Cyclic voltammetry and spectroelectrochemistry of cytochrome c(8) from Rubrivivax gelatinosus. Implications in photosynthetic electron transfer

We present here the electrochemical characterization of cytochrome c(8) from light-grown cells of the purple phototroph Rubrivivax gelatinosus. At 25 degrees C, cytochrome c(8) exhibits a quasi-reversible, diffusion-controlled, electrochemical process as observed by cyclic voltammetry (CV) using a pyrolitic graphite microelectrode (10 mM Tris . HCl buffer, 250 mM NaCl). The pH dependence of the reduction potential, investigated in the range 4.9-8.2, allows the calculation of E-m,E-7 = +294 mV, and further yields values of pK(OX) = 6.24 and pK(red) = 6.87, probably associated to the ionization of a heme propionate group in the oxidized and reduced form of cytochrome c(8), respectively. The calculated reduction potentials of completely protonated and deprotonated cytochrome c(8) are +322 and +284 mV, respectively. The equilibrium reduction potential of cytochrome c(8) was also determined at pH 8 by recording its visible absorption spectrum as a function of the applied potential using an optically transparent electrode (OTE) cell (E-m = +276 mV). These observations indicate that cytochrome c(8) might play a significant role in photosynthetic electron transport and associated proton translocation in purple bacteria.