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Lawrence Berkeley National Laboratory Recent Work Title Nanoscale Transforming Mineral Phases in Fresh Nacre Permalink
| Content Provider | Semantic Scholar |
|---|---|
| Author | DeVol, Ross T. Sun, Chang-Yu Marcus, Matthew A. Coppersmith, Susan N. Myneni, Satish C. B. Gilbert, Pupa U. P. A. |
| Copyright Year | 2015 |
| Abstract | Nacre, or mother-of-pearl, the iridescent inner layer of many mollusk shells, is a biomineral lamellar composite of aragonite (CaCO3) and organic sheets. Biomineralization frequently occurs via transient amorphous precursor phases, crystallizing into the final stable biomineral. In nacre, despite extensive attempts, amorphous calcium carbonate (ACC) precursors have remained elusive. They were inferred from non-nacre-forming larval shells, or from a residue of amorphous material surrounding mature gastropod nacre tablets, and have only once been observed in bivalve nacre. Here we present the first direct observation of ACC precursors to nacre formation, obtained from the growth front of nacre in gastropod shells from red abalone (Haliotis rufescens), using synchrotron spectromicroscopy. Surprisingly, the abalone nacre data show the same ACC phases that are precursors to calcite (CaCO3) formation in sea urchin spicules, and not proto-aragonite or poorly crystalline aragonite (pAra), as expected for aragonitic nacre. In contrast, we find pAra in coral. ■ INTRODUCTION Mollusk shell nacre is among the most studied biominerals due to its iridescent lamellar structure, its remarkable resistance to fracture, and its complex formation mechanisms. An amorphous precursor to nacre formation has been suspected to exist but has never been successfully identified, despite extensive attempts. Most fresh nacre studies thus far published focus on the organic matrix, only one of them has focused on the minerals. In recent years, amorphous calcium carbonate (ACC) minerals have been identified as the first phases deposited by living organisms to form their biominerals, which later become fully crystalline via a series of biologically controlled phase transitions. Sea urchin embryonic spicules were the first biominerals in which an amorphous precursor was discovered, followed by sea urchin spines and teeth, bone, tooth enamel, and a variety of other biominerals. These observations sparked tremendous interest in in situ and in silico experiments and simulations to understand mineral formation via ACC in abiotic and biological systems. Key in vitro experiments are beginning to reveal how the amorphous precursor phases may be stabilized by confinement in pores, between crossed cylinders, in picoliter droplets, within a silica coating, and in liposomes, all of which prevent release of or contact with water. In sea urchin embryos, such stabilization by confinement takes place in an intracellular vesicle in which ACC is first synthesized and then transported to the biomineralization site. Another intriguing possibility is that acidic polymers such as poly-aspartate stabilize ACC in a liquid precursor phase, as first demonstrated by the Gower group and later re-discovered by molecular dynamics simulations. In vivo, once hydrated amorphous calcium carbonate (ACCH2O) is released from the intracellular vesicle, it must be stabilized by other agents, possibly proteins, retarding its dehydration and crystallization. In sea urchin embryonic spicules this function may be carried out by SM50 or other proteins. This stabilizing function of proteins, yet to be identified, would prevent the deposited mineral in the forming biomineral from prematurely converting to calcite or aragonite (both naturally occurring crystal forms of calcium carbonate, CaCO3). The presence of ACC in mature nacre was demonstrated by Nassif et al., who observed by transmission electron microscopy (TEM) that a thin, 2 nm layer of ACC surrounds crystalline aragonite tablets. This is consistent with an ACC precursor to all the aragonite deposited but does not uniquely demonstrate it, because other mechanisms could generate such an ACC lining. For instance, if aragonite crystals grew from solution ionReceived: July 29, 2015 Published: September 24, 2015 Article |
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| Alternate Webpage(s) | https://cloudfront.escholarship.org/dist/prd/content/qt5j71h6rv/qt5j71h6rv.pdf?t=onlji1 |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
