Scientists from the U.S. Department of Energy’s (DOE) Argonne National Laboratory, working closely with collaborators at Washington University in St. Louis, have solved a critical part of the age-old mystery of photosynthesis, focusing on the initial, ultrafast events through which photosynthetic proteins capture light and use it to initiate a series of electron transfer reactions.
Plants have been harnessing the sun’s energy for hundreds of millions of years. Algae and photosynthetic bacteria have been doing the same for even longer, all with remarkable efficiency and resiliency.
It comes as no surprise then that scientists want to understand exactly how they do this hoping to use this knowledge to improve human-made devices such as solar panels and sensors.
Photosynthesis in plants
Photosynthesis is the process by which plants and other organisms convert light energy into chemical energy. It is then used to convert water, carbon dioxide, and minerals into oxygen and energy-rich organic compounds. In most cases, oxygen is also released as a waste product.
Most plants, most algae, and cyanobacteria perform photosynthesis; such organisms are called photoautotrophs. Photosynthesis is largely responsible for producing and maintaining the oxygen content of the Earth’s atmosphere and supplies most of the energy necessary for life on Earth.
“In order to understand how biology fuels all of its engrained activities, you must understand electron transfer,” said Argonne biophysicist Philip Laible. “The movement of electrons is crucial: it’s how work is accomplished inside a cell.”
In photosynthetic organisms, these processes begin when a photon absorbs light pigments localized in proteins. Each photon propels an electron across a membrane located inside specialized compartments within the cell.
Argonne biochemist Deborah Hanson said, “The separation of charge across a membrane –and stabilization of it — is critical as it generates energy that fuels cell growth.” The Argonne and Washington University research team has gained valuable insight with respect to the initial steps in this process: the electron’s journey.
When the first structure of these types of complexes was discovered nearly 35 years ago, scientists were surprised to know that after the absorption of light, the electron transfer processes faced a dilemma: there are two possible pathways for the electron to travel.
Plants, algae and photosynthetic bacteria use just one of them and the scientists had no idea why. What they did know was that the propulsion of the electron across the membrane – effectively harvesting the energy of the photon – required multiple steps.
The research team managed to interfere with each one of them to change the electron’s trajectory. “We’ve been on this trail for more than three decades, and it is a great accomplishment that opens up many opportunities,” said Dewey Holten, a chemist at Washington University.
The scientist’s article, “Switching sides – Reengineered primary charge separation in the bacterial photosynthetic reaction centre,” which was published in the Proceedings of the National Academy of Sciences, showed how they discovered an engineered version of this protein complex that switched the utilization of the pathways, enabling the one that was inactive while disabling the other.
“It is remarkable that we have managed to switch the direction of initial electron transfer,” said Christine Kirmaier, Washington University chemist and project leader. “In nature, the electron chose one path 100 per cent of the time. But through our efforts, we have been able to make the electron switch to an alternate path 90 per cent of the time. These discoveries pose exciting questions for future research.”
Owing to their efforts, the scientists are now closer than ever to being able to design electron transfer systems in which they can send an electron down a pathway of their choosing.
“This is important because we are gaining the ability to harness the flow of energy to understand design principles that will lead to new applications of abiotic systems,” Laible said. “This would allow us to greatly improve the efficiency of many solar-powered devices, potentially making them far smaller. We have a tremendous opportunity here to open up completely new disciplines of light-driven biochemical reactions, ones that haven’t been envisioned by nature. If we can do that, that’s huge.”