Control of Redox Transitions and Oxygen Species Binding in Mn Centers by Biologically Significant Ligands; Model Studies with [Mn]-bacteriochlorophyll a

Mn-superoxide dismutase (Mn-SOD), which protects the cell from the toxic potential of superoxide radicals (O2 -•), is the only type of SOD which resides in eukaryotic mitochondria. Up-to-date, the exact catalytic mechanism of the enzyme and the relationship between substrate moieties and the ligands within the active site microenvironment are still not resolved. Here, we set out to explore the possible involvement of hydroperoxyl radicals (•OOH) in the catalytic dismutaion by following the interplay of MnIII/MnII redox transitions, ligands binding, and evolution or consumption of superoxide radical, using a new model system. The model system encompassed an Mn atom chelated by a bacteriochlorophyll allomer macrocycle (BChl) in aerated aprotic media that contain residual water. The redox states of the Mn ion were monitored by the Q y electronic transitions at 774 and 825 nm for [MnII]- and [MnIII]-BChl, respectively (Geskes, C.; Hartwich, G.; Scheer, H.; Mantele, W.; Heinze, J. J. Am. Chem. Soc. 1995, 117, 7776) and confirmed by electron spin resonance spectroscopy. Evolution of •OOH radicals was monitored by the ESR spin-trap technique using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). The experimental data suggest that the [Mn]-BChl forms a (HO-)[MnIII]-BChl(OOH) complex upon solvation. Spectrophotometeric titrations with tetrabutylamonnium acetate (TBAA) and 1-methylimidazole (1-MeIm) together with ESI-MS measurements indicated the formation of a 1:1 complex with [Mn]-BChl for both ligands. The coordination of ligands at low concentrations to [MnIII]-BChl induced a release of a •OOH radical and a [MnIII]-BChl → [MnII]-BChl transition at higher concentrations. The estimated equilibrium constants for the total redox reaction are 1.9 × 104 ± 1 × 103 M-1 and 12.3 ± 0.6 M-1 for TBAA and 1-MeIm, respectively. The profound difference between the equilibrium constants agrees with the suggested key role of the ligand's basicity in the process. A direct interaction of superoxide radicals with [MnIII]-BChl in a KO2 acetonitrile (AN) solution also resulted in [MnIII]-BChl → [MnII]-BChl transition. Cumulatively, our data show that the MnIII center encourages the protonation of the O2 -• radical in an aprotic environment containing residual water molecules, while promoting its oxidation in the presence of basic ligands. Similar coordination and stabilization of the •OOH radical by the Mn center may be key steps in the enzymatic dismutation of superoxide radicals by Mn-SOD.