the MβLs represents a significant and interesting challenge, and much remains to be accomplished for the bioinorganic chemist. Like the many examples of metalloenzyme systems explored previously, such as the purple acid phosphatases [89,[123][124][125][126][127][128][129][130][131][132][133] and other metallohydrolases, [134][135][136][137] the biomimetic chemistry of th...…”