When photosynthetic cells absorb light from the sun, packets of energy called photons jump between a series of light-absorbing proteins until they reach the photosynthetic reaction center. There, the cells convert the energy into electrons, which eventually activate the production of sugar molecules.
This transfer of energy through the light-harvesting complex occurs with extremely high efficiency: Almost every photon of light absorbed generates an electron, a phenomenon known as near-unity quantum efficiency.
A new study from MIT chemists offers a potential explanation for how the proteins in the light-harvesting complex, also called the antenna, achieve such high efficiency. For the first time, researchers were able to measure the energy transfer between light-harvesting proteins, allowing them to discover that the disorganized arrangement of these proteins increases the efficiency of energy transfer. .
“For that antenna to work, you need long-distance energy transfer. Our main finding is that the disordered organization of light-harvesting proteins improves the efficiency of long-distance energy transfer,” said Gabriela Schlau-Cohen, an associate professor of chemistry at MIT and the senior author of the new study.
MIT postdocs Dihao Wang and Dvir Harris and former MIT graduate student Olivia Fiebig Ph.D. are the lead authors of the paper, published in Proceedings of the National Academy of Sciences. Jianshu Cao, an MIT professor of chemistry, is also an author on the paper.
Energy capture
For this study, the MIT team focused on purple bacteria, which are commonly found in oxygen-poor aquatic environments and are often used as a model for photosynthetic light-harvesting studies.
Within these cells, captured photons travel through light-harvesting complexes consisting of proteins and light-absorbing pigments such as chlorophyll. Using ultrafast spectroscopy, a technique that uses extremely short laser pulses to study events that occur on timescales of femtoseconds to nanoseconds, scientists were able to study how energy moves within the one of these proteins. However, studying how energy travels between these proteins has proven more difficult because it requires positioning many proteins in a controlled manner.
To create an experimental setup where they could measure how energy travels between two proteins, the MIT team designed synthetic nanoscale membranes with a composition similar to natural cell membranes. By controlling the size of these membranes, known as nanodiscs, they were able to control the distance between the two proteins embedded within the disks.
For this study, the researchers embedded two versions of the main light-harvesting protein found in purple bacteria, known as LH2 and LH3, into their nanodiscs. LH2 is the protein that is present during normal light conditions, and LH3 is a variant that is normally expressed only during low light conditions.
Using the cryo-electron microscope at the MIT.nano facility, the researchers were able to image their proteins embedded in the membrane and show that they are located at distances similar to those seen in the native membrane. They were also able to measure the distances between the light-harvesting proteins, which were on the scale of 2.5 to 3 nanometers.
Messy is better
Because LH2 and LH3 absorb slightly different wavelengths of light, it is possible to use ultrafast spectroscopy to observe the energy transfer between them. For proteins linked together, the researchers found that it takes about 6 picoseconds for a photon of energy to travel between them. For proteins farther apart, the transition takes up to 15 picoseconds.
Faster travel translates into more efficient energy transfer, because the longer the trip, the more energy is lost during the transfer.
“When a photon is absorbed, it takes a long time for you to lose energy through unwanted processes like nonradiative decay, so the faster it can be converted, the more efficient it is,” Schlau-Cohen said.
The researchers also found that proteins arranged in a lattice structure show less efficient energy transfer than proteins arranged in randomly organized structures, as they usually are in living things. cells.
“Ordered organization is actually less efficient than disordered organization in biology, which we think is very interesting because biology tends to be chaotic. It’s to its advantage,” Schlau-Cohen said.
Now that they have established the ability to measure inter-protein energy transfer, the researchers plan to examine energy transfer between other proteins, such as transfer between antenna proteins to reaction center proteins. They also plan to study energy transfer between antenna proteins found in organisms other than purple bacteria, such as green plants.
More information:
Wang, Dihao et al, Elucidating interprotein energy transfer dynamics within the antenna network from purple bacteria, Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.2220477120. doi.org/10.1073/pnas.2220477120
Provided by the Massachusetts Institute of Technology
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Citation: Chemists discover why photosynthetic light-harvesting is so efficient (2023, July 3) retrieved on July 4, 2023 from https://phys.org/news/2023-07-chemists-photosynthetic-light-harvesting- efficient.html
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