For the first time, they were able to measure the energy transfer between light-harvesting proteins.
In the realm of photosynthesis, the seemingly chaotic arrangement of proteins within light-harvesting complexes may actually be their secret weapon for superior efficiency.
Sunlight absorbed by photosynthetic cells triggers a dynamic chain reaction. Photons, energy packets, leap across a series of light-harvesting proteins before landing at the photosynthetic reaction center. Here, the energy is converted into electrons, eventually fueling the creation of sugar molecules.
Ultra-Efficient Photosynthetic Light-Harvesting
This energy migration within the light-harvesting complex boasts almost perfect efficiency. Nearly every absorbed photon generates an electron – a marvel known as near-unity quantum efficiency.
MIT chemists propose an intriguing explanation for this high efficiency in a new study. For the first time, they were able to measure the energy transfer between light-harvesting proteins. This revealed that the seemingly haphazard organization of these proteins enhances the energy transfer’s efficiency.
“The disordered organization of the light-harvesting proteins enhances the efficiency of that long-distance energy transduction,” explains Gabriela Schlau-Cohen, an associate professor of chemistry at MIT and the study’s senior author.
A Study on Purple Bacteria
This study primarily focuses on purple bacteria. These organisms are typically found in oxygen-poor aquatic environments and serve as popular models for photosynthetic light-harvesting studies.
Photos absorbed by these bacteria travel through complexes made up of proteins and light-absorbing pigments like chlorophyll. Through ultrafast spectroscopy, scientists can explore how energy moves within a single protein. However, studying the energy transfer between proteins has been a more significant challenge due to the need for precise positioning of multiple proteins.
A Unique Experimental Approach: Photosynthetic Light-Harvesting
To overcome this obstacle, the MIT team designed synthetic nanoscale membranes, mimicking the composition of natural cell membranes. By controlling the size of these membranes, they could manage the distance between two proteins within the discs.
The study featured two variants of the primary light-harvesting protein found in purple bacteria – LH2 and LH3. LH2 is present under normal light conditions, while LH3 usually appears during low light conditions.
The team could image their membrane-embedded proteins using the cryo-electron microscope at the MIT.nano facility. They demonstrated that these proteins were positioned at distances comparable to those in natural membranes. The distance between light-harvesting proteins was measured to be between 2.5 to 3 nanometers.
The Journey of Energy Transfer
The slightly different light wavelengths absorbed by LH2 and LH3 enabled the researchers to observe energy transfer between them using ultrafast spectroscopy. For closely spaced proteins, the photon energy transfer took around 6 picoseconds. For proteins farther apart, the transfer could take up to 15 picoseconds. Faster transfer implies more efficient energy transfer, as more energy is lost during a long journey.
“The faster it can get converted, the more efficient it will be,” says Schlau-Cohen.
Remarkably, proteins in a disorganized structure demonstrated more efficient energy transfer than those in an orderly lattice structure, mirroring the organization usually found in living cells.
Future Exploration of Photosynthetic Light-Harvesting
“Biology tends to be disordered. This finding tells us that that may not just be an inevitable downside of biology, but organisms may have evolved to take advantage of it,” Schlau-Cohen adds.
With this newfound ability to measure inter-protein energy transfer, the researchers plan to study energy transfer between other proteins and explore organisms beyond purple bacteria, like green plants.
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