Abstract
Over the last eight years new theories regarding nucleation, crystal growth, and polymorphism have emerged. Many of these theories were developed in response to observations in nature, where classical nucleation theory failed to account for amorphous mineral precursors, phases, and particle assembly processes that are responsible for the formation of invertebrate mineralized skeletal elements, such as the mollusk shell nacre layer (aragonite polymorph) and the sea urchin spicule (calcite polymorph). Here, we summarize these existing nucleation theories and place them within the context of what we know about biomineralization proteins, which are likely participants in the management of mineral precursor formation, stabilization, and assembly into polymorphs. With few exceptions, much of the protein literature confirms that polymorph-specific proteins, such as those from mollusk shell nacre aragonite, can promote polymorph formation. However, past studies fail to provide important mechanistic insights into this process, owing to variations in techniques, methodologies, and the lack of standardization in mineral assay experimentation. We propose that the way forward past this roadblock is for the protein community to adopt standardized nucleation assays and approaches that are compatible with current and emerging nucleation precursor studies. This will allow cross-comparisons, kinetic observations, and hopefully provide the information that will explain how proteins manage polymorph formation and stabilization.
Highlights
In nature, invertebrate organisms primarily utilize calcium carbonates as the building materials for extracellular skeletal elements such as the mollusk shell [1,2,3,4] sea urchin spicules [5,6,7,8,9,10,11] and corals [12,13,14]
Biological calcium carbonates can exist in an amorphous state [9,10,11,15,16,17,18,19] and as three different crystalline polymorphs: calcite, aragonite, and vaterite, where calcite is the more stable form and aragonite and vaterite are metastable relative to calcite [15,16]
The attraction of biological calcium carbonate polymorphism to the scientific community is that this process occurs largely under ambient conditions [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19] and represents a “game changer” for materials science and chemistry communities who have utilized non-ambient and sometimes extreme conditions to generate a given polymorph
Summary
Invertebrate organisms primarily utilize calcium carbonates as the building materials for extracellular skeletal elements such as the mollusk shell [1,2,3,4] sea urchin spicules [5,6,7,8,9,10,11] and corals [12,13,14]. What is often overlooked in the discussion of biological calcium carbonate polymorphism is the role and identity of extrinsic agents in the formation and stabilization processes. We believe that the challenge for the protein community will be to adopt consistent mineralization assay standards and approaches that are compatible with current and emergent nucleation precursor studies [15,16,29,30,31,32,33]. These studies will allow time-dependent, careful monitoring of protein effects on the early events in nucleation and polymorph selection processes that can yield information on mechanisms. Will we be able to determine what nucleation schemes are at work in nature, how proteins foster polymorph formation and stabilization, and how to apply this information to materials science
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