MSGID: 1:317/3 64a3a072   
   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Chemists discover why photosynthetic light-harvesting is so efficient   
    The disorganized arrangement of the proteins in light-harvesting   
   complexes is the key to their extreme efficiency    
      
     Date:   
         July 3, 2023   
     Source:   
         Massachusetts Institute of Technology   
     Summary:   
         Chemists have measured the energy transfer between photosynthetic   
         light- harvesting proteins. They discovered that the disorganized   
         arrangement of light-harvesting proteins boosts the efficiency of   
         the energy transduction.   
      
      
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   When photosynthetic cells absorb light from the sun, packets of energy   
   called photons leap between a series of light-harvesting proteins until   
   they reach the photosynthetic reaction center. There, cells convert   
   the energy into electrons, which eventually power the production of   
   sugar molecules.   
      
   This transfer of energy through the light-harvesting complex occurs with   
   extremely high efficiency: Nearly 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   
   proteins of the light-harvesting complex, also called the antenna,   
   achieve that high efficiency. For the first time, the researchers were   
   able to measure the energy transfer between light-harvesting proteins,   
   allowing them to discover that the disorganized arrangement of these   
   proteins boosts the efficiency of the energy transduction.   
      
   "In order for that antenna to work, you need long-distance energy   
   transduction.   
      
   Our key finding is that the disordered organization of the   
   light-harvesting proteins enhances the efficiency of that long-distance   
   energy transduction," says 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 PhD '22 are the lead authors of the paper, which will   
   appear in the Proceedings of the National Academy of Sciences. Jianshu   
   Cao, an MIT professor of chemistry, is also an author of the paper.   
      
   Energy capture For this study, the MIT team focused on purple bacteria,   
   which are often found in oxygen-poor aquatic environments and are commonly   
   used as a model for studies of photosynthetic light-harvesting.   
      
   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 happen on timescales   
   of femtoseconds to nanoseconds, scientists have been able to study how   
   energy moves within a single one of these proteins. However, studying how   
   energy travels between these proteins has proven much more challenging   
   because it requires positioning multiple proteins in a controlled way.   
      
   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 those of naturally occurring   
   cell membranes. By controlling the size of these membranes, known as   
   nanodiscs, they were able to control the distance between two proteins   
   embedded within the discs.   
      
   For this study, the researchers embedded two versions of the primary   
   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 usually expressed only   
   during low light conditions.   
      
   Using the cryo-electron microscope at the MIT.nano facility, the   
   researchers could image their membrane-embedded proteins and show that   
   they were positioned 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.   
      
   Disordered 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 spaced closely   
   together, the researchers found that it takes about 6 picoseconds for   
   a photon of energy to travel between them. For proteins farther apart,   
   the transfer takes up to 15 picoseconds.   
      
   Faster travel translates to more efficient energy transfer, because the   
   longer the journey takes, the more energy is lost during the transfer.   
      
   "When a photon gets absorbed, you only have so long before that energy   
   gets lost through unwanted processes such as nonradiative decay,   
   so the faster it can get converted, the more efficient it will be,"   
   Schlau-Cohen says.   
      
   The researchers also found that proteins arranged in a lattice structure   
   showed less efficient energy transfer than proteins that were arranged   
   in randomly organized structures, as they usually are in living cells.   
      
   "Ordered organization is actually less efficient than the disordered   
   organization of biology, which we think is really interesting because   
   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 says.   
      
   Now that they have established the ability to measure inter-protein   
   energy transfer, the researchers plan to explore energy transfer between   
   other proteins, such as the transfer between proteins of the antenna to   
   proteins of the reaction center. They also plan to study energy transfer   
   between antenna proteins found in organisms other than purple bacteria,   
   such as green plants.   
      
   The research was funded primarily by the U.S. Department of Energy.   
      
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   Journal Reference:   
      1. Dihao Wang, Olivia C. Fiebig, Dvir Harris, Hila Toporik, Yi Ji,   
      Chern   
         Chuang, Muath Nairat, Ashley L. Tong, John I. Ogren, Stephanie   
         M. Hart, Jianshu Cao, James N. Sturgis, Yuval Mazor, Gabriela   
         S. Schlau-Cohen.   
      
         Elucidating interprotein energy transfer dynamics within the antenna   
         network from purple bacteria. Proceedings of the National Academy   
         of Sciences, 2023; 120 (28) DOI: 10.1073/pnas.2220477120   
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   Link to news story:   
   https://www.sciencedaily.com/releases/2023/07/230703160002.htm   
      
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