June 14 () –
A novel experiment has revealed the quantum dynamics of one of nature’s most crucial processes by verifying that photosynthesis begins with a single photon.
Thanks to a complex cast of metallic pigments, proteins, enzymes, and coenzymes, photosynthetic organisms can convert light energy into chemical energy for life. And now, thanks to the new one, it is known that this organic chemical reaction it is sensitive to the least amount of light possible, that is, to a single photon.
The discovery, published in Nature, it solidifies our current understanding of photosynthesis and will help answer questions about how life works on the smallest of scales, where quantum physics and biology meet.
“A huge amount of work, theoretical and experimental, has been done all over the world to try to understand what happens after the absorption of a photon, but we realized that nobody was talking about the first step. That was a question that still needed to be answered in detail,” says study co-author Graham Fleming, senior scientist in the Biosciences Area of the Lawrence Berkeley National Laboratory (Berkeley Lab) and Professor of Chemistry at the University of Berkeley (United States).
In their study, Fleming, co-lead author Birgitta Whaley, a senior scientist in Berkeley Lab’s Energy Sciences Area, and their research groups demonstrated that a single photon can initiate the first step of photosynthesis in photosynthetic purple bacteria.
Since all photosynthetic organisms use similar processes and share an evolutionary ancestor, the team is confident that photosynthesis in plants and algae works in the same way. “Nature has invented a very clever trick”, says Fleming.
Based on the efficiency of photosynthesis in converting sunlight into energy-rich molecules, scientists have long assumed that a single photon was all it took to start the reaction, in which photons transmit energy to electrons, which in turn , are exchanged with electrons from different molecules, ultimately creating the precursor ingredients for the production of sugars. After all, the sun doesn’t provide that many photons–only a thousand photons reach a chlorophyll molecule per second on a sunny day– and yet the process occurs reliably all over the planet.
However, “no one had ever backed up that assumption with a demonstration,” recalls first author Quanwei Li, a joint postdoctoral researcher developing new experimental techniques with quantum light in the Fleming and Whaley groups.
And, to further complicate matters, much of the research that has unraveled precise details about the later steps of photosynthesis was carried out by activating photosynthetic molecules with powerful, ultra-fast laser pulses.
“There is a huge difference in intensity between a laser and sunlight: a typical concentrated laser beam is a million times brighter than sunlightLi explains.
“Even if you manage to produce a weak beam with an intensity equal to that of sunlight, they are still very different due to quantum properties of light called photon statistics. Since no one has seen how the photon is absorbed, we don’t know what There is a difference between the type of photon it is –he reasons–, but just as each particle must be understood to build a quantum computer, we need to study the quantum properties of living systems to truly understand them and make efficient artificial systems that generate renewable fuels”.
Photosynthesis, like other chemical reactions, began to be understood roughly, that is, we knew what the global inputs and outputs were and, from there, we could deduce what the interactions between individual molecules would be like.
In the 1970s and 1980s, technological advances allowed scientists to directly study individual chemical reactions. Now, scientists are beginning to explore the next frontier, the scale of the individual atom and subatomic particle, using even more advanced technologies.
Designing an experiment that allowed individual photons to be observed involved bringing together a unique team of theorists and experimenters who combined cutting-edge tools from quantum optics and biology. “It was new for those who study photosynthesis, because they don’t normally use these tools.and it was new to the quantum optics people, because we don’t normally think of applying these techniques to complex biological systems,” explains Whaley, who is also a professor of chemical physics at UC Berkeley.
The scientists set up a photon source that generates a single pair of photons through a process called spontaneous parametric downconversion. During each pulse, the first photon — “the harbinger” — was observed with a highly sensitive detector, which confirmed that the second photon was on its way to the assembled sample of light-absorbing molecular structures taken from photosynthetic bacteria.
Another photon detector was installed near the sample to measure the lowest energy photon emitted by the photosynthetic structure after absorbing the second “heralded” photon of the original pair.
The light-absorbing structure used in the experiment, called LH2, has been the subject of numerous studies.. Photons at the 800 nanometer (nm) wavelength are known to be absorbed by a ring of 9 bacteriochlorophyll molecules in LH2, causing energy to pass to a second ring of 18 bacteriochlorophyll molecules that can emit fluorescent photons at 850nm.
In native bacteria, the energy from the photons would continue to be transferred to subsequent molecules until it was used to initiate the chemistry of photosynthesis. But in the experiment, when the LH2 had been separated from other cellular machinery, the detection of the 850 nm photon it served as a definitive signal that the process had been activated.
“If you only have one photon, it’s terribly easy to lose it. So that was the fundamental difficulty of this experiment and that’s why we used the herald photon,” Fleming says. Scientists analyzed more than 17.7 billion heraldic photon detection events and 1.6 million heraldic fluorescent photon detection events to ensure that the observations could only be attributed to single photon absorption and that no other factors influenced the results.
“I think the first thing is that this experiment has shown that you can actually do things with single photons. That’s very, very important,” Whaley says. “The next thing is, what else can we do? Our goal is to study the transfer of energy of individual photons through the photosynthetic complex on the shortest possible spatial and temporal scales”.
