![]() “The sponge makes it possible to locally increase the ion concentration, which makes nucleation easier,” said Nico Sommerdijk, a collaborator in the study from Eindhoven University of Technology in Eindhoven, The Netherlands. Biomineralization then readily occurs where the calcium is clustered. Having monitored the crystallization process step by step, they concluded that the negatively charged polymer side chains act as a sponge for the positively charged calcium ions (or counterions), concentrating them in specific regions. Large charged molecules (red) lure in calcium ions (blue), which position carbonate ions (yellow and red) and form amorphous calcium carbonate (white ball). The finding suggested to the team that calcium binding to the organic macromolecules mediated how and where the ACC formed. Once the carbonate was added, ACC crystals only formed within the calcium-PSS globules, and stopped growing once the calcium ran out. The macromolecules clumped together, absorbing the calcium ions to form globules. To understand how the PSS scaffold interfered with vaterite formation, the researchers mixed the calcium with the macromolecules without the carbonate. ACC is a precursor to many biologically based minerals. ![]() This time, the mineralization process looked different: Amorphous calcium carbonate (ACC) formed first, then later transformed into vaterite. Then, they repeated the experiment with added polystyrene sulfonate (PSS), an organic polymer with negatively charged side chains that is structurally similar to the macromolecules that guide biomineralization in natural systems. ![]() They first observed vaterite and a little bit of calcite, two different calcium carbonate crystal structures, forming in a solution in the TEM. ![]() “This work is of great value in the realm of fundamental materials science-in particular in the world of living systems, where soft matter controls hard matter,” said Jim De Yoreo of Pacific Northwest National Laboratory.ĭe Yoreo and his colleagues used liquid-phase transmission electron microscopy (TEM)-a relatively new imaging technology that visualizes atomic-level activity in liquid samples-to monitor the crystallization process in real time at nanoscale resolution. The results, published recently in Nature Materials, challenge previous assumptions about the molecular-level mechanisms responsible for biomineralization. Now, an international team of materials researchers has demonstrated that these organic scaffolds influence the crystallization process by binding clusters of positively charged calcium ions, inducing mineral formation in specific locations. Understanding on a molecular level the way that inorganic minerals interact with a framework of biological macromolecules is a critical step toward mimicking the process in artificial systems-and one that has proven challenging. Through a process called biomineralization, organisms like mollusks, clams, and corals crystallize excess carbon in their environment into hard calcium carbonate shells. The seashells you pick up at the beach might not seem extraordinary, but they are a source of inspiration for researchers searching for efficient ways to store extra atmospheric carbon.
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