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.
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.
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.
pH-Dependent Schemes of Calcium Carbonate Formation in the Presence of Alginates
From recent studies on bone and shell formation, the importance of polysaccharides in biomineralization processes is gradually being recognized. Through ion-complexation and self-assembly properties, such macromolecules have remarkable effects on mineralization. However, their influences on the different regimes of crystallization including the interactions with precursor species are unclear. The present study therefore addresses calcium carbonate mineralization in the presence of alginates, a class of linear copolymeric saccharides composed of β-1,4 linked d-mannuronic and l-guluronic acid. During mineralization, this biopolymer is found to exert pH-dependent control over mineralization pathways in terms of the stability of prenucleation clusters, inhibitory effect toward nucleation and initially formed postnucleation products. Remarkably in the presence of this macromolecular additive, either amorphous or crystalline vaterite particles can be selectively nucleated in a pH-dependent manner. This is validated by electron microscopy wherein vaterite particles are intimately associated with alginate assemblies after nucleation at pH 9.75. At lower pH, aggregates of amorphous particles are formed. Thus, in addition to the general focus on biochemical properties of additives, solution pH, a physiologically fundamental parameter significantly alters the scheme of mineralization.
Mineral Nucleation : Stabilization of Mineral Precursors by Intrinsically Disordered Proteins
Biogenic crystallization reactions produce hybrid nanostructured materials under physiological conditions. In article 1802063, Helmut Cölfen and co‐workers identify the disorder to order transitions of biomacromolecules as a regulatory feature of additive‐controlled mineralization. This molecular conditioning generates conformational sub‐ensembles and supramolecular assemblies adept at controlling the pathways of nucleation and crystallization.
Colloidal Stabilization of Calcium Carbonate Prenucleation Clusters with Silica
Calcium carbonate precipitation proceeds via a complex multistage scenario involving neutral ion clusters as precursors and amorphous phases as intermediates, which finally transform to crystals. Although the existence of stable clusters in solution prior to nucleation has been demonstrated, the molecular mechanisms by which they precipitate are still obscure. Here, direct insight into the processes that drive the transformation of individual clusters into amorphous nanoparticles is provided by progressive colloidal stabilization of different transient states in silica-containing environments. Nucleation of calcium carbonate in the presence of silica can only take place via cluster aggregation at low pH values. At higher pH, prenucleation clusters become colloidally stabilized and cannot aggregate. Nucleation through structural reorganization within the clusters is not observed under these conditions, indicating that this pathway is blocked by kinetic and/or thermodynamic means. The degree of stabilization against nucleation is found to be sufficient to allow for a dramatic enrichment of solutions with prenucleation clusters and enable their isolation into the dry state. This approach renders direct analyses of the clusters by conventional techniques possible and is thus likely to facilitate deeper insight into the chemistry and structure of these elusive species in the future.