Insights into Chemistry of Biological Materials: Newly Discovered Silica-Aragonite-Chitin Biocomposites in Demosponges
Hermann EhrlichPaul SimonW. Carrillo‐CabreraVasilii V. BazhenovJoseph P. BottingMicha IlanAlexander EreskovskyGuilherme MuricyHartmut WorchA. MenschR. BornArmin SpringerK. KummerD. V. VyalikhС. Л. МолодцовDenis V. KurekMartin KammerSilvia PaaschEike Brunner
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Abstract:
Biological materials are a rewarding area of modern materials science, yielding both evolutionary insights and inspiration for biomimetic research. In particular, biocomposite structures are valuable sources of novel structures with unusual chemical properties, and they are very informative for the mechanisms of biomineralization. Here we describe a unique biocomposite of amorphous silica, crystalline aragonite, and chitin from species of the order Verongida, a group of marine sponges. The structures have been analyzed with a diverse suite of techniques, revealing a chitinous template for siliceous overgrowth containing aragonite-based crystal aggregates. Sponge chitin is an example of a specific template where two minerals in amorphous and crystalline forms are formed together with an organic molecule.Keywords:
Biocomposite
Biomimetic Materials
Amorphous calcium carbonate
Amorphous silica
Currently a basic tenet in biomineralization is that biominerals grow by accretion of amorphous particles, which are later transformed into the corresponding mineral phase. The globular nanostructure of most biominerals is taken as evidence of this. Nevertheless, little is known as to how the amorphous-to-crystalline transformation takes place. To gain insight into this process, we have made a high-resolution study (by means of transmission electron microscopy and other associated techniques) of immature tablets of nacre of the gastropod Phorcus turbinatus, where the proportion of amorphous calcium carbonate is high. Tablets displayed a characteristic nanoglobular structure, with the nanoglobules consisting of an aragonite core surrounded by amorphous calcium carbonate together with organic macromolecules. The changes in composition from the amorphous to the crystalline phase indicate that there was a higher content of organic molecules within the former phase. Within single tablets, the crystalline cores were largely co-oriented. According to their outlines, the internal transformation front of the tablets took on a complex digitiform shape, with the individual fingers constituting the crystalline cores of nanogranules. We propose that the final nanogranular structure observed is produced during the transformation of ACC into aragonite.
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Early-stage reaction mechanisms for aragonite-promoting systems are relatively unknown compared to the more thermodynamically stable calcium carbonate polymorph, calcite. Using cryoTEM and SEM, the early reaction stages taking place during aragonite formation were identified in a highly supersaturated solution using an alcohol-water solvent, and an overall particle attachment growth mechanism was described for the system. In vitro evidence is provided for the solid-state transformation of amorphous calcium carbonate to aragonite, demonstrating the co-existence of both amorphous and crystalline material within the same aragonite needle. This supports non-classical formation of aragonite within both a synthetic and biological context.
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Amorphous calcium carbonate (ACC) is an important precursor in biominerals such as shells, coral, foraminiferans, and urchin spine. However, the mechanism underlying the transition from ACC to stable biosynthetic crystals is still poorly understood. Herein, we identified a matrix protein referred to as Alv in Pinctada fucata, which has dramatically opposite functions during the different transition processes from ACC to stable crystals—calcite and aragonite in shell formation. The functions of Alv were studied by RNA interference, binding of recombinant Alv (rAlv) to chitin, calcite and aragonite assay, ACC transition, in vitro crystallization, calcium carbonate precipitation, and near-UV CD spectra. We found that rAlv could promote nucleation during ACC crystallization, stimulate the transition from ACC to calcite, but suppress transition from ACC to aragonite. It is concluded that Alv is involved in the transition of ACC, and plays a crucial role in the formation of shells. As far as we know, Alv is one of the few reported matrix proteins which play opposite roles in the transition of ACC to calcite and aragonite both in vivo and in vitro. This study could further enhance our understanding of the important regulatory role of biomacromolecules in biomineralization.
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Abstract Calcite and aragonite shell layers of the main classes of Molluscs are composed of monocrystalline units (prisms, tablets, laths or fibres). Scanning electron and atomic force microscopy studies show these units are composed of small round granules with a thin cortex (amorphous calcium carbonate and/or organic matrix). These granules are organo-mineral composites. A comparison of the size and shape of the granules in different taxa (Mollusca, Brachiopoda) suggests a possible relationship with taxonomy and/or phylogeny.
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Abstract Early‐stage reaction mechanisms for aragonite‐promoting systems are relatively unknown compared to the more thermodynamically stable calcium carbonate polymorph, calcite. Using cryoTEM and SEM, the early reaction stages taking place during aragonite formation were identified in a highly supersaturated solution using an alcohol–water solvent, and an overall particle attachment growth mechanism was described for the system. In vitro evidence is provided for the solid‐state transformation of amorphous calcium carbonate to aragonite, demonstrating the co‐existence of both amorphous and crystalline material within the same aragonite needle. This supports non‐classical formation of aragonite within both a synthetic and biological context.
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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.
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Magnesium doped Amorphous Calcium Carbonate was synthesised from precursor solutions containing varying amounts of calcium, magnesium, H2O and D2O. The Mg/Ca ratio in the resultant Amorphous Calcium Carbonate was found to vary linearly with the Mg/Ca ratio in the precursor solution. All samples crystallised as aragonite. No Mg was found in the final aragonite crystals. Changes in the Mg to Ca ratio were found to only marginally effect nucleation rates but strongly effect crystal growth rates. These results are consistent with a dissolution-reprecipitation model for aragonite formation via an Amorphous Calcium Carbonate intermediate.
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