Three billion years ago, light first passed through chlorophyll in tiny reaction centers, the first step that photosynthetic plants and bacteria took to convert light into food.
Heliobacteria, a type of bacteria that use photosynthesis to generate energy, have reaction centers considered to be similar to those of the common ancestors of all photosynthetic organisms. Now, a team from the University of Michigan has determined the first steps in converting light to energy for this bacteria.
“Our study highlights the different ways in which nature has used the basic architecture of the reaction center that emerged over 3 billion years ago,” said lead author and UM physicist Jennifer Ogilvie. “We ultimately want to understand how energy moves through the system and ultimately creates what we call ‘the state of separate charge.’ This state is the battery that drives the engine of photosynthesis. “
laser 2.jpeg Photosynthetic organisms contain “antenna” proteins that are filled with pigment molecules to harvest photons. The collected energy is then directed to “reaction centers” which fuel the initial stages that convert light energy into food for the body. These initial stages occur on incredibly fast time scales – femtoseconds, or a millionth of a billionth of a second. In the blink of an eye, this conversion occurs several quadrillion times.
Researchers want to understand how this transformation occurs. This allows us to better understand how plants and photosynthetic organisms transform light into nourishing energy. It also allows researchers to better understand how photovoltaics work and how to better build them.
When light reaches a photosynthetic organism, the pigments inside the antenna collect photons and direct energy to the reaction center. In the reaction center, energy pushes an electron to a higher energy level, from which it moves to a new location, leaving behind a positive charge. This is called a charge separation. This process occurs differently depending on the structure of the reaction center in which it occurs.
In the reaction centers of plants and most photosynthetic organisms, the pigments that orchestrate charge separation absorb similar colors of light, making it difficult to visualize charge separation. Using heliobacteria, the researchers identified which pigments initially donate the electron after being excited by a photon, and which pigments accept the electron.
Heliobacteria are a good model to look at, Ogilvie said, because their reaction centers contain a mixture of chlorophyll and bacteriochlorophyll, which means these different pigments absorb different colors of light. For example, she says, imagine trying to follow a person in a crowd – but everyone is wearing blue jackets, you are watching from a distance, and you can only take snapshots of the person moving through the crowd.
“But if the person you were looking at was wearing a red jacket, you could track them a lot easier. This system is kind of like this: it has separate markers,” said Ogilvie, professor of physics, biophysics and science and macromolecular engineering.
Previously, heliobacteria were difficult to understand because the structure of their reaction center was unknown. The structure of membrane proteins like reaction centers is notoriously difficult to determine, but Ogilvie co-author, Arizona State University biochemist Kevin Redding, has developed a way to solve the crystal structure of these reaction centers. reaction.
To probe reaction centers in heliobacteria, the Ogilvie team uses a type of high-speed spectroscopy called multidimensional electron spectroscopy, performed in Ogilvie’s lab by senior author and postdoctoral fellow Yin Song. The team is aiming for a sequence of very short, carefully timed laser pulses on a sample of bacteria. The shorter the laser pulse, the wider the light spectrum it can excite.
Each time the laser pulse hits the sample, the light excites the reaction centers within. The researchers vary the delay between pulses, then record how each of those pulses interacts with the sample. When the pulses hit the sample, its electrons are excited to a higher energy level. Pigments in the sample absorb specific wavelengths of laser-specific color light – and the colors that are absorbed give researchers information about the structure of the system’s energy level and how energy crosses it.
“That’s an important role of spectroscopy: when we just look at the structure of something, it’s not always obvious how it works. Spectroscopy allows us to follow a structure as it operates, as energy is absorbed and makes its way through them first. energy conversion steps, ”Ogilvie said. “Because the energies are quite distinct in this type of reaction center, we can really have a clear view of where the energy is going.”
Getting a clearer picture of this energy transport and charge separation allows researchers to develop more precise theories about how the process works in other reaction centers.
“In plants and bacteria, the mechanism of charge separation is thought to be different,” Ogilvie said. “The dream is to be able to take a structure and, if our theories are good enough, we should be able to predict how it works and what will happen in other structures – and rule out mechanisms that are incorrect.”