STRUCTURAL AND SPECTROSCOPIC MODELS OF THE MANGANESE CATALASE ACTIVE-SITE - ISOLATION AND STRUCTURES OF THE ASYMMETRIC [(H2O)MN-III(MU-O)-(MU-O2CR)(2)MN-III(L)] (L = CR2O72-, CH3OH) CORES - ANALOGS OF A SUBSTRATE-BOUND CATALASE ACTIVE-SITE INTERMEDIATE

The isolation and structural characterization are reported for two new unsymmetrical (mu-oxo)bis( mu-carboxylato) dimanganese(III) complexes , [Mn2O(O2CC2H5)(2)(H2O) (Cr2O7) (bPY)(2)] (1) and [Mn2O(O2CCH2Cl)(2)( H2O) (CH3OH) (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 tr eatment of equimolar Mn(II) and bpy with K2Cr2O7 in propionic acid ric h solution. Complex 2 was synthesized by oxidation of Mn(II) with KMnO 4 in pure methanol in the presence of bpy, chloroacetic acid, and sodi um chloroacetate. X-ray crystallographic studies of 1 and 2 establish the presence of a similar triply bridged dimanganese core with distinc t manganese sites in which one manganese atom contains an axial water molecule and the other is bonded to a terminal oxygen atom of dichroma te anion (1), or to the oxygen atom of methanol (2), a neutral substra te. Complex 1 represents the first structurally characterized instance in which the dichromate acts as a monodentate ligand, the Mn-O(dichro mate) bond of 2.132(23) Angstrom indicating strong anion coordination. Isolation of these model complexes and their structural characteristi cs suggest a preference of the [Mn2O(O2CR)(2)(H2O)(2)](2+) core toward s a replacement of one water molecule only, a feature that is suspecte d to operate in hydroperoxide binding to a dimanganese site of catalas es. The resonance Raman signatures with visible excitation (406.7-501. 7 nm) were obtained for a series of catalase model complexes containin g dimanganese cores bridged by one or two mu-oxo groups, including ( m u-oxo) bis( mu-carboxylato) Mn(III,III), and mixed-valent bis(mu-oxo) and bis( mu-oxo) mono( mu-carboxylato) Mn(III,TV) dimers. The two stru ctural types, which model the oxidized Mn(III,III) and superoxidized M n(III,IV) forms of catalase, are distinguished readily by the uniquely resonance-enhanced mu-O-18-sensitive Mn-O(oxo) stretching bands at si milar to 560 and similar to 700 cm(-1), respectively. (C) 1998 Elsevie r Science S.A. All rights reserved.