"Our study is a pioneering effort in imaging conformational structures of complex macromolecules. As this technology is effective for both crystalline and amorphous compounds, we believe this technology can also be applied for determination of structures of multinuclear peptides through complexation with tracer metal atoms."Takane Imaoka, Tokyo Institute of Technology
Coordination compounds possess molecular structures that consist of either one or more metal atoms at the center, surrounded by non-metal atoms. Their fascinating physical and chemical properties, which have significant applications in material science, depend largely on this molecular structure. Thus, a definitive analysis of their molecular structure is required not only for understanding their properties, but also for designing specific coordination compounds with targeted functions.
Though several analytical methods are available for determining the structure of coordination compounds, they each have their own limitations. For example, X-ray crystallography can only determine the structure of crystalline compounds, while nuclear magnetic resonance cannot provide accurate results when paramagnetic atoms are involved. A more recent microscopy technique called high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), which has revolutionized the field of molecular imaging with real-time visualization of single coordination molecules, is also limited to observations of simple and planar molecules.
As a consequence, determining the structure of various conformations (all possible spatial orientations of atoms) of both crystalline and amorphous polynuclear coordination molecules remains an unexplored area.
To bridge this gap, a team of researchers from Tokyo Institute of Technology in Japan, led by Kimihisa Yamamoto and Takane Imaoka, has developed a novel imaging method that uses a metal-atom tracer with HAADF-STEM to determine the conformational structures of complex and highly branched polynuclear coordination compounds. The researchers report this novel method in a paper in Science Advances.
"Using iridium as a metal tracer, because its high atomic number (77) will provide better visualization with HAADF-STEM, we synthesized iridium-fixated dendritic phenylazomethine (DPA) compounds," explained Imaoka. "Then we determined the optimum operating conditions for HAADF-STEM, under which the different conformations of these highly branched DPA compounds could be determined with the highest accuracy.”
To determine the optimum operating conditions for HAADF-STEM, the researchers studied samples of the iridium-DPA compound dispersed on the surface of graphene nanopowder under a variety of operating conditions. They found that reducing the beam current to 7pA and the exposure time per pixel to 8µs, together with low magnification, helped to reduce damage to the iridium-DPA compound and allowed the successful observation of its structure. The iridium atoms appear as bright spots in the resulting HAADF-STEM images, highlighting their position in the structure of the molecule.
Once the HAADF-STEM image of the iridium-DPA molecule was obtained using the optimum conditions, the researchers compared it to simulated images of all possible conformations of the molecule to find the closest match. They found that the structures captured in the experimental HAADF-STEM images fitted extremely well with the simulated conformational structures. Thus, the most accurate conformational orientation of a molecule could be easily determined by comparing the HAADF-STEM images with the simulated images.
The potential applications of this heavy-metal-guided HAADF-STEM technology are not just limited to analyzing the structure of coordination compounds. "Our study is a pioneering effort in imaging conformational structures of complex macromolecules," said Imaoka. "As this technology is effective for both crystalline and amorphous compounds, we believe this technology can also be applied for determination of structures of multinuclear peptides through complexation with tracer metal atoms, and work on this area is already under progress."
This story is adapted from material from Tokyo Institute of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.