J. Phys. Chem. A 2008, 112, 4929–4935
4929
Pulse Radiolysis Investigation on the Mechanism of the Catalytic Action of Mn(II)-Pentaazamacrocycle Compounds as Superoxide Dismutase Mimetics Andrej Maroz,† Geoffrey F. Kelso,‡ Robin A. J. Smith,§ David C. Ware,† and Robert F. Anderson* Department of Chemistry, The UniVersity of Auckland, PriVate Bag 92019, Auckland 1142, New Zealand, Centre for Green Chemistry, Monash UniVersity, P.O. Box 75, Clayton, Victoria 3800, Australia, Department of Chemistry, UniVersity of Otago, Box 56, Dunedin 9054, New Zealand ReceiVed: January 23, 2008; ReVised Manuscript ReceiVed: February 28, 2008
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 [20.3.1.04,9.014,9] hexacosa1(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 pKa of the transient adducts (pKa ) 5.65 ( 0.05; 5.3 ( 0.1 and
