On the biophysical regulation of mineral growth: Standing out from the crowd

Highly acidic pH facilitates enamel protein self-assembly, apatite crystal growth and enamel protein interactions in the early enamel matrix

Tooth enamel develops within a pH sensitive amelogenin-rich protein matrix. The purpose of the present study is to shed light on the intimate relationship between enamel matrix pH, enamel protein self-assembly, and enamel crystal growth during early amelogenesis. Universal indicator dye staining revealed highly acidic pH values (pH 3-4) at the exocytosis site of secretory ameloblasts. When increasing the pH of an amelogenin solution from pH 5 to pH 7, there was a gradual increase in subunit compartment size from 2 nm diameter subunits at pH 5 to a stretched configuration at pH6 and to 20 nm subunits at pH 7. HSQC NMR spectra revealed that the formation of the insoluble amelogenin self-assembly structure at pH6 was critically mediated by at least seven of the 11 histidine residues of the amelogenin coil domain (AA 46-117). Comparing calcium crystal growth on polystyrene plates, crystal length was more than 20-fold elevated at pH 4 when compared to crystals grown at pH 6 or pH 7. To illustrate the effect of pH on enamel protein self-assembly at the site of initial enamel formation, molar teeth were immersed in phosphate buffer at pH4 and pH7, resulting in the formation of intricate berry tree-like assemblies surrounding initial enamel crystal assemblies at pH4 that were not evident at pH7 nor in citrate buffer. Amelogenin and ameloblastin enamel proteins interacted at the secretory ameloblast pole and in the initial enamel layer, and co-immunoprecipitation studies revealed that this amelogenin/ameloblastin interaction preferentially takes place at pH 4-pH 4.5. Together, these studies highlight the highly acidic pH of the very early enamel matrix as an essential contributing factor for enamel protein structure and self-assembly, apatite crystal growth, and enamel protein interactions.

Biomineralization at fluid interfaces

Biomineralization is of paramount importance for life on Earth. The delicate balance of physicochemical interactions at the interface between organic and inorganic matter during all stages of biomineralization resembles an extremely high complexity. The coordination of this sophisticated biological machinery and physicochemical scenarios is certainly a wonderful show of nature. Understanding of the biomineralization processes is still far from complete. The recent advances in biomineralization research from the Colloid and Interface Science perspective are reviewed herein. The synergy between this two fields of research is demonstrated. The unique opportunities offered by purposefully designed fluid interfaces, mainly Langmuir monolayers are presented. Biomedical applications of biomineral-based nanostructures are discussed, showing their improved biocompatibility and on-demand delivery features. A brief guide to the array of state-of-the-art experimental techniques for unraveling the mechanisms of biomineralization using fluid interfaces is included. In summary, the fruitful and exciting crossroad between Colloid and Interface Science with Biomineralization is exhibited.

Microbe-Mediated Extracellular and Intracellular Mineralization: Environmental, Industrial, and Biotechnological Applications

Microbe-mediated mineralization is ubiquitous in nature, involving bacteria, fungi, viruses, and algae. These mineralization processes comprise calcification, silicification, and iron mineralization. The mechanisms for mineral formation include extracellular and intracellular biomineralization. The mineral precipitating capability of microbes is often harnessed for green synthesis of metal nanoparticles, which are relatively less toxic compared with those synthesized through physical or chemical methods. Microbe-mediated mineralization has important applications ranging from pollutant removal and nonreactive carriers, to other industrial and biomedical applications. Herein, the different types of microbe-mediated biomineralization that occur in nature, their mechanisms, as well as their applications are elucidated to create a backdrop for future research.

Changes in the properties of calcium-carbon inclusions during leaf development and their possible relationship with leaf functional maturation in three inclusion-bearing species

In many plant species, carbon-calcium inclusion (calcium oxalate crystals or cystoliths containing calcium carbonate) formation is a fundamental part of their physiology even necessary for normal growth and development. Despite the long-standing studies on carbon-calcium inclusions, the alterations in their properties during leaf development and their possible association with the maturation of the photosynthetic machinery have not been previously examined. In order to acquire more insights into this subject, we examined three of the most common species bearing abundant inclusions of different types, i.e., Amaranthus hybridus, Vitis vinifera, and Parietaria judaica. Results of our study showed that, irrespective of species and type of inclusion, similar patterns in the alterations of their properties are observed during leaf maturation, except for some differences in cell differentiation and distribution between raphides and druses in Vitis vinifera. As expected, inclusion formation has taken place at very early developmental stages and maximum density was observed in very young leaves. Inclusion properties are changing in a coordinated way with leaf area and these modifications are compatible with the concept that each idioblast or lithocyst "services" a finite number/area of adjacent cells. This tight coordination is also evident at the whole leaf level. Moreover, we observed an association of the properties of carbon-calcium inclusions and gas exchange, suggesting a possible implication of these structures in photosynthesis.

Bioinspired Silicification Reveals Structural Detail in Self-Assembled Peptide Cages

Understanding how molecules in self-assembled soft-matter nanostructures are organized is essential for improving the design of next-generation nanomaterials. Imaging these assemblies can be challenging and usually requires processing, e.g., staining or embedding, which can damage or obscure features. An alternative is to use bioinspired mineralization, mimicking how certain organisms use biomolecules to template mineral formation. Previously, we have reported the design and characterization of Self-Assembled peptide caGEs (SAGEs) formed from de novo peptide building blocks. In SAGEs, two complementary, 3-fold symmetric, peptide hubs combine to form a hexagonal lattice, which curves and closes to form SAGE nanoparticles. As hexagons alone cannot tile onto spheres, the network must also incorporate nonhexagonal shapes. While the hexagonal ultrastructure of the SAGEs has been imaged, these defects have not been observed. Here, we show that positively charged SAGEs biotemplate a thin, protective silica coating. Electron microscopy shows that these SiO2-SAGEs do not collapse, but maintain their 3D shape when dried. Atomic force microscopy reveals a network of hexagonal and irregular features on the SiO2-SAGE surface. The dimensions of these (7.2 nm ± 1.4 nm across, internal angles 119.8° ± 26.1°) are in accord with the designed SAGE network and with coarse-grained modeling of the SAGE assembly. The SiO2-SAGEs are permeable to small molecules (<2 nm), but not to larger biomolecules (>6 nm). Thus, bioinspired silicification offers a mild technique that preserves soft-matter nanoparticles for imaging, revealing structural details <10 nm in size, while also maintaining desirable properties, such as permeability to small molecules.

On mechanisms of mesocrystal formation: magnesium ions and water environments regulate the crystallization of amorphous minerals

Elucidating the emergence of crystalline superstructures from amorphous precursors, hydration environments and ionic constituents can guide transformation and structuration reactions towards distinct micro- and nano-structures.

Enhanced growth of tellurium nanowires under conditions of macromolecular crowding

The time-evolution of the mean excitonic wavelength demonstrating the enhanced ripening and growth rates of tellurium nanowires at increasing concentrations of the PVP crowder.

Mineralization and non-ideality: on nature's foundry

Understanding how ions, ion-clusters and particles behave in non-ideal environments is a fundamental question concerning planetary to atomic scales. For biomineralization phenomena wherein diverse inorganic and organic ingredients are present in biological media, attributing biomaterial composition and structure to the chemistry of singular additives may not provide a holistic view of the underlying mechanisms. Therefore, in this review, we specifically address the consequences of physico-chemical non-ideality on mineral formation. Influences of different forms of non-ideality such as macromolecular crowding, confinement and liquid-like organic phases on mineral nucleation and crystallization in biological environments are presented. Novel prospects for the additive-controlled nucleation and crystallization are accessible from this biophysical view. In this manner, we show that non-ideal conditions significantly affect the form, structure and composition of biogenic and biomimetic minerals.

Identification of Specific Hydroxyapatite {001} Binding Heptapeptide by Phage Display and Its Nucleation Effect

With recent developments of molecular biomimetics that combine genetic engineering and nanotechnology, peptides can be genetically engineered to bind specifically to inorganic components and execute the task of collagen matrix proteins. In this study, using biogenous tooth enamel as binding substrate, we identified a new heptapeptide (enamel high-affinity binding peptide, EHBP) from linear 7-mer peptide phage display library. Through the output/input affinity test, it was found that EHBP has the highest affinity to enamel with an output/input ratio of 14.814 × 10⁻⁷, while a random peptide (RP) displayed much lower output/input ratio of 0.00035 × 10⁻⁷. This binding affinity was also verified by confocal laser scanning microscopy (CLSM) analysis. It was found that EHBP absorbing onto the enamel surface exhibits highest normalized fluorescence intensity (5.6 ± 1.2), comparing to the intensity of EHBP to enamel longitudinal section (1.5 ± 0.9) (p < 0.05) as well as to the intensity of a low-affinity binding peptide (ELBP) to enamel (1.5 ± 0.5) (p < 0.05). Transmission electron microscopy (TEM), Attenuated total Reflection-Fourier transform infrared spectroscopy (ATR-FTIR), and X-ray Diffraction (XRD) studies further confirmed that crystallized hydroxyapatite were precipitated in the mineralization solution containing EHBP. To better understand the nucleation effect of EHBP, EHBP was further investigated on its interaction with calcium phosphate clusters through in vitro mineralization model. The calcium and phosphate ion consumption as well as zeta potential survey revealed that EHBP might previously adsorb to phosphate (PO₄³⁻) groups and then initiate the precipitation of calcium and phosphate groups. This study not only proved the electrostatic interaction of phosphate group and the genetically engineering solid-binding peptide, but also provided a novel nucleation motif for potential applications in guided hard tissue biomineralization and regeneration.