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

Reactions of imidazolate-bridged Cu^II-Zn^II heterodinuclear and Cu^II-Cu^II homodinuclear complexes, [Cu^IIZn^II(bdpi)(CH₃CN)₂](ClO ₄)₃ · 2 CH₃CN (1) and [Cu^II₂(bdpi)(CH₃CN)₂]-(ClO ₄)₃·CH₃CN·3 H₂O (2) (Hbdpi = 4,5-bis(di(2-pyridylmethyl)aminomethyl)imidazole), with the p-benzosemiquinone radical anion (Q^.-) have been examined to provide mechanistic insight into the role of the Zn^II ion in copper-zinc superoxide dismutase (Cu,Zn-SOD). The addition of less than one equivalent of Q^.- to a deaerated solution of 1 or 2 in propionitrile at -80°C results in the oxidation of Q^.- accompanied by the appearance of a new absorption band at 585 nm due to the Cu^I-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, which can be assigned to a CO stretching vibration in the Cu^I-Q complexes. Further addition of Q^.- to a deaerated solution of 3 in propionitrile results 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^I ion in 3. The coordination of O₂^.- and Q^.- to the Zn^II ion has been confirmed by the electronic and ESR spectra of the O₂^.- 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-imidazolylmethyl)bis(2-pyridylmethyl)amine, MeIm(Me)₂ = (1-methyl-4-imidazolylmethyl)bis(6-methyl-2-pyridylmethyl)amine). The binding energies of O₂^.- with the Zn^II ion in 5 and 6 have been evaluated from the deviation of the g_∥ values of the ESR spectra from the free spin value.