Synthesis of a Model System for the Primary Energy Conversion Reactions in Photosynthesis

A model system is constructed which, like the photosynthetic apparatus of green plants, can convert light to chemical free energy through pigment-sensitized photooxidation of water. The system has two light-harvesting subunits connected electrically in series. Each subunit is made of a multimolecular layer of Zn(II)-tetraphenylporphyrin deposited on a clean aluminum surface and immersed in an aqueous mixture of potassium ferri- and ferrocyanide. Upon illumination by amber light, charge transfer takes place across more than 70 molecular layers of the pigment with a photoelectromotive force of 1.1 to 1.3 volts per subunit. With NADP as the electron acceptor and NADP-reductase as a mediator, the system can photooxidize water to oxygen gas. With these model experiments as a guide, a molecular mechanism for the primary energy conversion reactions in photosynthesis is formulated that offers a unified interpretation of most of the relevant observations reported in the literature.