Characterization of Mn(III) complexes of linear and cyclic desferrioxamines as mimics of superoxide dismutase activity.

The green complex of Mn(III) with desferrioxamine B has been prepared by air oxidation of Mn(II) in the presence of the ligand or by reaction of Mn(OH)3, or of MnO2, with the ligand. The resultant complex was purified by ion-exchange chromatography and shown by electrospray ionization mass spectrometry to be a 1:1 complex of Mn(III) with desferrioxamine B. A similar complex was prepared from the macrocyclic Tris hydroxamate desferrioxamine E. Mn(III) desferrioxamine B was not stable to equimolar EDTA while the Mn(III) desferrioxamine E was stable to a fivefold molar excess of EDTA. Chelex-100 decomposed the desferrioxamine B complex, but not the desferrioxamine E complex. These complexes prevented the reduction of cytochrome c by O2- but were less active than free Mn(II). The second-order rate constants for reaction with O2- at 25 degrees C and at pH 7.5 were 3.0 x 10(6), 1.0 x 10(6), and 9.5 x 10(4) for Mn(II), Mn(III) desferrioxamine B, and Mn(III) desferrioxamine E, respectively.