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.
Smirnov
Valeriy V
VV
Department of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, USA.
Roth
Justine P
JP
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