Classical nucleation theory (CNT) is based on the notion of critical nuclei serving as transition states between supersaturated solutions and growing particles. Their excess standard free energy depends on supersaturation, and determines the height of the barrier for phase separation. However, predictions of CNT nucleation rates can deviate from experimental observations by many orders of magnitude. We argue that this is due to oversimplifications within CNT, rendering the critical nucleus essentially a conceptual notion, rather than a truly existing physical entity. Still, given adequate parametrization, CNT is useful for predicting and explaining nucleation phenomena, since it is currently the only quantitative framework at hand. In the recent years, we have been introducing an alternative theory, the so-called pre-nucleation cluster (PNC) pathway. The truly “non-classical” aspect of the PNC pathway is the realization that critical nuclei, as defined within CNT, are not the key to nucleation, but that the transition state relevant for phase separation is based on a change in dynamics of PNCs rather than their size. We provide a summary of CNT and the PNC pathway, thereby highlighting this major difference. The discussion of recent works claiming to provide scientific evidence against the existence of PNCs reveals that such claims are indeed void. Moreover, we illustrate that an erroneous interpretation of the concentration dependence of the free energy has led to a postulated rationalization of the standard free energy of ion pairs and stable ion associates within CNT, which is not sustainable. In fact, stable ion associates are stuck in a free energy trap from the viewpoint of CNT and cannot be considered in a straightforward manner. On the other hand, the notions of the PNC pathway, by dismissing the idea of the CNT-type critical nucleus as a required transition state, overcome this issue. While a quantitative theory of the PNC pathway is eagerly anticipated, the rationalization of experimental observations that are inconsistent with CNT proves its qualitative explanatory power, underpinning great promise towards a better understanding of, for instance, polymorph selection and crystallization control by additives.
The formation of the sea urchin spicule involves the stabilization and transformation of amorphous calcium carbonate (ACC) and assembly of ACC nanoparticle precursors into a mesoscale single crystal of fracture-resistant calcite. This process of particle assembly or attachment is under the control of a family of proteins known as the spicule matrix [Strongylocentrotus purpuratus (SpSM)] proteome. Recently, two members of this proteome, SpSM50 and the glycoprotein SpSM30B/C-G (in recombinant forms), were found to interact together via SpSM30B/C-G oligosaccharide–SpSM50 protein interactions to form hybrid protein hydrogels with unique physical properties. In this study, we investigate the mineralization properties of this hybrid hydrogel alongside the hydrogels formed by SpSM50 and SpSM30B/C-G individually. We find that the SpSM50 + SpSM30B/C-G hybrid hydrogel is synergistic with regard to surface modifications and intracrystalline inclusions of existing calcite crystals, the inhibition of ACC formation, and the kinetic destabilization of ACC to form a crystalline phase. Most importantly, the hybrid hydrogel phase assembles and organizes mineral particles into discrete clusters or domains within in vitro mineralization environments. Thus, the interactions of SpSM50 and SpSM30B/C-G, mediated by carbohydrate–protein binding, reflect the need for protein cooperativity for the ACC-to-crystalline transformation, intracrystalline void formation, and guided mineral particle assembly processes that are instrumental in spicule formation.
14.08.2018 | We are featured on the back cover of CrystEngComm.
Biogenic nucleation and crystallization occur in confined spaces with defined interfacial properties. However, the regulatory functions of organic players in the stabilization and transport of inorganic precursors such as ion clusters, liquid‐condensed phases, and amorphous particles are unclear. Given the prevalence of unstructured proteins in biogenic materials, the present study investigates the effects of biomineral‐associated, intrinsically disordered protein domains with simple and repetitive amino acid compositions on mineral nucleation and their capability to form distinct supramolecular assemblies. The quantitative assessment and structural evaluation of the nucleation process reveal that disordered regions confine hydrated mineral precursors within vesicles, transiently suppressing mineral precipitation. Stabilization of the amorphous mineral is attributed to protein self‐association and restructuration toward β‐configurations, triggered by specific bioinorganic interactions. In consequence, the conditioned macromolecules localize at phase boundaries formed upon liquid–liquid demixing of mineral precursors and stabilize the fluidic mineral precursors against crystallization. Thus, the conformational plasticity and self‐association of intrinsically disordered sequences in response to crystallization environments mediates the selection of functional macromolecular subensembles dedicated to biomaterial growth.
27.06.2018 | The Special Issue reprint book "Nucleation of Minerals: Precursors, Intermediates and Their Use in Materials Chemistry" has been published online and is freely accessible on the MDPI Books platform.
Nature produces hierarchical, functional materials by shaping amorphous mineral precursors under physiological conditions. Although biominerals inspire the architectures of synthetic counterparts, the biogenic phase transformations yielding precise crystalline forms, polymorphs and structures are unclear. Elucidating the transformation and structuration of amorphous minerals, herein we show distinct crystallization and structuration schemes synergistically controlled by environmental water contents and the Mg/Ca atomic ratio within amorphous mixed metal carbonates. Control of phase transformation, as well as resultant crystalline micro- and nano-structures, reflects the significance of the amorphous precursors of biominerals as disordered by design. Thereby, we complement the literature-known, suggested (bio)polymer-mediated ‘divide and protect’ mechanism of amorphous mineral stabilization by a Mg2+-based ‘unite and protect’ strategy. Altogether, this allows delineating a novel mechanism for mesocrystal formation based on the interface-coupled dissolution re-precipitation of mesoscale amorphous precursors, which appear important in biomineralization. In the latter case, the recruitment of environmentally abundant Mg2+ species can also supplement the functions of biomolecules.
15.05.2018 | Denis Gebauer receives the 2018 LBS-Environment Award! Together with the Landesbausparkasse Südwest (state home loans and savings bank), the foundation Stiftung „Umwelt und Wohnen an der Universität Konstanz“ presents an annual science award. This environment award is presented to scientists and academics at the University of Konstanz, who carry out successful research projects on the topic of environment with a special focus on habitation, living, and society.
13.05.2018 | The Sino-German Young-Scientist Forum "Polymer Chemistry in Functional Materials: Developments and Challenges" co-organised by Shengtong Sun and Denis Gebauer in Shanghai (Donghua University) has been a great success with 30 talks from invited German and Chinese participants. Thanks Everyone and hope to see you soon again!
The interactions between additives and mineral precursors and intermediates are at the heart of additive-controlled crystallisation, which is of high importance for various fields. In this commentary, we reflect on potential modes of additive control according to classical nucleation theory on one hand, and from the viewpoint of the so-called pre-nucleation cluster pathway on the other. This includes a brief review of the corresponding literature. While the roles of additives are discussed generally, i.e., without specific chemical or structural details, corresponding properties are outlined where possible. Altogether, our discussion illustrates that “non-classical” nucleation pathways promise an improved understanding of additive-controlled scenarios, which could be utilised in targeted applications in various fields, ranging from scale inhibition to materials chemistry.
In the nacre layer of the Pinctada fucata oyster shell there exists a multimember proteome, known as the framework family, which regulates the formation of the aragonite mesoscale tablets and participates in the creation of an organic coating around each tablet. Several approaches have been developed to understand protein-associated mechanisms of nacre formation, yet we still lack insight into how protein ensembles or proteomes manage nucleation and crystal growth. To provide additional insights we have created a proportionally defined combinatorial model consisting of two recombinant framework proteins, r-Pif97 (containing a von Willebrand Factor Type A domain (vWA)) and r-n16.3 (containing an EGF-like domain), whose individual in vitro mineralization functionalities are distinct from one another. We find that at 1:1 molar ratios r-Pif97 and r-n16.3 exhibit little or no synergistic activity regarding modifying existing calcite crystals. However, during the early stages of nucleation in solution, we note synergistic effects on nucleation kinetics and ACC formation/stability (via dehydration) that are not observed for the individual proteins. This selective synergism is generated by Ca2+-mediated protein–protein interactions (∼4 molecules of r-n16.3 per 1 molecule of r-Pif97) which lead to the formation of nucleation-responsive hybrid hydrogel particles in solution. Interestingly, in the absence of Ca2+ there are no significant interactions occurring between the two proteins. This unique behavior of the framework-associated n16.3 and Pif97 proteins suggests that the Asp/Glu-containing regions of the vWA and EGF-like domains may play a role in both nacre matrix formation and mineralization.
Calcium carbonate is an abundant biomineral that is of great importance in industrial or geological contexts. In recent years, many studies of the precipitation of CaCO3 have shown that amorphous precursors and intermediates are widespread in the biomineralization processes and can also be exploited in bio-inspired materials chemistry. In this work, the thorough investigation of a urinary stone of a guinea pig suggests that amorphous calcium carbonate (ACC) can play a role in pathological mineralization. Importantly, certain analytical techniques that are often applied in the corresponding analyses are sensitive only to crystalline CaCO3 and can misleadingly exclude the relevance of calcium carbonate during the formation of urinary stones. Our analyses suggest that ACC is the major constituent of the particular stone studied, which possibly precipitated on struvite nuclei. Minor amounts of urea, other stable inorganics, and minor organic inclusions are observed as well.
Water is a fundamental solvent sustaining life, key to the conformations and equilibria associated with solute species. Emerging studies on nucleation and crystallization phenomena reveal that the dynamics of hydration associated with mineral precursors are critical in determining material formation and growth. With certain small molecules affecting the hydration and conformational stability of co-solutes, this study systematically explores the effects of these chaotropes and kosmotropes as well as certain sugar enantiomers on the early stages of calcium carbonate formation. These small molecules appear to modulate mineral nucleation in a class-dependent manner. The observed effects are finite in comparison to the established, strong interactions between charged polymers and intermediate mineral forms. Thus, perturbations to hydration dynamics of ion clusters by co-solute species can affect nucleation phenomena in a discernable manner.
Crystal formation is a highly debated problem. We show that the crystallization of L-(+)-tartaric acid from water follows a non-classical path involving intermediate hydrated states. Analytical ultracentrifugation indicates solution clusters of the initial stages aggregate to form an early intermediate. Terahertz spectroscopy performed during water evaporation highlights a transient increase in the absorption during nucleation. This indicates the recurrence of water molecules which are expelled from the intermediate phase. Besides, a transient resonance at 750 GHz that can be assigned to a natural vibration of large hydrated aggregates vanishes after the final crystal has formed. Furthermore, THz data reveal the vibration of nanosized clusters in the dilute solution indicated by analytical ultracentrifugation. Infrared spectroscopy and wide-angle X-ray scattering highlight that the intermediate is not a crystalline hydrate. Our results demonstrate that nanoscopic intermediate units assemble to form the first solvent-free crystalline nuclei upon dehydration.
copyright [Chem. Eur. J. (2017); DOI: 10.1002/chem.201702218]
In the nacre or aragonitic layer of the oyster pearl there exists a 12-member proteome which regulates both the early stages of nucleation and nano-to-mesoscale assembly of nacre tablets and calcitic crystals from mineral nanoparticle precursors. Several approaches have been developed to understand protein-associated mechanisms of pearl nacre formation, yet we still lack insight into how protein ensembles or proteomes manage nucleation and crystal growth. To provide additional insights we have created a proportionally-defined combinatorial model consisting of two pearl nacre-associated proteins, PFMG1 and PFMG2 (shell oyster pearl nacre, P. fucata) whose individual in vitro mineralization functionalities are distinct from one another. Using SEM, AFM, Ca(II) potentiometric titrations and QCM-D quantitative analyses, we find that at 1:1 molar ratios, rPFMG2 and rPFMG1 co-aggregate in specific molecular ratios to form hybrid hydrogels that affect both the early and later stages of in vitro calcium carbonate nucleation. Within these hybrid hydrogels rPFMG2 plays a role in defining protein co-aggregation and hydrogel dimension, whereas rPFMG1 defines participation in non-classical nucleation processes, and both proteins exhibit synergy with regard to surface and subsurface modifications to existing crystals. The interactions between both proteins is enhanced by Ca(II) ions and may involve Ca(II)-induced conformational events within the EF-hand rPFMG1 protein, as well as putative interactions between the EF-hand domain of rPFMG1 and the calponin-like domain of rPFMG2. Thus, the pearl-associated PFMG1 and PFMG2 proteins interact and exhibit mineralization functionalities in specific ways, which may be relevant for pearl formation.
A key requirement for the understanding of crystal growth is to detect how new layers form and grow at the nanoscale. Multistage crystallization pathways involving liquid-like, amorphous or metastable crystalline precursors have been predicted by theoretical work and have been observed experimentally. Nevertheless, there is no clear evidence that any of these precursors can also be relevant for the growth of crystals of organic compounds. Herein, we present a new growth mode for crystals of DL-glutamic acid monohydrate that proceeds through the attachment of preformed nanoscopic species from solution, their subsequent decrease in height at the surface and final transformation into crystalline 2D nuclei that eventually build new molecular layers by further monomer incorporation. This alternative mechanism provides a direct proof for the existence of multistage pathways in the crystallization of molecular compounds and the relevance of precursor units larger than the monomeric constituents in the actual stage of growth.
[Nat. Commun. (2017); DOI: 10.1038/ncomms15933]
09.06.2017 | PhD Defense of Stefan Wolf.
16.05.2017 | PhD Defense of Masoud Farhadi Khouzani.
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.
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.
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.
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.
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.
copyright [J. Mater. Chem. A (2017); DOI: 10.1039/C6TA09524K]