Hydroxyapatite crystals, the main component of tooth enamel, captured in a microscope image (left) shows only mineral shape and orientation, while the APT image on the right captures protein (red) trapped within between crystals. (Image courtesy of Sandra Taylor, Pacific Northwest National Laboratory.)Biominerals such as bone, teeth, and shells possess unique hierarchical structures with exceptional physical properties. These structures are created during biomineralization and are regulated by interfacial interactions between organic macromolecules and growing inorganic crystallites. Macromolecules exert control over crystallites’ size, structure, morphology, aggregation, and orientation. But fundamental processes occurring at the interface between them is poorly understood and technically challenging to explore. Now researchers have demonstrated how atom probe tomography (APT) can be used to reveal organic-inorganic interfacial interactions during mineralization [Taylor et al., Materials Today Advances 18 (2023) 100378, http://doi.org/10.1016/j.mtadv.2023.100378].
“The interaction of biomolecules, like proteins, with crystals as they grow leads to the unique structures and physicochemical properties in biominerals,” explain Sandra Taylor and Arun Devaraj of Pacific Northwest National Laboratory, who led the work. “[For example], the association of the protein amelogenin with calcium phosphate minerals as they grow could make all the difference between having strong, healthy teeth or structurally weak, abnormal teeth in humans,” they point out.
Amelogenin guides the nucleation, growth, and assembly of amorphous calcium phosphate into long, thin ribbon-like structures that transform into hydroxyapatite (HAP), weaving together to form tooth enamel – one of nature’s hardest and most crack-resistant materials. To understand this process and also how abnormalities might arise, the researchers’ developed an approach utilizing APT to explore how proteins like amelogenin interact at the nanoscale with HAP. They grew HAP in the presence of amelogenin in vitro and used APT to map the protein distribution on the mineral surface with near atomic resolution. The analysis can be carried out at different points in the biomineralization process to unravel the underlying crystal growth mechanism.
“The analytical techniques we developed allows us to deduce unique protein-mineral interactions that could influence tooth enamel formation,” says Taylor.
For example, the researchers observed that the protein distribution over the inorganic surface is not uniform but becomes trapped within aggregating crystal structures. The ability to determine protein distributions on inorganic interfaces in vitro will likely provide other insights into interactions at different stages of mineral growth by, for example, looking at the effects of other ions, additives, or organic macromolecules.
“We also anticipate that our work will be valuable for analyzing polymer composites and other organic engineered materials, especially where interactions between organic and inorganic structures are key to their performance,” adds Devaraj.