Complexation, Structure, and Superoxide Dismutase Activity of the Imidazolate-Bridged Dinuclear Copper Moiety with β-Cyclodextrin and Its Guanidinium-Containing Derivative

An imidazolate-bridged homodinuclear complex, {[Cu(L)(H2O)]2(im)}(ClO4)3 (1), assembled with β-cyclodextrin (βCD) and its guanidinium-containing derivative (βGCD), and thus a helical inclusion complex, {[Cu(L)(H2O)(βCD)]2(im)}(ClO4)3 (2), were successfully isolated and structurally characterized. Structural analysis showed that each Cu(II) ion has a distorted square pyramidal N4Ow coordination sphere and forms a chiral chain through hydrogen-bonding and hydrophobic interactions. The UV−vis data showed that such a chain can provide the imidazolate bridge additional stability and results in the dissociation equilibrium taking place at the physiological pH. The obtained IC50 value for 2 (0.23 μM) showed a high superoxide dismutase (SOD) activity, which corresponds to a highly stable imidazolate bridge. Interestingly, the guanidinium-containing 1/βGCD system showed higher SOD activity (IC50 = 0.16 μM), which is enhanced at least by 30% in comparison with that of guanidinium-lacking 2. This result supports that the positive guanidinium plays a role in the catalytic mechanism of Cu,Zn−SOD by ensuring that superoxide enters and peroxide leaves rapidly from the coordination sphere of the copper ion.