Redox Potentials of Quinones in Aqueous Solution: Relevance to Redox Potentials in Protein Environments

Abstract

Quinones serve as redox-active cofactors in photosynthetic reaction centers. To understand the energetics of electron transfer along the electron transfer pathways in protein environments, the redox potentials (Em) of the cofactors in water versus normal hydrogen electrode (NHE) are required. However, ubiquinone, menaquinone (phylloquinone), and plastoquinone, which are found in photosynthetic reaction centers, have insoluble hydrophobic isoprene side chains, and thus far only Em in dimethylformamide (DMF) versus saturated calomel electrode (SCE) had been reported. Recently, Em in water versus NHE was reported for the quinone species of photosynthetic reaction centers. These results confirmed that Em(Q/Q-) in water versus NHE was more relevant to Em(Q/Q-) in protein environments than Em(Q/Q-) in DMF versus SCE. It has also been demonstrated that Em for one-electron reduction can also be calculated based on the lowest unoccupied molecular orbital (LUMO) level of the quinone molecules.