Cölfen, Helmut
0000-0002-1148-0308 Berufsbeschreibung
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New insights into the nucleation of magnesium hydroxide and the influence of poly(acrylic acid) during the early stages of Mg(OH)2 crystallisation
Nucleation is a unique process with broad relevance across a wide range of scientific disciplines and applications. While considerable progress in the understanding of the mechanisms underlying the nucleation of minerals from solution has been made for popular model systems such as calcium carbonate, corresponding detailed insights are still missing for other, less prominent minerals. Here, we present a potentiometric titration-based method that allows the early stages of the crystallisation of brucite, Mg(OH)2, to be monitored and quantified. Together with complementary characterisation provided by (cryogenic) transmission electron microscopy, the collected data shed novel light on the species occurring prior to, during, and after nucleation of brucite. In the second part of the work, the newly developed approach was applied to investigate the effects of added poly(acrylic acid) on the different stages of the crystallisation process. The polymer is found to stabilise brucite nanoplatelets and co-precipitate with the inorganic phase, yielding a composite material. The methodology established in this study can readily be used to screen other chemistries for their ability to prevent magnesium hydroxide scaling and/or afford brucite nanomaterials with tailored properties.
Role of Water in CaCO3 Biomineralization
Biomineralization occurs in aqueous environments. Despite the ubiquity and relevance of CaCO3 biomineralization, the role of water in the biomineralization process has remained elusive. Here, we demonstrate that water reorganization accompanies CaCO3 biomineralization for sea urchin spine generation in a model system. Using surface-specific vibrational spectroscopy, we probe the water at the interface of the spine-associated protein during CaCO3 mineralization. Our results show that, while the protein structure remains unchanged, the structure of interfacial water is perturbed differently in the presence of both Ca2+ and CO32– compared to the addition of only Ca2+. This difference is attributed to the condensation of prenucleation mineral species. Our findings are consistent with a nonclassical mineralization pathway for sea urchin spine generation and highlight the importance of protein hydration in biomineralization.
On Biomineralization : Enzymes Switch on Mesocrystal Assembly
Cellular machineries guide the bottom-up pathways toward crystal superstructures based on the transport of inorganic precursors and their precise integration with organic frameworks. The biosynthesis of mesocrystalline spines entails concerted interactions between biomolecules and inorganic precursors; however, the bioinorganic interactions and interfaces that regulate material form and growth as well as the selective emergence of structural complexity in the form of nanostructured crystals are not clear. By investigating mineral nucleation under the regulation of recombinant proteins, we show that SpSM50, a matrix protein of the sea urchin spine, stabilizes mineral precursors via vesicle-confinement, a function conferred by a low-complexity, disordered region. Site-specific proteolysis of this domain by a collagenase initiates phase transformation of the confined mineral phase. The residual C-type lectin domain molds the fluidic mineral precursor into hierarchical mesocrystals identical to structural crystal modules constituting the biogenic mineral. Thus, the regulatory functions of proteolytic enzymes can guide biomacromolecular domain constitutions and interfaces, in turn determining inorganic phase transformations toward hybrid materials as well as integrating organic and inorganic components across hierarchical length scales. Bearing striking resemblance to biogenic mineralization, these hybrid materials recruit bioinorganic interactions which elegantly intertwine nucleation and crystallization phenomena with biomolecular structural dynamics, hence elucidating a long-sought key of how nature can orchestrate complex biomineralization processes.
Crystallization Caught in the Act with Terahertz Spectroscopy : Non-Classical Pathway for L-(+)-Tartaric Acid
Crystal formation is a highly debated problem. This report shows 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 that are expelled from the intermediate phase. Besides, a transient resonance at 750 GHz, which 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. These results demonstrate that nanoscopic intermediate units assemble to form the first solvent-free crystalline nuclei upon dehydration.
Highly hydrated paramagnetic amorphous calcium carbonate nanoclusters as an MRI contrast agent
Amorphous calcium carbonate plays a key role as transient precursor in the early stages of biogenic calcium carbonate formation in nature. However, due to its instability in aqueous solution, there is still rare success to utilize amorphous calcium carbonate in biomedicine. Here, we report the mutual effect between paramagnetic gadolinium ions and amorphous calcium carbonate, resulting in ultrafine paramagnetic amorphous carbonate nanoclusters in the presence of both gadolinium occluded highly hydrated carbonate-like environment and poly(acrylic acid). Gadolinium is confirmed to enhance the water content in amorphous calcium carbonate, and the high water content of amorphous carbonate nanoclusters contributes to the much enhanced magnetic resonance imaging contrast efficiency compared with commercially available gadolinium-based contrast agents. Furthermore, the enhanced T1 weighted magnetic resonance imaging performance and biocompatibility of amorphous carbonate nanoclusters are further evaluated in various animals including rat, rabbit and beagle dog, in combination with promising safety in vivo. Overall, exceptionally facile mass-productive amorphous carbonate nanoclusters exhibit superb imaging performance and impressive stability, which provides a promising strategy to design magnetic resonance contrast agent.
Non-stoichiometric hydrated magnesium-doped calcium carbonate precipitation in ethanol
The effect of Mg2+ on the precipitation pathway of CaCO3 in absolute ethanol has been studied to investigate the role of ion solvation in the crystallization process. Our data reveal that high concentrations of Mg2+ promote the precipitation of an amorphous transient phase together with non-stoichometric hydrated phases of calcium carbonate.
Flüssige metastabile Vorstufen von Ibuprofen als Zwischenprodukt der Nukleation in wässriger Lösung
Der Nukleationsmechanismus von Kristallen kleiner organischer Moleküle wurde durch Computersimulationen postuliert, doch bisher mangelt es an experimentellen Nachweisen. Hier wurde ein experimenteller Ansatz entwickelt, mit dem die frühen Stadien der Kristallisation von Ibuprofen als Modellsystem aufgezeigt werden. Ibuprofen vollzieht eine flüssig-flüssig Entmischung. Die binodalen und spinodalen Grenzen dieser Mischungslücke wurden analysiert und der flüssige Charakter der Phase wurde bestätigt. Ein Anstieg der Viskosität hält die kinetische Stabilität der dichten flüssigen Phase aufrecht, die intermolekularen Abstände entsprechen denen in den Kristallstrukturen. Die dichte flüssige Phase wurde als Vorläuferphase der Nukleation von Ibuprofen identifiziert, bei der zunächst eine Verdichtung stattfindet und sich erst später eine strukturelle Ordnung ausbildet. Diese Entdeckung verspricht die Möglichkeit, schwerlösliche Wirkstoffe in wässriger Lösung über klassische Löslichkeitsgrenzen hinaus zu befördern.
Crystal Nucleation and Growth of Inorganic Ionic Materials from Aqueous Solution : Selected Recent Developments, and Implications
In this review article, selected, latest theoretical, and experimental developments in the field of nucleation and crystal growth of inorganic materials from aqueous solution are highlighted, with a focus on literature after 2015 and on non-classical pathways. A key point is to emphasize the so far underappreciated role of water and solvent entropy in crystallization at all stages from solution speciation through to the final crystal. While drawing on examples from current inorganic materials where non-classical behavior has been proposed, the potential of these approaches to be adapted to a wide-range of systems is also discussed, while considering the broader implications of the current re-assessment of pathways for crystallization. Various techniques that are suitable for the exploration of crystallization pathways in aqueous solution, from nucleation to crystal growth are summarized, and a flow chart for the assignment of specific theories based on experimental observations is proposed.
Liquid Metastable Precursors of Ibuprofen as Aqueous Nucleation Intermediates
The nucleation mechanism of crystals of small organic molecules, postulated based on computer simulations, still lacks experimental evidence. In this study we designed an experimental approach to monitor the early stages of the crystallization of ibuprofen as pharmaceutically eminent molecule and a model system for small organic molecules. We found that ibuprofen undergoes liquid-liquid phase separation prior to nucleation. The binodal and spinodal limits of the corresponding liquid-liquid miscibility gap were localized by combining potentiometric titration with 1 H NMR spectroscopy and additional analyses. We confirmed the liquid character of this initially formed phase by applying PFG-STE self-diffusion experiments ( 1 H NMR) and found an increase in viscosity sustaining the kinetic stability of the dense liquid intermediate. Intermolecular distances of ibuprofen within the dense liquid phase were found to be similar to those in the crystal forms, according to 2D 1 H- 1 H NOESY measurements. Hence, this dense liquid phase is identified as a precursor phase within a nucleation pathway of ibuprofen, in which densification is followed by structural order generation. Fundamentally, this discovery bears the opportunity and promise to enrich poorly soluble pharmaceuticals beyond classical solubility limitations in aqueous environments.
Stabilization of Mineral Precursors by Intrinsically Disordered Proteins
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.