Electron Transfer Reactions of Redox Cofactors in Spinach Photosystem I Reaction Center Protein in Lipid Films on Electrodes

Thin film voltammetry was used to obtain direct, reversible, electron transfer between electrodes and spinach Photosystem I reaction center (PS I) in lipid films for the first time. This reaction center (RC) protein retains its native conformation in the films, and AFM showed that film structure rearranges during the first several minutes of rehydration of the film. Two well-defined chemically reversible reduction−oxidation peaks were observed for native PS I in the dimyristoylphosphatidylcholine films, and were assigned to phylloquinone, A1 (E m = −0.54 V) and iron−sulfur clusters, FA/FB (E m = −0.19 V) by comparisons with PS I samples selectively depleted of these cofactors. Observed E m values may be influenced by protein−lipid interactions and electrode double-layer effects. Voltammetry was consistent with simple kinetically limited electron transfers, and analysis of reduction−oxidation peak separations gave electrochemical rate constants of 7.2 s-1 for A1 and 65 s-1 for FA/FB. A catalytic process was observed in which electrons were injected from PS I in films to ferredoxin in solution, mimicking in vivo electron shuttle from the terminal FA/FB cofactors to soluble ferredoxin during photosynthesis.