Chemoselective N‑Acylation of Amines with Acylsilanes under Aqueous Acidic Conditions

We report a facile method for forming amide bonds between acylsilanes and a wide range of amines in the presence of a mild chlorinating agent under aqueous acidic conditions. The reaction is highly chemoselective, as exemplified by the late-stage modification of a panel of approved drugs and natural products containing reactive functionalities

Synthesis and Characterization of Monodispersed Spheres of Amorphous Calcium Carbonate and Calcite Spherules

The isotropic property of amorphous calcium carbonate (ACC) is useful for the controlled synthesis of calcium carbonate based biomaterials. A simple and efficient strategy for the synthesis of monodispersed microspheres of ACC is reported using a low-temperature precipitation of calcium carbonate in the presence of magnesium ions. The room-temperature aging of this amorphous phase yielded superstructures of self-assembled calcite crystals. The stability of the precipitated ACC is proportional to an increase in the concentration of the magnesium ions and the precipitation time. The ACC with high magnesium content was stable for a one month period at room temperature under dry conditions. The ACC aged in solution was stable for up to 48 h. It is believed that the low temperature and the presence of magnesium ions facilitated the formation of stable monodispersed spheres of ACC. The morphological studies and characterizations were carried out using SEM, FTIR, XRD, electron diffraction, and Raman spectroscopy. We expect that the low-temperature synthesis of ACC followed by structural manipulation would be an efficient method for the controlled synthesis of interesting calcium-rich biomaterials

Purification and Characterization of a Vaterite-Inducing Peptide, Pelovaterin, from the Eggshells of <i>Pelodiscus </i><i>s</i><i>inensis</i> (Chinese Soft-Shelled Turtle)

Proteins play a crucial role in the biomineralization of hard tissues such as eggshells. We report here the purification, characterization, and in vitro mineralization studies of a peptide, pelovaterin, extracted from eggshells of a soft-shelled turtle. It is a glycine-rich peptide with 42 amino acid residues and three disulfide bonds. When tested in vitro, the peptide induced the formation of a metastable vaterite phase. The floret-shaped morphology formed at a lower concentration (∼1 μM) was transformed into spherical particles at higher concentrations (>500 μM). The solution properties of the peptide are investigated by circular dichroism (CD), fluorescence emission spectroscopy, and dynamic light scattering (DLS) experiments. The conformation of pelovaterin changed from an unordered state at a low concentration to a β-sheet structure at high concentrations. Fluorescence emission studies indicated that the quantum yield is significantly decreased at higher concentrations, accompanied by a blue shift in the emission maximum. At higher concentrations a red-edge excitation shift was observed, indicating the restricted mobility of the peptide. On the basis of these observations, we discuss the presence of a peptide concentration-dependent monomer−multimer equilibrium in solution and its role in controlling the nucleation, growth, and morphology of CaCO3 crystals. This is the first peptide known to induce the nucleation and stabilization of the vaterite phase in solution

Formation of Transient Amorphous Calcium Carbonate Precursor in Quail Eggshell Mineralization: An In Vitro Study

To understand the mechanism of quail eggshell biomineralization, we have performed two CaCO3 precipitation experiments. In the reprecipitation experiments, supersaturated Ca(HCO3)2 was prepared by bubbling CO2 through a slurry of biogenic CaCO3 obtained from bleach-treated eggshell followed by filtration to obtain a clear solution for crystallization experiments. The nucleated crystals were collected at various time intervals and analyzed. In the second experiment, the extracted SOM from the bleach-treated eggshell was added to the supersaturated clear solution of Ca(HCO3)2 solution obtained by bubbling CO2 gas through a slurry of synthetic CaCO3 followed by filtration. The crystals/precipitates collected at various time intervals were analyzed. Both experiments showed that amorphous CaCO3 (ACC) was precipitated in the early stages, which then transformed to the most stable crystalline calcite phase. Amino acid analysis of the soluble organic matrixes (SOM) indicated the presence of high amounts of Glx and Asx amino acids. Ovomucoidan acidic glycoprotein, and lysozymea basic protein, are the two major components along with a few low molecular weight peptides present in the SOM of quail eggshell matrix. Both ovomucoid and lysozyme did not induce precipitation of the ACC phase in in vitro conditions, while the fraction containing low molecular weight peptides induced the precipitation of ACC, suggesting that the latter play an important role in the eggshell biomineralization. Thus, organisms can produce inorganic minerals which assume nonequilibrium morphologies and intricate architecture by precipitating transient ACC, which then transformed into the crystalline phase. Altogether, these observations further demonstrate that this strategy may be common in both vertebrate and invertebrate mineralized structures

The Tooth Enamel Protein, Porcine Amelogenin, Is an Intrinsically Disordered Protein with an Extended Molecular Configuration in the Monomeric Form

Amelogenins make up a class of proteins associated with the formation of mineralized enamel in vertebrates, possess highly conserved N- and C-terminal sequence regions, and represent an interesting model protein system for understanding biomineralization and protein assembly. Using bioinformatics, we report here the identification of molecular traits that classify 12 amelogenin proteins as members of the intrinsically disordered or unstructured protein family (IDPs), a group of proteins that normally exist as unfolded species but are capable of transformation to a folded state as part of their overall function. Using biophysical techniques (CD and NMR), we follow up on our bioinformatics studies and confirm that one of the amelogenins, recombinant porcine rP172, exists in an extended, unfolded state in the monomeric form. This protein exhibits evidence of conformational exchange between two states, and this exchange may be mediated by Pro residues in the sequence. Although the protein is globally unfolded, we detect the presence of local residual secondary structure [α-helix, extended β-strand, turn/loop, and polyproline type II (PPII)] that may serve several functional roles within the enamel matrix. The extended, labile conformation of rP172 amelogenin is compatible with the known functions of amelogenin in enamel biomineralization, i.e., self-assembly, associations with other enamel matrix proteins and with calcium phosphate biominerals, and interaction with cell receptors. It is likely that the labile structure of this protein facilitates interactions of amelogenin with other macromolecules or with minerals for achievement of internal protein stabilization

Schematic diagram of a Descemet's membrane (DM) with and without fibrin glue (FG) used for nanoindentation by atomic force microscopy (AFM).

<p>Indentation tests were performed on DM mounted on PDMS substrate while flexural tests were performed on DM mounted on PDMS substrate circular holes of diameters 1.2 mm or 1.5 mm. (A) shows a setup for a native DM and (B) shows the DM sprayed with FG with the glue facing downwards and attached to PDMS substrate.</p

Relative hysteresis measured from the force curves with the area under the approach and retract curves.

<p>From these force curves, hysteresis was compared between the samples by calculating the difference in the area of the approach and retract curve by trapezoidal law. Values were compared between DM, DM+FG, and FG alone, indented over 1.2 mm (A) and 1.5 mm diameter holes (B) and between 1.2 and 1.5 mm groups (C). Each data represents the average hysteresis obtained from 10–12 force curves. The error bar represents SEM. * and ** indicate the significant differences at <i>P</i><0.05 and <i>P</i><0.001, respectively.</p

Representative force curves obtained after indenting DM with and without FG suspended on the punched holes of diameter 1.2 mm and 1.5 mm on a PDMS substrate to determine the hysteresis in a sample.

<p>The curves were plotted between separation or indentation (δ = z–d) where z is the piezo-displacement and d is the cantilever deflection in x-axis and force applied by the indenter on y-axis for DM (A), DM sprayed with FG (B), and FG alone (C), in the 1.2 mm diameter group. Similar force curves were plotted for DM (D), DM+FG (E), and FG alone (F), obtained from the 1.5 mm diameter group. The black solid line indicates the approach curve when the tip contacts the sample and the red line represents the retract curve when the tip moves away from the sample.</p

Average of nanosphere radii in rp(H)M180 and chimeric proteins.

<p>The difference in radii is significant using student's t-test (samples were normally distributed).</p

Atomic force microscope (AFM) images showing surface topography of native DM (DM; A–D) and DM with fibrin glue (FG) facing up (E–H).

<p>A and E: Height data; B and F: Amplitude data; C and G: Phase data. D and H represent a 3D presentation of topographical map of native DM and DM+FG, respectively. Image scale = 10 µm×10 µm.</p