State-of-the-art experiments have revealed the quantum mechanics underlying one of nature’s most important processes.
Photosynthetic organisms can convert light energy into chemical energy for life using a complex cast of metal-encrusted pigments, proteins, enzymes and coenzymes. In a recently published study, Nature It turns out that this natural chemical process is sensitive to the smallest possible amount of light. photon.
This discovery confirms our current understanding. photosynthesis And it helps answer questions about how life works on the smallest scales that quantum physics and biology meet.
“There has been a huge amount of theoretical and experimental work done around the world to understand what happens after a photon is absorbed. But no one is talking about the first steps.” It was a question that still needed to be answered in detail,” said co-first author, senior researcher in the Biological Sciences Division at Lawrence Berkeley National Laboratory (Berkeley Lab) and a researcher at the University of California, Berkeley. said Graham Fleming, a professor of chemistry.
In their study, Fleming, Birgitta Waley, co-lead author and senior scientist in the Energy Sciences Division at Berkeley Lab, and their research group found that a single photon is actually the first of photosynthesis in photosynthetic purple bacteria. Indicated that the step can be started. Because all photosynthetic organisms use similar processes and share an evolutionary ancestry, the research team is confident that photosynthesis in plants and algae works similarly. “Nature has invented some very clever tricks,” says Fleming.
How Living Systems Use Light
Based on the efficiency of photosynthesis in converting sunlight into energy-rich molecules, scientists have long assumed that only a single photon is sufficient to initiate a reaction. A photon transfers energy to an electron, which then replaces it with an electron in another molecule, ultimately a precursor component to produce a sugar. After all, the sun doesn’t provide that many photons. Only 1,000 photons reach a single chlorophyll molecule per second on a clear day, yet this process occurs reliably across the globe.
But “no one had empirically backed up that hypothesis,” says lead author Quan Wei Lee, a joint postdoctoral fellow developing new experimental techniques using quantum light in the Fleming and Whaley groups. said Mr.
And to further complicate matters, many of the studies that have revealed precise details about the late stages of photosynthesis have been done by triggering photosynthetic molecules with intense, ultrafast laser pulses.
“There’s a big difference in intensity between lasers and sunlight. A typical focused laser beam is a million times brighter than sunlight,” Lee said. Even if we could produce a weak beam with an intensity that matches that of sunlight, the intensity would still vary widely due to a quantum property of light called photon statistics. No one has ever seen a photon absorbed, so we don’t know what difference it makes or what kind of photon it is, he explained. “But in the same way that we need to understand each particle to build a quantum computer, to truly understand living systems and create efficient artificial systems that produce renewable fuels, life We need to study the quantum properties of the system.”
Photosynthesis, like other chemical reactions, was initially understood holistically. So now we know what the overall inputs and outputs are, and from that we can infer what the interactions between individual molecules are like. In the 1970s and his 80s, technological advances allowed scientists to study individual chemicals directly during reactions. Scientists are now next frontierindividual atomsubatomic particle scale using even more advanced techniques.
From Assumption to Fact
Designing experiments that allow the observation of individual photons meant bringing together a unique team of theorists and experimenters who combined state-of-the-art tools from quantum optics and biology. “For people studying photosynthesis, this was refreshing because they don’t usually use these tools. I hadn’t thought about it, so that was refreshing,” Professor Whaley said. He is also Professor of Chemical Physics at the University of California, Berkeley.
Scientists set up a photon source that produces a pair of photons through a process called spontaneous parametric downconversion. During each pulse, the first photon (“messenger”) is observed by a sensitive detector, confirming that the second photon is en route to an assembled sample of light-absorbing molecular structures taken from photosynthetic bacteria. it was done. Another photon detector near the sample was installed to measure the low-energy photons emitted by the photosynthetic structure after absorbing his second ‘heralded’ photon of the original pair.
The light-absorbing structure used in the experiment, called LH2, has been extensively studied. A photon with a wavelength of 800 nanometers (nm) is absorbed by a ring of 9 bacteriochlorophyll molecules within LH2 and the energy is transferred to a second ring of 18 bacteriochlorophyll molecules that can emit a fluorescence photon at 850 nm. known to be In natural bacteria, the energy from a photon continues to be transferred to subsequent molecules until it is used to initiate the chemistry of photosynthesis. However, in our experiments, when LH2 was isolated from other cellular machinery, the detection of photons at 850 nm served as a definitive indication that this process was activated.
“If you only have one photon, it’s very easy to lose it. That’s the fundamental difficulty of this experiment, and that’s why we use the Herald Photon,” Fleming said. Scientists analyzed more than 17.7 billion Herald photon detection events and more than 1.6 million Herald fluorescence photon detection events, and found that the observations were due solely to the absorption of single photons, and that other factors contributed to the results. confirmed that it did not affect
“I think first of all, this experiment shows that you can actually do something with individual photons, so that’s a very, very important point,” Whaley said. . “The next question is what else can we do? Our goal is to study the energy transfer from individual photons through photosynthetic complexes on the shortest possible temporal and spatial scales.”
Reference: “Absorption and Emission of Single Photons from Natural Photosynthetic Complexes” Quanwei Li, Kaydren Orcutt, Robert L. Cook, Javier Sabines-Chesterking, Ashley L. Tong, Gabriela S. Schlau-Cohen, Xiang Zhang, Graham R. Fleming and K. Birgitta Waley, 14 June 2023, Nature.
DOI: 10.1038/s41586-023-06121-5