Wrighton and others have been able to find and to build multi-component molecules with ''a donor and an acceptor system covalently linked to a light absorber, an assembly,'' he pointed out, "that does indeed resemble the heart of the Z-scheme" found in plants. But their system does not produce energy as efficiently as they would like, because of the timing of the reactions: "In solution the energy-rich molecules lose the stored energy by either intermolecular or intramolecular back electron transfer. In nature, the movement of the carriers away from one another is crucial to high efficiency. The unidirectional movement of electrons in the Z-scheme is a consequence of the components of the charge transport chain, and how they are arranged, both geometrically and energetically," Wrighton explained. Work in his group, he continued, did lead to construction of a complex molecule with all of the functional components, but the photoexcitation test showed that a 10-nanosecond time was required for the donor to deliver the molecule to the acceptor. "This is very sluggish compared to the 4 picoseconds demonstrated [in the natural systems]," Wrighton summarized, "and so one of the challenges is to prepare a molecule that will have more zip than our 10-nanosecond time." Thus chemists explore the quantum world, said Wrighton, narrowing in on several factors that might elucidate the transfer rates of the electrons: "the energetics for electrons, the distance dependence of electron transfer, and the structures of the donor and acceptor and their relationship" in space. George McLendon of the University of Rochester, said Wrighton, "has made important progress in understanding such factors."
McLendon is a chemist specializing in the quantum processes of moving electrons from one molecule to another. Not focusing exclusively on photosynthesis, he usually works with proteins and biological systems, but his laboratory has demonstrated phenomena crucial to all electron transfer systems. The basic physics involves the concept of conservation of energy, which, explained McLendon, shows that an electron's rate of transfer varies with the energy force driving it. Essential to the first step in photosynthesis, this relationship between rate and energy was analyzed theoretically some years ago by Rudy Marcus (1956) at Caltech, who predicted an anomaly that was first confirmed by John Miller at Argonne National Laboratory, and verified subsequently by McLendon and others. Up to a certain level of energy, the rate of electron transfer increases with the force driving it, but the initially proportional relationship changes. After the peak level is reached, additional driving force actually slows the electron down. "A funny thing," said McLendon, "is that you can have too much of a good thing."