E. Alan Salter

Single‐Molecule Determination of the Face‐Specific Adsorption of Amelogenin’s C‐Terminus on Hydroxyapatite

The energetics of protein–mineral interactions is a crucial but poorly characterized factor underlying the hierarchical structure of mineralized tissue. During mineralization, organized protein matrices direct formation of mineral components. As with all assembly processes, the free-energy change provides the underlying thermodynamic driver, in this case reflecting protein interactions with the nascent mineral. However, despite the importance of obtaining face-specific free energies of mineral binding to establish a molecular-level understanding of biomineral organization, to date no direct measurements have been reported. Computational approaches struggle with the complexities of proteins, the inadequacies of model water potentials and effects of background electrolytes. Herein we present a novel application of force spectroscopy in which an atomic force microscopy (AFM) tip, functionalized with Amelogenin protein (Amel) C-terminal fragment, is used to directly determine the singlemolecule, face-specific free energy DGB of Amel binding to hydroxyapatite (HAp), the mineral phase in tooth enamel. [1] We then use complementary molecular dynamics (MD) simulations to compare binding energies at different faces and surface terminations and to identify the key interactions controlling face-specific binding and crystal morphology. Amelogenin (Table 1), a largely hydrophobic protein rich

Energetic basis for the molecular-scale organization of bone

Significance The remarkable mechanical properties of bone are determined by the organization and strength of binding at the mineral–collagen interface. Although the process through which collagen becomes mineralized has been extensively studied, little is known about the mechanisms or energetics that underlie the organization of this mineral–matrix composite. Combining molecular-scale imaging and analyses of collagen adsorption on four bone-related calcium phosphate phases, single-molecule force measurements and molecular simulations of collagen binding to hydroxyapatite, and electron microscopy analyses of bone and dentine, we determine the magnitude and chemistry of collagen–hydroxyapatite binding and show that calcium-deficient apatite is the only phase consistent with observed structural relationships. The remarkable properties of bone derive from a highly organized arrangement of coaligned nanometer-scale apatite platelets within a fibrillar collagen matrix. The origin of this arrangement is poorly understood and the crystal structures of hydroxyapatite (HAP) and the nonmineralized collagen fibrils alone do not provide an explanation. Moreover, little is known about collagen–apatite interaction energies, which should strongly influence both the molecular-scale organization and the resulting mechanical properties of the composite. We investigated collagen–mineral interactions by combining dynamic force spectroscopy (DFS) measurements of binding energies with molecular dynamics (MD) simulations of binding and atomic force microscopy (AFM) observations of collagen adsorption on single crystals of calcium phosphate for four mineral phases of potential importance in bone formation. In all cases, we observe a strong preferential orientation of collagen binding, but comparison between the observed orientations and transmission electron microscopy (TEM) analyses of native tissues shows that only calcium-deficient apatite (CDAP) provides an interface with collagen that is consistent with both. MD simulations predict preferred collagen orientations that agree with observations, and results from both MD and DFS reveal large values for the binding energy due to multiple binding sites. These findings reconcile apparent contradictions inherent in a hydroxyapatite or carbonated apatite (CAP) model of bone mineral and provide an energetic rationale for the molecular-scale organization of bone.

Building a bridge between aprotic and protic ionic liquids

ILs of dications with linked protic and aprotic centres have been prepared. They have rich H-bonding and a Bronsted acid–base character like small ΔpKa protic ILs, but very low apparent vapour pressures, more akin to aprotic ILs.

Growth inhibition of calcium oxalate monohydrate crystal by linear aspartic acid enantiomers investigated by in situ atomic force microscopy

The inhibitory effect of linear enantiomers of L- and D-Asp6 on the growth of calcium oxalate monohydrate crystal has been investigated using in situ atomic force microscopy. The inhibitory magnitude of D-Asp6 on the growth of the [00] step on the (010) face is about 10% larger than that of L-Asp6. While no chiral effect is observed or expected on the growth of the [0] step on the (01) face by both enantiomers, their inhibitory effect on this step is much stronger than that on the [00] step on the (010) face. In both cases, the step morphology indicates that these enantiomers create the impurity pinning along the steps, while the dependence of step speed on supersaturation shows that they also produce a reduction of the step kinetic coefficients. Analysis of the step speed data within the context of an existing model for step pinning and kink blocking shows that the major impact of Asp6 is to block active kink sites. The larger inhibition of the [00] step growth by D-Asp6 over L-Asp6 and the substantially larger inhibition of the [0] step over the [00] step by both enantiomers both result from larger affinity for adsorption to the (010) face and the (01) face, respectively. This is because the larger adsorption leads to a higher density of blocking kink sites along the steps. The estimated difference in binding energy of L- and D-Asp6 to the respective faces from the kinetics model is consistent with the trend predicted by our molecular modeling of the enantiomer binding to the faces.

Exploiting isolobal relationships to create new ionic liquids: novel room-temperature ionic liquids based upon (N-alkylimidazole)(amine)BH2+“boronium” ions

Readily prepared imidazole-based boronium ions form stable, hydrophobic, room-temperature ionic liquids (RTIL) with unique electronic and spectroscopic characteristics.

A simple and rapid route to novel tetra(4-thiaalkyl)ammonium bromides

A simple approach for the preparation of symmetrical quaternary ammonium bromides employing thiol–ene click chemistry is used to synthesize tetra(4-thiaalkyl)ammonium bromides. This approach allows the incorporation of a variety of alkyl moieties onto the nitrogen center with a one-step synthesis involving easy work-up, no side reactions and environmentally friendly reagents. To elucidate information regarding the behaviour of this novel class of compounds, comparisons to tetraalkylammonium analogues have been made. These include melting points, activity as phase-transfer catalysts, and conformational predictions from computational modelling. All results are consistent in indicating stronger bonding between the quaternary cation and the anion for the salts with 4-thiaalkyl chains as compared to those with n-alkyl chains.

Suppression of β-catenin/TCF transcriptional activity and colon tumor cell growth by dual inhibition of PDE5 and 10

Previous studies suggest the anti-inflammatory drug, sulindac inhibits tumorigenesis by a COX independent mechanism involving cGMP PDE inhibition. Here we report that the cGMP PDE isozymes, PDE5 and 10, are elevated in colon tumor cells compared with normal colonocytes, and that inhibitors and siRNAs can selectively suppress colon tumor cell growth. Combined treatment with inhibitors or dual knockdown suppresses tumor cell growth to a greater extent than inhibition from either isozyme alone. A novel sulindac derivative, ADT-094 was designed to lack COX-1/-2 inhibitory activity but have improved potency to inhibit PDE5 and 10. ADT-094 displayed >500 fold higher potency to inhibit colon tumor cell growth compared with sulindac by activating cGMP/PKG signaling to suppress proliferation and induce apoptosis. Combined inhibition of PDE5 and 10 by treatment with ADT-094, PDE isozyme-selective inhibitors, or by siRNA knockdown also suppresses β-catenin, TCF transcriptional activity, and the levels of downstream targets, cyclin D1 and survivin. These results suggest that dual inhibition of PDE5 and 10 represents novel strategy for developing potent and selective anticancer drugs.

Thermally robust: triarylsulfonium ionic liquids stable in air for 90 days at 300 °C

Select triarylsulfonium salts constitute ionic liquids with outstanding long-term, high-temperature aerobic stability (no mass loss in 90 days at 300 °C in air), making them among the most thermally stable organic materials known. A detailed analysis of their thermophysical properties reveals that lowering melting points in these salts by increasing ion size or lowering ion symmetry cannot be assumed, but remains an iterative process.

An evaluation of anion suitability for use in ionic liquids with long-term, high-temperature thermal stability

The stability of fourteen different PPN+ salts has been studied in 96 hour tests, in air, at temperatures of 200 °C, 250 °C, and 300 °C. The results have enabled generation of a ranking of their stabilities, and by extension, that of their anionic components. This data should prove especially useful in ongoing efforts to formulate ionic liquids for high-temperature applications.