Scientists zoom in on the atomic structure of man-made proteins


October 30, 2021

(Nanowerk NewsScientists have created thin, paper-like crystalline sheets using a synthetic protein-like molecule called a polypeptoid. These nanosheets are only one molecule thick, the molecules being arranged in a very specific way. Scientists take images of these nanosheets using electron microscopes under cryogenic conditions.

Until recently, these images were blurry due to the small number of electrons that can pass through the leaves without causing damage. In this study (PNAS, “Engineering and imaging at the atomic level of polypeptoid crystal lattices”), the researchers used machine learning-based algorithms to process around 500,000 independent images. The result is the first clear, real image of individual atoms in a flexible synthetic material.

Cryogenic imaging combined with machine learning allowed scientists to derive the chemical structure of short peptoid polymers (green) from micrographs (grayscale image) and observe bromine atoms on side chains ( magenta). (Image: Lawrence Berkeley National Laboratory)

Synthetic polymers are essential for many products we take for granted. These range from plastic furniture to the fuselages of modern airplanes. They are also at the heart of devices such as fuel cells and rechargeable batteries. These devices are becoming increasingly important in the emerging clean energy landscape.

All the important properties of synthetic polymers depend on the arrangement of their atoms. The ability of scientists to position individual atoms in polymeric materials will improve our understanding of the bottlenecks that limit the performance of synthetic polymers. This research also marks an important step in the whole of nanoscience.

For the first time, scientists have revealed the atomic structural details of a flexible synthetic material. Peptoid diblock copolymers consist of two different protein-like chains that are linked together. These materials were designed to fit closely together to form highly organized crystalline sheets in water.

The individual molecules and their relative orientations within the nanosheets were directly observed by cryogenic transmission electron microscopy (cryo-TEM), revealing atomic details in positional space inaccessible by conventional scattering techniques. The ultra-cold temperature used to quickly freeze the nanosheets effectively locked the molecules in place.

Imaging the sample under cryogenic conditions prevented energetic electrons from destroying the structure. To better protect soft materials from the electron beam, the researchers used fewer electrons per image. The images obtained under these conditions were processed using sophisticated mathematical tools and machine learning algorithms to produce high resolution images of the structure at the atomic scale.

The combined precision synthesis of peptoid polymers, atomic imaging of cryo-MET, and computer modeling have helped scientists understand polymer structures at the atomic level. Researchers are now able to perform atomic-level modifications to design targeted molecules. This paves the way for the rational engineering of sophisticated functions in flexible materials through systematic control of their chemical structure. The research was carried out in part at the molecular foundry and the advanced light source.

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