Coordination of semiquinone and superoxide radical anions to the zinc ion in SOD model complexes that act as the key step in disproportionation of the radical anions

Reactions of imidazolate-bridged Cu-II-Zn-II heterodinuclear and Cu-II-Cu-I I homodinuclear complexes, [Cu-II-Zn-II(bdpi)(CH3CN)(2)](ClO4)(3). 2CH(3),C N (1) and [CU2II(bdpi)(CH3CN)(2)]-(ClO4)(3). CH3CN . 3H(2)O (2) (Hbdpi = 4, 5-bis(di(2-pyridylmethyl)aminomethyl)imidazole), with the p-benzosemiquinon e radical anion (Q(.-)) have been examined to provide mechanistic insight i nto the role of the Zn-II ion in copper-zinc superoxide dismutase (Cu,ZnSOD ). The addition of less than one equivalent of Q(.-) to a deaerated solutio n of 1 or 2 in propionitrile at - 80 degreesC results in the oxidation of Q (.-) accompanied by the appearance of a new absorption band at 585 nm due t o the Cu(1-)Q complex (3 or 4), the absorbance of which increases linearly with the increase in Q(.-) concentration. Both the resonance Raman spectra of 3 and 4 exhibit a strong resonance-enhanced Raman band at 1580 cm(-1), w hich can be assigned to a CO stretching vibration in the Cu(1-)Q complexes. Further addition of Q(.-) to a deaerated solution of 3 in propionitrile re sults in the reduction of Q(.-), whereas no reduction of Q(.-) occurs with 4. which does not contain the Zn-II ion. Thus, the coordination of Q(.-) to the Zn-II ion is essential for the reduction of Q(.-) by the Cu-1 ion in 3 . The coordination of O-2(.-) and Q(.-) to the Zn-II ion has been confirmed by the electronic and ESR spectra of the O-2(.-) and Q(.-) complexes with mononuclear Zn-II complexes, [Zn-II(MeIm(Py)(2)}(CH3CN)](ClO4)(2) (5) and [ Zn-II{MeIm(Me)(2)}-(H2O)](ClO4)(2) (6) (MeIm(Py)(2) = (1-methyl-4-imidazoly lmethyl)bis(2-pyridyl- methyl)amine, MeIm(Me)(2) = (1-methyl-4-imidazolylme thyl)bis(6-methyl-2-pyri- dylmethyl)amine). The binding energies of O-2(.-) with the Zn-II ion in 5 and 6 have been evaluated from the deviation of th e g(parallel to) values of the ESR spectra from the free spin value.