Manganese(III) biliverdin IX dimethyl ester: A powerful catalytic scavenger of superoxide employing the Mn(III)/Mn(IV) redox couple

A manganese(III) complex of biliverdin IX dimethyl ester, {(MnBVDME)-B-III} (2), was prepared and characterized by elemental analysis, UV/vis spectrosc opy, cyclic voltammetry, chronocoulometry, electrospray mass spectrometry, freezing-point depression, magnetic susceptibility, and catalytic dismuting of superoxide anion (O-2(.-)). In a dimeric conformation each trivalent ma nganese is bound to four pyrrolic nitrogens of one biliverdin dimethyl este r molecule and to the enolic oxygen of another molecule. This type of coord ination stabilizes the +4 metal oxidation state, whereby the +3/+4 redox cy cling of the manganese in aqueous medium was found to be at E-1/2 = +0.45 V vs NHE. This potential allows the Mn(III)/Mn(IV) couple to efficiently cat alyze the dismutation of O-2(.-) with the catalytic rate constant of k(cat) = 5.0 x 10(7) M-1 s(-1) (concentration calculated per manganese) obtained by cytochrome c assay at pH 7.8 and 25 degreesC. The fifth coordination sit e of the manganese is occupied by an enolic oxygen, which precludes binding of NO., thus enhancing the specificity of the metal center toward O-2(.-). For the same reason the {(MnBVDME)-B-III}(2) is resistant to attack by H2O 2. The compound also proved to be an efficient SOD mimic in vivo, facilitat ing the aerobic growth of SOD-deficient Escherichia coli.