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October 4, 2004
Looking to INCITE for Insight into Photosynthesis

Solar power remains the ultimate Olympic gold-medal dream of a clean, efficient and sustainable source of energy. The problem has been that in order to replace fossil fuels, we need to get a lot more proficient at harvesting sunlight and converting it into energy. Nature has solved this problem through photosynthesis; all we have to do is emulate it. But first we need a much better understanding of how photosynthesis works at the molecular and electronic levels.

Seeking to unlock the secrets of photosynthesis are, from left, Alán Aspuru-Guzik, Graham Fleming, Romelia Salomón-Ferrer, Brian Austin, Harsha Vaswani, William Lester, and Ricardo Oliva.

"After working on the problem for about 3 billion years, nature has achieved an energy transfer efficiency of approximately 97 percent," says Graham Fleming, director of Berkeley Lab's Physical Biosciences Division and an internationally acclaimed leader in spectroscopic studies of photosynthetic processes. "If we can get a complete understanding as to how this is done, creating artificial versions of photosynthesis should be possible."

Toward this end, Fleming has teamed his capabilities in ultrafast spectroscopic experiments with the computational expertise of theoretical chemist William Lester, Jr., of the Chemical Sciences Division. Their collaboration received one of three inaugural INCITE Awards (Innovative and Novel Computational Impact on Theory and Experiment) from the U.S. Department of Energy's Office of Science. The INCITE program is aimed at advancing a select number of computationally intensive scientific projects with high-impact potential by providing a substantial amount of time on the supercomputers at NERSC, the National Energy Research Scientific Computing Center based at Berkeley Lab.

"The theory behind energy transfer in photosynthesis is more than 50 years old, but it has never been fully tested," Fleming says. "Some aspects of this theory are beyond current experimental testing capabilities but can be simulated at NERSC."

Says Lester, "Before we had computational capabilities such as those at NERSC, it was not possible to model the energy and electron transfer processes we want to study. NERSC can provide us with the computers and software support that will enable us to run our codes to give us the information we need and could not otherwise obtain."

Photosynthesis, the biochemical process by which green plants and certain bacteria are able to convert the energy of light into chemical energy, is responsible for the production of all of Earth's oxygen.

Life on Earth is dependent upon the photosynthetic reactions that green plants and cyanobacteria use to convert energy from sunlight into chemical energy. Among other things, these reactions are responsible for the production of all of our planet's oxygen. In high-school biology, students learn that nature uses chlorophyll, the family of green-pigment molecules, as a light absorber and energy-transfer agent, but the chemistry behind this process is extremely complicated. What's more, photosynthetic chemistry takes place on a femtosecond timescale, a femtosecond being one millionth of a billionth of a second.

"According to the first law of photosynthetic economics, a photon saved is a photon earned," Fleming says. "Nature has designed one of the most exquisitely effective systems for harvesting light, with the reactions happening too fast for any light to be wasted as heat. Current synthetic light-harvesting devices, however, aren't following nature's model."

Fleming has been using femtosecond spectroscopy techniques to shed scientific light on nature's light-harvesting and energy-transferring secrets. Photosynthesis in plants starts with a light harvesting system, which consists of two protein complexes, Photosystem I and Photosystem II. Each complex features light-absorbing antennas made up of members from two families of pigment molecules, chlorophylls and carotenoids. These pigment antennas are able to capture photons of sunlight over a wide spectral and spatial cross section.

The light harvesting system in plants and bacteria consists of rings of molecules, complex 1 and complex 2, which feature pigment antennas of clorophyll and carotenoid molecules. Light energy is funneled to the reaction center, where it is converted to chemical energy.

The chlorophyll and carotenoid molecules gain extra "excitation" energy from the captured photons, which is immediately funneled from one neighboring molecule to the next, until it arrives at another molecular complex that serves as a reaction center for converting solar energy solar to chemical energy. This transferal of excitation energy involves several hundred molecules and hundreds of individual steps along different electronic pathways — yet still transpires within 30 picoseconds for Photosystem I and 200 picoseconds for Photosystem II. (A picosecond is a trillionth of a second.) By human standards of time, that's instantaneous.

"If we can follow the steps in transferring energy from donor to acceptor molecules, we might be able to design new and much more effective strategies for synthetic light harvesters," Fleming says.

Because the extra energy being transferred from one molecule to the next changes the way each molecule absorbs and emits light, the flow of energy can be spectroscopically followed. To do this, however, Fleming and his experimental research team need to know what spectroscopic signals they should be looking for.

This is where the INCITE grant will help. Working with NERSC staff members, most prominently David Skinner, and using NERSC's IBM SP computer, Lester has developed and is running a "quantum Monte Carlo" computer code to predict the optimal electronic pathways for photosynthetic energy transfer. A quantum Monte Carlo code is a statistical model for studying strongly correlated systems such as electrons.

Says Lester, "Most people have long thought of computational chemistry as only being able to tackle simple systems reliably, but we've come a long way with improved implementation of our algorithms in recent years."

With sufficient understanding of the energy conversion in photosynthesis, fields of expensive solar cells could be replaced with much cheaper green plants, making commercial solar power feasible.

With the INCITE grant, which will provide them with a million processor hours, Fleming and Lester will study the electronic structures behind a defense mechanism within the photosynthetic system, which protects plants from absorbing more solar energy than they can immediately utilize and suffering from oxidation damage as a result. The focus will be on the carotenoids in Photosystem II, which appear to be the controlling elements behind this photoprotective mechanism.

"The photosynthetic light-harvesting system works less efficiently when sunlight is intense, and more efficiently when the available sunlight is lower," says Fleming. "This system is so sensitive to changing light conditions, it will even respond to the passing of clouds overhead. It is one of nature's supreme examples of nanoscale engineering."

Studying the way in which carotenoids regulate photon absorption through spectroscopic experiments is made exceptionally difficult by how closely packed together the carotenoid molecules are. This tight spacing changes the way one molecule "sees" and interacts with its neighbors. A previous attempt to calculate the electronic structures and energy pathways that are involved in the regulation process was off by several orders of magnitude.

"These will be one of the largest electronic and molecular calculations ever performed at such a rigorous level of theory," says Lester, "but the project should show the research community that with our powerful algorithms and a computational resource like NERSC, we can model highly complex chemical systems."

Other researchers working with Fleming and Lester on this project include Alán Aspuru-Guzik, Romelia Salomón-Ferrer, Brian Austin, Harsha Vaswani and Ricardo Oliva.

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