Structural and spectroscopic models of the manganese catalase active site. Isolation and structures of the asymmetric [(H 2O)Mn III(μ-O)-(μ-O 2CR) 2Mn III(L)] (L  Cr 2O 72−, CH 3OH) cores: Analogs of a substrate-bound catalase active site intermediate

Abstract

The isolation and structural characterization are reported for two new unsymmetrical (μ-oxo)bis(μ-carboxylato)dimanganese(III) complexes, [Mn 2O(O 2CC 2H 5) 2(H 2O)(Cr 2O 7)(bpy) 2] ( 1) and [Mn 2O(O 2CCH 2Cl) 2(H 2O)(CH 2OH)(bpy) 2] 2− ( 2) (bpy = 2,2′-bipyridine) that mimic end-on substrate and/or inhibitor coordination to only one manganese atom of the dimeric active site in bacterial manganese catalases. Crystals of 1 were self assembled spontaneously in aqueous medium following treatment of equimolar Mn(II) and bpy with K 2Cr 2O, in propionic acid rich solution. Complex 2 was synthesized by oxidation of Mn(II) with KMnO 4 in pure methanol in the presence of bpy, chloroacetic acid, and sodium chloroacetate. X-ray crystallographic studied of 1 and 2 establish the presence of a similar triply bridged dimanganese core with distinct manganese sites in which one manganese atom contains an axial water molecule and the other is bonded to a terminal oxygen atom of dichromate anion ( 1), or to the oxygen atom of methanol ( 2), a neutral substrate. Complex 1 represents the first structurally characterized instance in which the dichromate acts as a monodentate ligand, the MnO(dichromate) bond of 2.132(23) A indicating strong anion coordination. Isolation of these model complexes and their structural characteristics suggest a preference of the [Mn 2O(O 2CR) 2(H 2O) 2] 2− core towards a replacement of one water molecule only a feature that is suspected to operate in hydroperoxide binding to a dimanganese site of catalases. The resonance Raman signatures with visible excitation (406.7–501.7 nm) were obtained for a series of catalase model complexes containing dimanganese cores bridged by one or two μ-oxo groups, including (μ-oxo)bis(μ-carboxylato) Mn(III.III), and mixed-valent bis (μ-oxo) and bis (μ-oxo)mono(μ-carboxylato)Mn(III.IV) dimers. The two structural types, which model the oxidized Mn(III,III) and superoxidized Mn(III,IV) forms of catalase, are distinguished readily by the uniquely resonance-enhanced μ- 18O-sensitive MnO(oxo) stretching bands at ≈ 560 and ≈ 700 cm, respectively.