Vibrations deep within spinach leaves enhance the efficiency of photosynthesis—the energy conversion process that powers life on our planet.
The discovery could potentially help engineers make more efficient solar cells and energy storage systems. It also injects new evidence into an ongoing “quantum biology” debate over exactly how photosynthesis manages to be so efficient.
Through photosynthesis, plants and some bacteria turn sunlight, water, and carbon dioxide into food for themselves and into oxygen for animals to breathe. It’s perhaps the most important biochemical process on Earth and scientists don’t yet fully understand how it works.
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The study’s findings identify specific molecular vibrations that help enable charge separation—the process of kicking electrons free from atoms in the initial steps of photosynthesis.
“Both biological and artificial photosynthetic systems take absorbed light and convert it to charge separation. In the case of natural photosynthesis, that charge separation leads to biochemical energy,” explains Jennifer Ogilvie, an associate professor of physics and biophysics at the University of Michigan and lead author of a paper published in Nature Chemistry.
“In artificial systems, we want to take that charge separation and use it to generate electricity or some other useable energy source such as biofuels.”
Charge separation
It takes about one-third of a second to blink your eye. Charge separation happens in roughly one-hundredth of a billionth of that amount of time. Ogilvie and her research group developed an ultrafast laser pulse experiment that can match the speed of these reactions. By using carefully timed sequences of ultrashort laser pulses, Ogilvie and coworkers were able to initiate photosynthesis and then take snapshots of the process in real time.
The researchers worked with Charles Yocum, a professor emeritus, to extract what’s called the photosystem II reaction centers from the leaves.
Located in the chloroplasts of plant cells, photosystem II is the group of proteins and pigments that does the photosynthetic heavy lifting. It’s also the only known natural enzyme that uses solar energy to split water into hydrogen and oxygen.
Spinach leaves
To get a sample, the researchers bought a bag of spinach leaves from a grocery store. “We removed the stems and veins, put it in the blender and then performed several extraction steps to gently remove the protein complexes from the membrane while keeping them intact.
“This particular system is of great interest to people because the charge separation process happens extremely efficiently,” she says. “In artificial materials, we have lots of great light absorbers and systems that can create charge separation, but it’s hard to maintain that separation long enough to extract it to do useful work. In the photosystem II reaction center, that problem is nicely solved.”
The researchers used their unique spectroscopic approach to excite the photosystem II complexes and examine the signals that were produced.
Exciting molecules
“We can carefully track what’s happening,” Ogilvie says. “We can look at where the energy is transferring and when the charge separation has occurred.”
The spectroscopic signals they recorded contained long-lasting echoes, of sorts, that revealed specific vibration motions that occurred during charge separation.
“What we’ve found is that when the gaps in energy level are close to vibrational frequencies, you can have enhanced charge separation,” Ogilvie says. “It’s a bit like a bucket-brigade: how much water you transport down the line of people depends on each person getting the right timing and the right motion to maximize the throughput. Our experiments have told us about the important timing and motions that are used to separate charge in the photosystem II reaction center.”
She envisions using this information to reverse engineer the process—to design materials that have appropriate vibrational and electronic structure to mimic this highly efficient charge separation process.
The US Department of Energy, the National Science Foundation, and the University of Michigan Center for Solar and Thermal Energy Conversion, as well as the Research Council of Lithuania funded the research.
Source: University of Michigan
