Evidence for Cu-O2 intermediates in superoxide oxidations by biomimetic copper(II) complexes.
The mechanism by which [Cu(II)(L)](OTf)2 and [Cu(II)N3(L)](OTf) (L = TEPA: tris(2-pyridylethyl)amine or TMPA: tris(2-pyridylmethyl)amine; OTf = trifluoromethanesulfonate) react with superoxide (O2*-) to form [Cu(I)(L)](OTf) and O2 is described. Evidence for a CuO2 intermediate is presented based on stopped-flow experiments and competitive oxygen (18O) kinetic isotope effects on the bimolecular reactions of (16,16)O2*- and (18,16)O2*- ((16,16)k/(18,16)k). The (16,16)k/(18,16)k fall within a narrow range from 0.9836 +/- 0.0043 to 0.9886 +/- 0.0078 for reactions of copper(II) complexes with different coordination geometries and redox potentials that span a 0.67 V range. The results are inconsistent with a mechanism that involves either rate-determining O2*- binding or one-step electron transfer. Rather a mechanism involving formation of a CuO2 intermediate prior to the loss of O2 in the rate-determining step is proposed. Calculations of similar inverse isotope effects, using stretching frequencies of CuO2 adducts generated from copper(I) complexes and O2, suggest that the intermediate has a superoxo structure. The use of 18O isotope effects to relate activated oxygen intermediates in enzymes to those derived from inorganic compounds is discussed.
Department of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, USA.
Valeriy V
VV
Smirnov
Justine P
JP
Roth
D040761:Biomimetic Materials / Q000737:chemistry; D003300:Copper / Q000737:chemistry*; D004563:Electrochemistry; D004798:Enzymes / Q000737:chemistry; D007700:Kinetics; D006899:Mixed Function Oxygenases / Q000737:chemistry; D009942:Organometallic Compounds / Q000737:chemistry; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D011725:Pyridines / Q000737:chemistry*; D013481:Superoxides / Q000737:chemistry*
D004798:Enzymes; D009942:Organometallic Compounds; D011725:Pyridines; C431843:tris(2-pyridylmethyl)amine; D013481:Superoxides; D003300:Copper; D006899:Mixed Function Oxygenases; D010100:Oxygen