Have you ever had a photo ruined by someone who moved too quickly? Scientists have the same problem when they record images of proteins that change in response to light. For years, researchers have tried to capture the details of this process. They have been stopped by how fast it happens.
A team of researchers from the University of Wisconsin Milwaukee and the Center for Free-Electron Laser Science in Germany have combined machine learning and quantum mechanical calculations to get the most precise record yet of structural changes in a photoactive yellow protein. They were able to make movies of processes that took less than a second.
PYP rearranges itself when it absorbs light. Whenever PYP folds or bends after being illuminated, this causes huge changes to the structure of the cell. Abbas Ourmazd, a physicist at UWM and co-author of the study, says that one important example of the interaction between light and proteins is in plants. Petra Fromme is the director of the Biodesign Center for Applied Structural Discovery at Arizona State University and she said that PYP is similar to a proteins in our eyes that help us see at night. Somebacteria can detect blue light that may be damaging to their genes so they can move away from it.
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Scientists have been unable to get details of isomerization for years. The isomerization is instant upon light, says Fromme. The changes in the structure of a molecule can happen in a single second. 32 million years is to a second, says Fromme.
Scientists probe these incredibly short timescales with similarly short flashes of X-rays. The data obtained in this way was used in the new study by the team led by UWM physicist Marius Schmidt. The researchers illuminated PYP with light. They hit it with an X-ray burst. The diffracted X-rays reflected the most recent structure in the same way that light reflected from objects helps make a picture. The briefness of the pulse allowed scientists to get a snapshot of the positions of all the atoms as they moved, like a camera with a very fast shutter can do.
The illustration depicts an experiment at the lab that showed how a bacterium changes shape in response to light.
Even the shortest X-ray flashes can not be used to get a record of aProteins shape change. Shaul Mukamel, a chemist at the University of California, Irvine who was not part of the study, says that the X-ray source is noisy. The X-ray flash always causes some blurriness. Imagine a contortionist folding itself into a pretzel. Using X-rays, scientists can get a clear image of its relaxed pose immediately after it absorbs the light energy that spurs the contortion and of its limbs intertwined at the end. Any images of its in-between motions would be fuzzy.
