An Electrochemical Investigation of the Redox Properties of Bacteriochlorophyll and Bacteriopheophytin in Aprotic Solvents

Knowledge of solvent effects on the redox properties of bacteriochlorophyll (BChl) and bacteriopheophytin (BPheo) is important for understanding their possible role(s) as intermediate electron acceptors in the primary photochemistry of photosynthetic bacteria. In the present study, an investigation of the electrochemical behavior of these compounds by cyclic voltammetry (CV) and cyclic differential pulse voltammetry (CDPV) in several aprotic solvents has shown that BChl aggregation and ligation interactions have a significant effect on its redox potentials. In methylene chloride, the one-electron reduction potential of BChl was found to shift positively by 200 mV to a value nearly identical with that of BPheo in the same solvent. The shift is most readily explained by the presence of BChl aggregates in this solvent. The one-electron oxidation potential is relatively unaffected by aggregation. In contrast, the formation of six-coordinate BChl in tetrahydrofuran (two molecules of solvent coordinated to the Mg atom of BChl) affects both the one-electron reduction and one-electron oxidation potential, with the greatest effect on the latter. Solvent effects on the redox properties of BPheo were found to be much smaller, a finding consistent with its inability to undergo aggregation and coordination interactions similar to those of BChl.