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+ {"source_url": "https://source.wustl.edu", "url": "https://source.wustl.edu/2019/12/switching-tracks/", "title": "Washington University in St. Louis", "top_image": "https://source.wustl.edu/wp-content/uploads/2019/12/shutterstock_8299315_760.jpg", "meta_img": "https://source.wustl.edu/wp-content/uploads/2019/12/shutterstock_8299315_760.jpg", "images": ["https://source.wustl.edu/wp-content/themes/thesource-theme/_assets/icons/search.svg", "https://source.wustl.edu/wp-content/uploads/2018/12/PARC_Blankenship_Pakrasi_Sun-300x300.jpg", "https://source.wustl.edu/wp-content/themes/thesource-theme/_assets/img/washu-logo.svg", "https://source.wustl.edu/wp-content/themes/thesource-theme/_assets/icons/close.svg", "https://source.wustl.edu/wp-content/uploads/2019/12/shutterstock_8299315_760.jpg", "https://source.wustl.edu/wp-content/uploads/2019/12/Chem_Kirmaier_C-300x200.jpg"], "movies": [], "text": "Think of a train coming down the tracks to a switch point where it could go either to the right or the left \u2014 and it always goes to the right.\n\nPhotosynthetic organisms have a similar switch point. After sunlight is absorbed, energy transfers rapidly to a protein called the reaction center. From this point, the electrons could move either to an A-branch (or \u201cright-track\u201d) set of molecules, or to a B-branch (\u201cleft-track\u201d) set of identical molecules.\n\nNew research from Washington University in St. Louis and Argonne National Laboratory coaxes electrons down the track that they typically don\u2019t travel \u2014 advancing understanding of the earliest light-driven events of photosynthesis. The findings were published Dec. 31 in the Proceedings of the National Academy of Sciences (PNAS).\n\n\u201cIn the bacterial reaction center, an electron goes to the A-branch of molecules 100% of the time. We have made it go to the B-branch molecules 90% of the time,\u201d said Christine Kirmaier, research professor of chemistry in Arts & Sciences.\n\n\u201cAfter all, if you think you understand how the train and the tracks work, why shouldn\u2019t you be able to make the train go to the left rather than the right? That\u2019s essentially what we\u2019ve done,\u201d Kirmaier said.\n\n\u201cWhy two tracks have evolved is still an open question, but the ability to control which track is utilized is exciting,\u201d said Philip D. Laible, a biophysicist in the biosciences division at Argonne National Laboratory and another lead author on the paper.\n\n\u201cWe would like to make the switching between them a more well understood phenomenon so that we could readily conduct electrons (pardon the pun) to any destination in a biological process,\u201d he said. \u201cRight now, we are controlling features that allows for electrons to transverse a biological membrane \u2014 the first step in making energy from sunlight in this organism.\u201d\n\nRe-engineering a pathway\n\nPlants, algae and photosynthetic bacteria convert the energy of sunlight into charge-separated units that they use to power life processes on Earth. And they do it in a very specific way: The reaction centers in these organisms feature two mirror image-like arrangements of protein and pigment cofactors, the A and B sides. Only one of these chains is active \u2014 the A side \u2014 while the B side is silent.\n\nKirmaier, with collaborator Dewey Holten, professor of chemistry at Washington University, and the team at Argonne National Laboratory have designed many iterations of photosynthetic mutants with the goal of achieving charge separation using the B branch instead. The new research re-engineers a pathway in a purple photosynthetic bacteria, one of nature\u2019s solar cells.\n\n\u201cUsing molecular biology, we\u2019ve been changing the amino acids around the pigments to try and find the magic combination to make the B branch work,\u201d she said.\n\nThe game was to make structural changes that de-tune, or make less optimal, electron transfers along the A side or normal path \u2014 and then, at the same time, speed up the reactions along the B side.\n\nThe researchers were able to step up this trial-and-error process by testing all possible amino acids at a specific target site on the A or B side, finding one or more that improve the B-side yield. They then carried that \u201chit\u201d forward in the mutant background to probe the next target site, and so on.\n\n\u201cIt was unexpected,\u201d Kirmaier said. \u201cWe picked a site, and in one of our best mutant backgrounds, placed all 20 amino acids there \u2014 and one of them gave us a 90% yield.\u201d\n\n\u201cThis is a breakthrough achievement and something that [everyone in] the field has been actively trying to figure out for decades \u2014 ever since we first set eyes on the two tracks in a high-profile structural study in Nature nearly 35 years ago,\u201d said Deborah K. Hanson of the biosciences division, Argonne National Laboratory, another lead author of the PNAS paper.\n\nRethinking the history of photosynthesis\n\nThe new work illuminates basic structure-function principles that govern efficient, light-induced electron transfer.\n\nThis knowledge can aid design of biohybrid and bioinspired systems for energy conversion and storage, the researchers said. The findings also will provoke additional experiments and analysis.\n\n\u201cThe results raise lots of questions about what is required to get unidirectional charge separation,\u201d Holten said.\n\nIn nature, purple bacteria do initial charge separation with a two-step process that takes place in several trillionths of a second. But the team\u2019s new B-branch solution gets almost the same yield, even though it uses a tandem one-step process that takes 5-10 times longer.\n\n\u201cIn the original history of photosynthesis, maybe such a combination of a fast two-step and slower one-step processes gave a 80 or 90% yield \u2014 and then, over time, it optimized,\u201d Holten said.\n\nFunding: This research is funded by the U.S. Department of Energy", "keywords": [], "meta_keywords": [""], "tags": ["chemistry", "Photosynthesis", "electron transfer", "Arts & Sciences"], "authors": [], "publish_date": "Wed Jan 1 00:13:58 2020", "summary": "", "article_html": "", "meta_description": "", "meta_lang": "en", "meta_favicon": "https://source.wustl.edu/wp-content/themes/thesource-theme/_assets/img/favicon/apple-touch-icon.png?v=20160912", "meta_data": {"viewport": "width=device-width, initial-scale=1, shrink-to-fit=no", "msapplication-config": "https://source.wustl.edu/wp-content/themes/thesource-theme/_assets/img/favicon/browserconfig.xml?v=20160912", "robots": "max-snippet:-1, max-image-preview:large, max-video-preview:-1", "og": {"locale": "en_US", "type": "article", "title": "Switching tracks | The Source | Washington University in St. Louis", "description": "Chemists in Arts & Sciences have re-engineered one of nature's solar cells to drive electrons down an alternate path. This work advances the understanding of the earliest light-driven events of photosynthesis and is published in the Proceedings of the National Academy of Sciences.", "url": "https://source.wustl.edu/2019/12/switching-tracks/", "site_name": "The Source", "image": {"identifier": "https://source.wustl.edu/wp-content/uploads/2019/12/shutterstock_8299315_760.jpg", "secure_url": "https://source.wustl.edu/wp-content/uploads/2019/12/shutterstock_8299315_760.jpg", "width": 760, "height": 507}}, "article": {"tag": "Photosynthesis", "section": "Science & Technology", "published_time": "2020-01-01T00:13:58+00:00", "topic": "Science & Technology", "school": "Arts & Sciences", "channel": "Newsroom"}, "twitter": {"card": "summary_large_image", "description": "Chemists in Arts & Sciences have re-engineered one of nature's solar cells to drive electrons down an alternate path. This work advances the understanding of the earliest light-driven events of photosynthesis and is published in the Proceedings of the National Academy of Sciences.", "title": "Switching tracks | The Source | Washington University in St. Louis", "image": "https://source.wustl.edu/wp-content/uploads/2019/12/shutterstock_8299315_760.jpg"}}, "canonical_link": "https://source.wustl.edu/2019/12/switching-tracks/"}