Research Group of
Denis Gebauer

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Journal of Structural Biology article available online "Retrosynthesis of CaCO3 via amorphous precursor particles using gastroliths of the Red Claw lobster (Cherax quadricarinatus)"


Gastroliths are highly calcified structures formed in the cardiac stomach wall of crustaceans for the temporary storage of amorphous CaCO3 (ACC). The gastrolithic ACC is stabilized by the presence of biomolecules, and represents a novel model for research into biomineralization. For the first time, an in vitro biomimetic retrosynthesis of scaffolds of gastrolithic matrices with CaCO3 is presented. With the help of synthetic polyacrylic (PAA) and phytic (PA) acids, amorphous precursor particles were stabilized in double (DD) and gas (GD) diffusion crystallization assays. The presence of these synthetic molecules as efficient inhibitors of nucleation and growth of CaCO3, and the use of biological gastrolith scaffolds as confined reaction environments determined the kinetics of crystallization, and controlled the morphogenesis of CaCO3. The formation of ACC particles was demonstrated and their crystallization was followed by light microscopy, scanning and transmission electron microscopy, and electron diffraction. [J. Struct. Biol. (2017); DOI: 10.1016/j.jsb.2017.05.004]




Biochemistry article available online "A model sea urchin spicule matrix protein, rSpSM50, is a hydrogelator that modifies and organizes the mineralization process"


In the purple sea urchin Strongylocentrotus purpuratus the formation and mineralization of fracture-resistant skeletal elements such as the embryonic spicule requires the combinatorial participation of numerous spicule matrix proteins such as SpSM50. However due to limited abundance and solubility issues it has been difficult to pursue extensive in vitro biochemical studies of SpSM50 protein and deduce its role in spicule formation and mineralization. To circumvent these problems we expressed a tag-free bacterial model recombinant spicule matrix protein rSpSM50. Bioinformatics and biophysical experiments confirm that rSpSM50 is an intrinsically disordered, aggregation-prone C-type lectin-like (CTLL)-domain containing protein that forms dimensionally and internally heterogeneous protein hydrogels that control the in vitro mineralization process in three ways: 1) kinetically stabilize the aqueous calcium carbonate system against nucleation and thermodynamically destabilize the initially formed ACC in bulk solution; 2) promote and organize faceted single crystal calcite and polycrystalline vaterite nanoparticles; and 3) promote surface texturing of calcite crystals and induce subsurface nanoporosities and channels within both calcite and vaterite crystals. Many of these features are also common to mollusk shell nacre proteins and the sea urchin spicule matrix glycoprotein, SpSM30B/C, and we conclude that rSpSM50 is a spiculogenesis hydrogelator protein that exhibits traits found in other calcium carbonate mineral-modification proteins. [Biochemistry (2017); DOI: 10.1021/acs.biochem.7b00083]







Angewandte Chemie International Edition article available online "Alignment of amorphous iron oxide clusters: A non-classical mechanism for magnetite formation"


Despite numerous studies on the nucleation and crystallization of iron (oxyhydr)oxides, the roles of species developing during the early stages, especially primary clusters and intermediate amorphous particles, are still poorly understood. Herein, both ligand-free and ligand-protected amorphous iron oxide (AIO) clusters (<2 nm) were synthesized as precursors for magnetite formation. Thermal annealing can crystallize the clusters into magnetite particles, and AIO bulk phases with domains of pre-aligned clusters are found to be direct precursors to crystals, suggesting a non-classical aggregation-based pathway that differs from the reported oriented attachment or particle accretion mechanisms. [Angew. Chem. Int. Ed. (2017); DOI: 10.1002/anie.201610275]



Ausrichtung amorpher Eisenoxid-Cluster: ein nichtklassischer Mechanismus für die Magnetitbildung


Obwohl schon viele Studien auf dem Gebiet der Nukleation und Kristallisation von Eisen(oxyhydr)oxiden durchgeführt wurden, ist die Rolle der Spezies, die sich in den frühen Stadien entwickeln, besonders primäre Cluster und amorphe Übergangspartikel, immer noch schlecht verstanden. Hier wurden sowohl ligandenfreie als auch ligandengeschützte amorphe Eisenoxid-Cluster (“amorphous iron oxide”, AIO, <2 nm) als Vorstufen für die Magnetitbildung synthetisiert. Durch Tempern können die Cluster zu Magnetitpartikeln kristallisieren, und es wurde festgestellt, dass die AIO-Bulkphase mit Domänen von vororientierten Clustern eine direkte Vorstufe zum Kristall ist. Dies weist auf einen nichtklassischen aggregationsbasierten Reaktionspfad hin, welcher sich von der bisher berichteten orientierten Anlagerung oder dem Partikelanlagerungsmechanismus unterscheidet. [Angew. Chem. (2017); DOI: 10.1002/ange.201610275]




Journal of Materials Chemistry A article available online "A CaCO3/nanocellulose-based bioinspired nacre-like material"


Nacre continues to be an inspiration for the fabrication of strong and tough materials from renewable and Earth-abundant raw materials. Here, we show how a nacre-like hybrid material based on nanocellulose (NC) and CaCO3 can be prepared by sequential infiltration of polymer-stabilised CaCO3 liquid precursors into layers of pre-deposited NC films. Layer-by-layer assembly of NC films followed by controlled spreading and infiltration with liquid CaCO3 precursors generated a lamellar material with an architecture and iridescent appearance similar to nacre. The wettability of the NC films towards the liquid CaCO3 precursors was controlled by hydroxyl and carboxyl functionalization of the NC fibrils, and the addition of magnesium ions. The combination of a high stiffness and plasticity of the nacre-like NC/CaCO3 hybrid materials shows that excellent mechanical properties can be obtained employing a fibrillar organic constituent that is relatively hard. The fabrication of a nacre-like hybrid material via an aqueous route of assembly and infiltration processing demonstrates how a sustainable composite material with outstanding properties can be produced using the most abundant biopolymer and biomineral on Earth. [J. Mater. Chem. A (2017); DOI: 10.1039/C6TA09524K]






Angewandte Chemie International Edition communication available online "Water Dynamics from THz Spectroscopy Reveal the Locus of a Liquid–Liquid Binodal Limit in Aqueous CaCO3 Solutions"


Many phenomena depend on CaCO3 nucleation where the role of water remains enigmatic. Changes in THz absorption during the early stages of CaCO3 nucleation evidence altered coupled motions of hydrated calcium and carbonate ions. The direct link between these changes and the continuous development of the ion activity product reveals the locus of a liquid–liquid binodal limit. The data strongly suggest that proto-structured amorphous CaCO3 forms through solidification of initially liquid precursors. Furthermore, polycarboxylates, which stabilize liquid precursors of CaCO3, significantly enhance the kinetic stability of the metastable liquid–liquid state, but they do not affect the locus of the binodal limit. The importance of water network dynamics in phase separation mechanisms can be understood based on the notions of the pre-nucleation cluster pathway, and is likely to be more general for aqueous systems. [Angew. Chem. Int. Ed. (2017); DOI: 10.1002/anie.201610554]



THz-Spektroskopie erlaubt Rückschlüsse auf die Wasserdynamik und die Lage einer flüssig-flüssig-binodalen Grenze in wässrigen CaCO3-Lösungen


Viele Naturphänomene hängen mit der Nukleation von CaCO3 zusammen, wobei die Rolle des Wassers bis jetzt weitgehend unklar war. Änderungen in der Absorption im THz-Frequenzbereich während der frühen Stadien der CaCO3-Nukleation belegen Veränderungen in den gekoppelten Bewegungen der hydratisierten Calcium- und Carbonat-Ionen. Der direkte Zusammenhang zwischen diesen Effekten und der kontinuierlichen Entwicklung des Ionenaktivitätsproduktes zeigt die Lage einer flüssig-flüssig-binodalen Grenze. Die Daten lassen stark darauf schließen, dass sich proto-strukturiertes, amorphes CaCO3 über die Verfestigung von anfänglich flüssigen Vorstufen bildet. Außerdem erhöhen Polycarboxylate, die flüssige Vorstufen von CaCO3 stabilisieren, die kinetische Stabilität des metastabilen flüssig-flüssigen Zustands signifikant, aber sie beeinflussen nicht die Lage der binodalen Grenze. Die mechanistische Bedeutung der Dynamik von Wassernetzwerken bei der Phasentrennung kann als alternativer Weg der Nukleation über Pränukleationscluster rationalisiert werden und ist wahrscheinlich allgemein für wässrige Systeme gültig. [Angew. Chem. (2017); DOI: 10.1002/ange.201610554]




Our edited book is now available "New Perspectives on Mineral Nucleation and Growth"


In the last decade, numerous studies have demonstrated the existence of alternative pathways to nucleation and crystallisation that oppose the classical view. Such proposed scenarios include multistage reactions proceeding via various precursor species and/or intermediate phases. The aim of this book is to review and discuss these recent advances in our understanding of the early stages of mineralisation through a series of contributions that address both experimental and theoretical studies about the formation and nature of initial precursor species (e.g., prenucleation clusters, dense liquid phases, amorphous nanoparticles, etc.) as well as their transformations leading to the stable mineral phase. Several chapters are devoted to cutting-edge analytical techniques used for investigating the above processes in situ, in real time and at conditions relevant to both natural and industrial processes. At the end of the book, the editors summarize the key questions that still need to be addressed in order to establish a complete picture of the nucleation and growth processes involved during the formation of minerals. [DOI: 10.1007/978-3-319-45669-0]





Chemistry of Materials article available online "Osteopontin Stabilizes Metastable States Prior to Nucleation during Apatite Formation"


Osteopontin, a phosphoprotein with strong ties to in vivo bone mineralization, is shown to change the precipitation pathway of calcium phosphate. We show that the presence of the phosphoprotein even in minute concentrations can stabilize an otherwise oversaturated mixture against precipitation. At moderate concentrations we find that the protein introduces a new intermediate state into the reaction pathway leading to apatite formation. This new intermediate was found to share many characteristics of a coacervate or Polymer Induced Liquid-like Precursor (PILP) phase. Our results show that these kinds of complex phases should be considered when discussing the mechanisms of bone mineralization on a sub-cellular level. [Chem. Mater. (2016); DOI: 10.1021/acs.chemmater.6b01088]





Jonathan Avaro has joined the team as a postdoctoral researcher. Welcome!


Jonathan's project is a collaboration with Karin Hauser within the framework of the SFB 1214.



Chemical Science article available online "A general strategy for colloidal stable ultrasmall amorphous mineral clusters in organic solvents"


While nature exerts precise control over the size and chemical composition of minerals, this is still a challenging task for artificial syntheses. Despite its significance, up to now, there are still no reports on colloidal mineral nanoparticles in the subnanometer range. Here we developed a general gas diffusion strategy using 10,12-pentacosadiynoic acid as ligand and ethanol as solvent to fabricate stable amorphous mineral clusters with a core size of less than 2 nm. First discovered for CaCO3, the method was successfully extended to produce monolayer protected clusters of MgCO3, SrCO3, Eu2(CO3)3, Tb2(CO3)3, Ce2(CO3)3, Cax(PO4)y, CaC2O4 and their hybrid minerals, CaxMgy(CO3)z and Cax(CO3)y(PO4)z. All the mineral clusters can be well dispersed in organic solvents like toluene, and are stable for a long period without further crystallization. Our work paves a way for the artificial synthesis of colloidal mineral clusters, which may gain various uses in both fundamental research and industry. [Chem. Sci. (2016); DOI: 10.1039/C6SC02333A]





Journal of Chemical Physics article available online "Polyaspartic acid facilitates oxolation within iron(iii) oxide pre-nucleation clusters and drives the formation of organic-inorganic composites"


The interplay between polymers and inorganic minerals during the formation of solids is crucial for biomineralization and bio-inspired materials, and advanced material properties can be achieved with organic-inorganic composites. By studying the reaction mechanisms, basic questions on organic-inorganic interactions and their role during material formation can be answered, enabling more target-oriented strategies in future synthetic approaches. Here, we present a comprehensive study on the hydrolysis of iron(iii) in the presence of polyaspartic acid. For the basic investigation of the formation mechanism, a titration assay was used, complemented by microscopic techniques. The polymer is shown to promote precipitation in partly hydrolyzed reaction solutions at the very early stages of the reaction by facilitating iron( iii) hydrolysis. In unhydrolyzed solutions, no significant interactions between the polymer and the inorganic solutes can be observed. We demonstrate that the hydrolysis promotion by the polymer can be understood by facilitating oxolation in olation iron(iii) pre-nucleation clusters. We propose that the adsorption of olation pre-nucleation clusters on the polymer chains and the resulting loss in dynamics and increased proximity of the reactants is the key to this effect. The resulting composite material obtained from the hydrolysis in the presence of the polymer was investigated with additional analytical techniques, namely, scanning and transmission electron microscopies, light microscopy, atomic force microscopy, zeta potential measurements, dynamic light scattering, and thermogravimetric analyses. It consists of elastic, polydisperse nanospheres, ca. 50-200 nm in diameter, and aggregates thereof, exhibiting a high polymer and water content. [J. Chem. Phys. (2016); DOI: 10.1063/1.4963738]




Angewandte Chemie International Edition communication available online "Distinct Short-Range Order Is Inherent to Small Amorphous Calcium Carbonate Clusters (<2 nm)"


Amorphous intermediate phases are vital precursors in the crystallization of many biogenic minerals. While inherent short-range orders have been found in amorphous calcium carbonates (ACCs) relating to different crystalline forms, it has never been clarified experimentally whether such orders already exist in very small clusters less than 2 nm in size. Here, we studied the stability and structure of 10,12-pentacosadiynoic acid (PCDA) protected ACC clusters with a core size of ca. 1.4 nm consisting of only seven CaCO3 units. Ligand concentration and structure are shown to be key factors in stabilizing the ACC clusters. More importantly, even in such small CaCO3 entities, a proto-calcite short-range order can be identified but with a relatively high degree of disorder that arises from the very small size of the CaCO3 core. Our findings support the notion of a structural link between prenucleation clusters, amorphous intermediates, and final crystalline polymorphs, which appears central to the understanding of polymorph selection. [Angew. Chem. Int. Ed. (2016); DOI: 10.1002/anie.201604179]





Denis Gebauer serves as the Guest Editor of the Minerals Special Issue "Nucleation of Minerals: Precursors, Intermediates and Their Use in Materials Chemistry"


Nucleation is the key event in mineralization, but a general molecular understanding of phase separation mechanisms is still missing, despite more than 100 years of research in this field. In the recent years, many studies have highlighted the occurrence of precursors and intermediates, which seem to challenge the assumptions underlying classical theories of nucleation and growth. This is especially true for the field of biomineralization, whereas bio-inspired strategies take advantage of the precursors' and intermediates' special properties for the generation of advanced materials. All of this has led to the development of "non-classical" frameworks, which, however, often lack quantitative expressions for the evaluation and prediction of phase separation, growth and ripening processes, and are under considerable debate. It is, thus, evident that there is a crucial need for research into the early stages of mineral nucleation and growth, designed for the testing, refinement, and expansion of the different existing notions. This Special Issue aims to bring together corresponding studies from all these areas, dealing with precursors and intermediates in mineralization processes. We welcome fundamental physical chemical studies, experimental, as well as theoretical, but also detailed analyses and characterizations of the formation mechanisms of both biogenic and bio-inspired, mineral-based (hybrid) materials. We also solicit methodological studies employing cutting-edge in situ analytics. The hope is that this Special Issue will contribute to the achievement of a better understanding of nucleation precursors and intermediates, and their target-oriented use in materials chemistry.


Manuscripts should be submitted online at by registering and logging in to the website. Manuscripts can be submitted until the deadline of 31 May 2017. Papers will be published continuously (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as communications are invited.





Johanna's Letter is in the Spotlights! Read the feature here.





ChemPhysChem article available online "Entropy drives calcium carbonate ion association"


The understanding of the molecular mechanisms underlying the early stages of crystallisation is still incomplete. In the case of calcium carbonate, experimental and computational evidence suggests that phase separation relies on so-called pre-nucleation clusters (PNCs). A thorough thermodynamic analysis of the enthalpic and entropic contributions to the overall free energy of PNC formation derived from three independent methods demonstrates that solute clustering is driven by entropy. This can be quantitatively rationalized by the release of water molecules from ion hydration layers, explaining why ion association is not limited to simple ion pairing. The key role of water release in this process suggests that PNC formation should be a common phenomenon in aqueous solutions. [ChemPhysChem (2016); DOI: 10.1002/cphc.201600653]





Cristina Ruiz Agudo has joined the team as a postdoctoral researcher. Welcome!


Cristina's project is a collaboration with Andreas Marx within the framework of the SFB 1214.




The Journal of Physical Chemistry Letters article available online "The molecular mechanism of iron(III) oxide nucleation"


A molecular understanding of the formation of solid phases from solution would be beneficial for various scientific fields. However, nucleation pathways are still not fully understood, whereby the case of iron (oxyhydr)oxides poses a prime example. We show that in the pre-nucleation regime, thermodynamically stable solute species up to a few nanometers in size are observed, which meet the definition of pre-nucleation clusters. Nucleation then is not governed by a critical size, but rather by the dynamics of the clusters that are forming at the distinct nucleation stages, based on the chemistry of the linkages within the clusters. This resolves a longstanding debate in the field of iron oxide nucleation, and the results may generally apply to oxides forming via hydrolysis and condensation. The (molecular) understanding of the chemical basis of phase separation is paramount for e.g. tailoring size, shape and structure of novel nanocrystalline materials. [J. Chem. Phys. Lett. (2016); DOI: 10.1021/acs.jpclett.6b01237]




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