2,5-Diamino-p-benzoquinone Derivatives as Photosystem I Electron Acceptors: Synthesis and Electrochemical and Physicochemical Properties

A series of 2,5-diamino-p-benzoquinone derivatives have been prepared and their physicochemical properties studied. The sensitivity of their photoreduction potential to 3-(3,4-dichlorophenyl)-1,1-dimethylurea, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, and KCN, as well as the photosystem I (PSI) activity, suggests that the reduction of 2,5-diamino-p-benzoquinone derivatives in the illuminated thylakoid is at the primary electron acceptor of PSI and it is reversible. The half-wave potentials of these compounds to their corresponding radical anions in an aprotic medium such as acetonitrile were found to be comparable with the midpoint potential values of the electron transport carriers at the reducing site of PSI. The strong reductant produced by PSI is really more accessible to the strong lipophilic electron acceptor. These lipophilic p-benzoquinone derivatives can reach the carriers inside the thylakoid membrane more easily than the ionic electron acceptor. The accepting electron properties of these compounds at PSI are similar to those of the bipyridinium herbicides.