Promotion of the polarographic reduction of ferredoxins by divalent metal cations: Effect of pH

Abstract

Differential pulse polarography has been used to study the reduction of spinach and Clostridium thermocellum ferredoxins. In the presence of divalent metal cations (Mg 2+, Ca 2+, Ba 2+ and Co 2+), an additional reduction peak corresponding to the reduction of native molecules is detected. The existence of a reduction-reoxidation process is confirmed by cyclic voltammetry at the hanging mercury drop electrode in the case of C. thermocellum ferredoxin. Direct reduction-reoxidation of C. thermocellum ferredoxin is also observed at the basal-plane pyrolytic graphite electrode in the presence of Ba 2+ cations. The effect of pH on the reduction process and on the stability of the ferredoxins has been investigated by differential pulse polarography and spectrophotometry. Below pH 6.5, two reduction peaks are detected for C. thermocellum ferredoxin. The reduction process of ferredoxins is complicated by adsorption phenomena and by the destruction of native molecules at the mercury electrode-solution interface. It is suggested that the high positive charge density generated by metal cations assits the electron transfer between the mercury electrode and negatively charged molecules.