The mechanism for the catalytic dismutation of superoxide by the Mn(II) pentaazamacrocyclic compound M40403 ([manganese(II) dichloro-(4R,9R, 14R, 19R)-3,10,13,20,26 pentaazatetracyclo [20.3.1.0(4,9).0(14,9)] hexacosa-1(26),-22(23),24-triene], SODm1) and two 2,21-dimethyl analogues has been investigated using pulse radiolysis. The initial rate of reaction between superoxide and the manganese compounds was found to be dependent on structure and pH, with the resulting transient adducts possessing spectral characteristics of the metal center being oxidized to Mn(III). Values for the pK(a) of the transient adducts (pK(a) = 5.65 +/- 0.05; 5.3 +/- 0.1 and < 5 for SODm1, SODm2 and SODm3, respectively) were obtained from spectrophotometric and conductivity measurements. Reaction of these transient adducts with further superoxide was highly structure dependent with the 2S,21S-dimethyl derivative (SODm2) being highly catalytically active at pH 7.4 (k(cat) = 2.35 x 10(8) M-1 s(-1)) compared to SODm1 (k(cat) = 3.55 x 10(6) M-1 s(-1)). In contrast the 2R,21R-dimethyl derivative (SODm3) showed no dismutation catalysis at all. The reaction rates of the initial complexes with HO2 center dot were significantly lower than with O-2(center dot-), and it is proposed that O-2(center dot-) is the main reactant in the catalytic cycle at pH 7.4. Variable temperature studies revealed major differences in the thermodynamics of the catalytic cycles involving SODm2 or SODm1. In the case of SODm2, the observed high entropic contribution to the activation energy is indicative of ligand conformational changes during the catalytic step. These results have provided the basis for a new mechanism for the catalytic dismutation of superoxide by Mn(II)-pentaazamacrocycle SOD mimetics.