(Nanowerk News) Using complex arrays of metal-encrusted pigments, proteins, enzymes, and coenzymes, photosynthetic organisms can convert the energy in light into the chemical energy for life. And now, thanks to a study published in Natural (“Absorption and emission of single photons from natural photosynthetic complexes”), we know that these organic chemical reactions are sensitive to the smallest possible amount of light – one photon.
This discovery reinforces our current understanding of photosynthesis and will help answer questions about how life works at the smallest scales, where quantum physics and biology meet.
“A tremendous amount of work, theoretically and experimentally, has been done around the world trying to understand what happens after a photon is absorbed. But we realized that no one was talking about the first step. That is still a question that needs to be answered in detail,” said lead author Graham Fleming, a senior faculty scientist in the Area of Biosciences at Lawrence Berkeley National Laboratory (Berkeley Lab) and professor of chemistry at UC Berkeley.
In their study, Fleming, co-lead author Birgitta Whaley, a senior faculty scientist in the Energy Sciences Area at Berkeley Lab, and their research group demonstrated that a single photon can indeed initiate the first step of photosynthesis in photosynthetic purple bacteria. Because all photosynthetic organisms use similar processes and share a common evolutionary ancestor, the team believes that photosynthesis in plants and algae works the same way. “Nature invented a very clever trick,” said Fleming.
How living systems use light
Based on how efficient photosynthesis is at converting sunlight into energy-rich molecules, scientists have long assumed that only one photon is needed to start a reaction, in which the photon passes energy to an electron which then swaps places with the electron in a different molecule, eventually creating the precursor material. for sugar manufacture. After all, the sun doesn’t provide many photons – only a thousand of them arriving at one chlorophyll molecule per second on a clear day – but the process occurs reliably all over the planet.
However, “no one has ever supported that assumption with demonstrations,” said first author Quanwei Li, a joint postdoctoral researcher who developed the new experimental technique with quantum light in Fleming and Whaley’s group.
And, complicating matters, many studies are uncovering precise details about the next steps of photosynthesis being carried out by igniting photosynthetic molecules with very fast and powerful laser pulses.
“There is a huge difference in intensity between a laser and sunlight – a typical focused laser beam is one million times brighter than sunlight,” said Li. Even if you managed to generate weak rays of intensity matching that of sunlight, they would still be very different due to a quantum property of light called photon statistics. Because no one has seen a photon being absorbed yet, we don’t know how different it is, he explained. “But just as you need to understand every particle to build a quantum computer, we need to study the quantum properties of living systems to truly understand them, and to create efficient artificial systems that produce renewable fuels.”
Photosynthesis, like other chemical reactions, was first understood in bulk – meaning that we know what the overall input and output are like, and from that we can infer what the interactions between individual molecules are like. In the 1970s and 80s, advances in technology allowed scientists to directly study individual chemicals during reactions. Now, scientists are starting to explore the next frontier, the individual atomic and subatomic particle scales, using more advanced technologies.
From assumptions to facts
Designing experiments that allow the observation of individual photons means bringing together a unique team of theorists and experimenters that combine the cutting-edge tools of quantum optics and biology. “That’s new to people studying photosynthesis, because they don’t usually use this tool, and it’s new to people in quantum optics because we don’t usually think about applying this technique to complex biological systems,” said Whaley, who is also a professor of chemical physics. at UC Berkeley.
The scientists set up a photon source that produces one pair of photons through a process called spontaneous parametric down-conversion. During each pulse, the first photon – the “herald” – is observed by a highly sensitive detector, which confirms that the second photon is heading to a sample of the light-absorbing molecular structure assembled from a photosynthetic bacterium. Another photon detector near the sample was installed to measure the low-energy photons emitted by the photosynthetic structure after absorbing the second “heralded” photon of its original counterpart.
The light-absorbing structure used in the experiment, called LH2, has been studied extensively. It is known that a photon at a wavelength of 800 nanometers (nm) is absorbed by the ring of 9 bacteriochlorophyll molecules in LH2, causing energy to be passed to a second ring of 18 bacteriochlorophyll molecules that can emit fluorescent photons at 850 nm. In the original bacteria, the energy from the photons would continue to be transferred to the next molecule until it was used to initiate photosynthetic chemistry. In experiments however, when LH2 has been dissociated from the rest of the cellular machinery, the detection of an 850 nm photon serves as the definitive sign that the process has been activated.
“If you only have one photon, it’s very easy to lose it. So that’s the fundamental difficulty in this experiment and that’s why we used herald photons,” said Fleming. The scientists analyzed more than 17.7 billion herald photon detection events and 1.6 million heralded fluorescent photon detection events to ensure that the observations could only be attributed to the absorption of a single photon, and no other factors influenced the results.
“I think the first thing is that this experiment has shown that you can actually do something with individual photons. So that’s a very, very important point,” Whaley said. “The next thing is, what else can we do? Our goal is to study the energy transfer of individual photons through photosynthetic complexes at the shortest possible temporal and spatial scales.”