Substituent Effect on Proton Affinity of Imidazole in Cu,Zn-Superoxide Dismutase

To investigate whether the proton-accepting ability of imidazole in Cu,Zn-superoxide dismutase （SOD） was possibly modulated by Zn（Ⅱ） or not, the proton affinity （Ap） of N^3 in imidazole group was calculated by density functional theory （DFT） with B3LYP functional. It was found that Zn（Ⅱ） attenuates the Ap, because of its electron-withdrawing effect, while the three ligands connected with Zn（Ⅱ） （residues of two His and one Asp） exert an opposite effect, owing to their electron-donating ability. This finding suggested that the three ligands should play a role in the normal function of Cu,Zn-SOD and should be taken into consideration in the future study.