from a molecule in solution to interact with molecules that have been chemically attracted to the surface of the assembly; the second step involves the product of that step reacting with molecules inside the tunnel altogether. "So we have a self-assembling, three-molecule chain, where the viologen is inside, porphyrin is on the outside, and EDTA is ion-paired with the porphyrin out there," Mallouk explained. The assembly worked, in that it propelled the electron into the tunnel in 30 picoseconds, only 10 times slower than with natural photosynthesis. The efficiency of the electron-hole separation was very poor, however, said Mallouk, illustrating Wrighton's emphasis on the importance of back electron transfer pathways.
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."
"It would be the biological equivalent of a tandem photovoltaic cell," said Robert Blankenship, one of the lead authors in the paper who studies photosynthesis at Washington University in St. Louis. "And those can have very high efficiencies."
''Natural photosynthesis is a process by which light from the sun is converted to chemical energy," began Mark Wrighton in his presentation to the Frontiers symposium. Wrighton directs a laboratory at the Massachusetts Institute of Technology's Chemistry Department, where active research into the development of workable laboratory synthesis of the process is under way. As chemists have known for many decades, the chemical energy he referred to comes from the breakdown of carbon dioxide (CO2) and water (H2O), driven by photons of light, and leads to production of carbohydrates that nourish plants and of oxygen (O2), which is vital to aerobic organisms. What is not known in complete detail is how this remarkable energy-conversion system works on the molecular level. However, recent advances in spectroscopy, crystallography, and molecular genetics have clarified much of the picture, and scientists like Wrighton are actively trying to transform what is known about the process into functional, efficient, synthetic systems that will tap the endless supply of energy coming from the sun. "Photosynthesis works," said Wrighton, "and on a large scale." This vast natural phenomenon occurring throughout the biosphere and producing an enormous amount of one kind of fuel—food for plants and animals—Wrighton described as "an existence proof that a solar conversion system can produce [a different, though] useful fuel on a scale capable of meeting the needs'' of human civilization. Photovoltaic (PV) cells already in use around the world provide a functional (if more costly per kilowatt-hour)
Wrighton's presentation, "Photosynthesis—Real and Artificial," was a closely reasoned, step-by-step discussion of the crucial stages in the chemical and molecular sequence of photosynthesis. His colleagues in the session were chosen for their expertise in one or another of these fundamental specialized areas of photosynthesis research. By the end of the session, they had not only provided a lucid explanation of the process, but had also described firsthand some of the intriguing experimental data produced. Douglas Rees of the California Institute of Technology (on the molecular details of biological photosynthesis), George McLendon of the University of Rochester (on electron transfer), Thomas Mallouk of the University of Texas (on the arrangement of materials to facilitate multielectron transfer chemistry), and Nathan Lewis of the California Institute of Technology (on synthetic systems using liquid junctions) all supplemented Wrighton's overview with reports about findings in their own area of photosynthesis research.
Titled "Comparing Photosynthetic and Photovoltaic Efficiencies and Recognizing the Potential for Improvement," (LINK HERE) the article combines lessons learned from evolutionary photobiology and modern solar cells to make the case for a potentially huge boost in the efficiency of the solar production of biofuels.
The multi-junction tandem solar cell initially developed at NREL proved to be an important strategy to understand how to boost the efficiency of corn, grasses, algae, and other plants that use photosynthesis to produce stored solar energy.
Photosynthesis does use two gaps based on chlorophyll molecules to provide enough energy to drive the photosynthesis reaction. But the two gaps have the same energy value, which means they don't help each other to produce energy over a wider stretch of the spectrum of solar light and enhance conversion efficiency.
One of NREL's roles at the DOE workshop was to help make it clear how the efficiency of photosynthesis could be improved by re-engineering the structure of plants through modern synthetic biology and genetic manipulation based on the principles of high efficiency photovoltaic cells, Nozik said. In synthetic biology plants can be built from scratch, starting with amino acid building blocks, allowing the formation of optimum biological band gaps.
The enabling paradigm was developed by British chemist John Dalton, who proposed the atomic theory of matter around the turn of the 19th century. Notwithstanding subsequent refinements due to quantum physics and to scientists' increasing ability to probe and examine these reactions directly, Dalton's basic description of the behavior and transfer of protons and electrons among and between elements and compounds—the opening salvo fired at every high school chemistry student—still sets the stage for the most advanced chemical research. Photosynthesis provides a vivid example of the type of drama that is played out effortlessly in nature but reenacted elaborately in chemical laboratories with painstaking concern for the intri-
A U.S. Department of Energy (DOE) workshop that drew a prestigious collection of 18 scientists to compare the efficiency of plants and photovoltaic solar cells led to an important and provocative scholarly article in today's issue of the journal . Two of the scientists are from DOE's National Renewable Energy Laboratory (NREL), Arthur J. Nozik and Maria Ghirardi,
In the case of research on artificial photosynthesis, such work could lead to the economical production of an alternative to the dwindling supply of fossil fuels. And a further benefit might be a reduction in the sulfurous products emitted by the combustion of carbon-based fuels. Wrighton explained that these fuels are themselves "the result of photosynthetic processes integrated over the ages." It must be kept in mind that long before plants developed the ability to produce molecular oxygen as a byproduct of photosynthesis, they were always about their real business of converting carbon dioxide from the atmosphere into carbohydrates for their own sustenance. In fact some of these anaerobic plants still exist today in certain specialized ecological niches. The system for photosynthesis evolved to its present state during the earth's natural history, and it exploits materials that are naturally abundant and inexpensive, Wrighton pointed out. As designed by nature, it is the ultimate recycling process—since it uses the planet's two most abundant resources, CO2 and H2O, providing fuel and breaking down a pollutant.
If one of Earth's abundant natural resources could be energized by sunlight to produce (probably by the breakdown and release of one of its elements) a source that could be used for fuel, the entire fossil fuel cycle and the problems associated with it might be obviated. If that resource were water, for example, and the resultant fuel source were hydrogen, burning liquid hydrogen in the air would produce only water as a combustion product. Liquid hydrogen is already in use as a fuel source and has always been the primary fuel powering space vehicles, since it produces more heat per gram of weight than any other known fuel. If a photosynthetic system delivering usable hydrogen could be developed, the process would regenerate the original water source, and an entirely new recycling of natural resources could be established. This time, however, cultural rather than natural evolution would call the shots. With such a major new fuel production process, science would hopefully be able to provide a methodology to better anticipate and control the global impact of any byproducts or emissions.