Jinhui Tao

Reconciling disparate views of template-directed nucleation through measurement of calcite nucleation kinetics and binding energies

Significance Organisms use specialized macromolecules to direct the timing and placement of crystals during biomineral formation. This phenomenon has inspired synthetic approaches to templating but remains poorly understood. One view holds that the organic matrix promotes nucleation through stereochemical matching to guide the organization of solute ions, while another equates binding strength to promotion of nucleation. Our study reconciles these views with a mechanistic explanation for template-directed nucleation. Through measurements of calcite nucleation kinetics and substrate–crystal binding we show that nucleation barriers and binding free energies are linearly related for all functional group chemistries and conformations as predicted from classical nucleation theory. This model reconciles long-standing concepts of stereochemical matching with the conventional wisdom that good binders are good nucleators. The physical basis for how macromolecules regulate the onset of mineral formation in calcifying tissues is not well established. A popular conceptual model assumes the organic matrix provides a stereochemical match during cooperative organization of solute ions. In contrast, another uses simple binding assays to identify good promoters of nucleation. Here, we reconcile these two views and provide a mechanistic explanation for template-directed nucleation by correlating heterogeneous nucleation barriers with crystal–substrate-binding free energies. We first measure the kinetics of calcite nucleation onto model substrates that present different functional group chemistries (carboxyl, thiol, phosphate, and hydroxyl) and conformations (C11 and C16 chain lengths). We find rates are substrate-specific and obey predictions of classical nucleation theory at supersaturations that extend above the solubility of amorphous calcium carbonate. Analysis of the kinetic data shows the thermodynamic barrier to nucleation is reduced by minimizing the interfacial free energy of the system, γ. We then use dynamic force spectroscopy to independently measure calcite–substrate-binding free energies, ΔGb. Moreover, we show that within the classical theory of nucleation, γ and ΔGb should be linearly related. The results bear out this prediction and demonstrate that low-energy barriers to nucleation correlate with strong crystal–substrate binding. This relationship is general to all functional group chemistries and conformations. These findings provide a physical model that reconciles the long-standing concept of templated nucleation through stereochemical matching with the conventional wisdom that good binders are good nucleators. The alternative perspectives become internally consistent when viewed through the lens of crystal–substrate binding.

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.

Self-Assembly of Collagen on Flat Surfaces: The Interplay of Collagen–Collagen and Collagen–Substrate Interactions

Fibrillar collagens, common tissue scaffolds in live organisms, can also self-assemble in vitro from solution. While previous in vitro studies showed that the pH and the electrolyte concentration in solution largely control the collagen assembly, the physical reasons why such control could be exerted are still elusive. To address this issue and to be able to simulate self-assembly over large spatial and temporal scales, we have developed a microscopic model of collagen with explicit interactions between the units that make up the collagen molecules, as well as between these units and the substrate. We have used this model to investigate assemblies obtained via molecular dynamics deposition of collagen on a substrate at room temperature using an implicit solvent. By comparing the morphologies from our molecular dynamics simulations with those from our atomic-force microscopy experiments, we have found that the assembly is governed by the competition between the collagen-collagen interactions and those between collagen and the substrate. The microscopic model developed here can serve for guiding future experiments that would explore new regions of the parameter space.

Formation of Reversible Solid Electrolyte Interface on Graphite Surface from Concentrated Electrolytes

Li-ion batteries (LIB) have been successfully commercialized after the identification of ethylene-carbonate (EC)-containing electrolyte that can form a stable solid electrolyte interphase (SEI) on carbon anode surface to passivate further side reactions but still enable the transportation of the Li+ cation. These electrolytes are still utilized, with only minor changes, after three decades. However, the long-term cycling of LIB leads to continuous consumption of electrolyte and growth of SEI layer on the electrode surface, which limits the batterys life and performance. Herein, a new anode protection mechanism is reported in which, upon changing of the cell potential, the electrolyte components at the electrode-electrolyte interface reorganize reversibly to form a transient protective surface layers on the anode. This layer will disappear after the applied potential is removed so that no permanent SEI layer is required to protect the carbon anode. This phenomenon minimizes the need for a permanent SEI layer and prevents its continuous growth and therefore may lead to largely improved performance for LIBs.

Matrix metalloproteinase-20 mediates dental enamel biomineralization by preventing protein occlusion inside apatite crystals

Reconstruction of enamel-like materials is a central topic of research in dentistry and material sciences. The importance of precise proteolytic mechanisms in amelogenesis to form a hard tissue with more than 95% mineral content has already been reported. A mutation in the Matrix Metalloproteinase-20 (MMP-20) gene results in hypomineralized enamel that is thin, disorganized and breaks from the underlying dentin. We hypothesized that the absence of MMP-20 during amelogenesis results in the occlusion of amelogenin in the enamel hydroxyapatite crystals. We used spectroscopy and electron microscopy techniques to qualitatively and quantitatively analyze occluded proteins within the isolated enamel crystals from MMP-20 null and Wild type (WT) mice. Our results showed that the isolated enamel crystals of MMP-20 null mice had more organic macromolecules occluded inside them than enamel crystals from the WT. The crystal lattice arrangements of MMP-20 null enamel crystals analyzed by High Resolution Transmission Electron Microscopy (HRTEM) were found to be significantly different from those of the WT. Raman studies indicated that the crystallinity of the MMP-20 null enamel crystals was lower than that of the WT. In conclusion, we present a novel functional mechanism of MMP-20, specifically prevention of unwanted organic material entrapped in the forming enamel crystals, which occurs as the result of precise amelogenin cleavage. MMP-20 action guides the growth morphology of the forming hydroxyapatite crystals and enhances their crystallinity. Elucidating such molecular mechanisms can be applied in the design of novel biomaterials for future clinical applications in dental restoration or repair.

Hierarchically Porous Graphitic Carbon with Simultaneously High Surface Area and Colossal Pore Volume Engineered via Ice Templating

Developing hierarchical porous carbon (HPC) materials with competing textural characteristics such as surface area and pore volume in one material is difficult to accomplish, particularly for an atomically ordered graphitic carbon. Herein we describe a synthesis strategy to engineer tunable HPC materials across micro-, meso-, and macroporous length scales, allowing the fabrication of a graphitic HPC material (HPC-G) with both very high surface area (>2500 m2/g) and pore volume (>11 cm3/g), the combination of which has not been attained previously. The mesopore volume alone for these materials is up to 7.53 cm3/g, the highest ever reported, higher than even any porous carbons total pore volume, which for our HPC-G material was >11 cm3/g. This HPC-G material was explored for use both as a supercapacitor electrode and for oil adsorption, two applications that require either high surface area or large pore volume, textural properties that are typically exclusive to one another. We accomplished these high textural characteristics by employing ice templating not only as a route for macroporous formation but as a synergistic vehicle that enabled the significant loading of the mesoporous hard template. This design scheme for HPC-G materials can be utilized in broad applications, including electrochemical systems such as batteries and supercapacitors, sorbents, and catalyst supports, particularly supports where a high degree of thermal stability is required.

Using Biomimetic Polymers in Place of Noncollagenous Proteins to Achieve Functional Remineralization of Dentin Tissues

In calcified tissues such as bones and teeth, mineralization is regulated by an extracellular matrix, which includes non-collagenous proteins (NCP). This natural process has been adapted or mimicked to restore tissues following physical damage or demineralization by using polyanionic acids in place of NCPs, but the remineralized tissues fail to fully recover their mechanical properties. Here we show that pre-treatment with certain amphiphilic peptoids, a class of peptide-like polymers consisting of N-substituted glycines that have defined monomer sequences, enhances ordering and mineralization of collagen and induces functional remineralization of dentin lesions in vitro. In the vicinity of dentin tubules, the newly formed apatite nano-crystals are co-aligned with the c-axis parallel to the tubular periphery and recovery of tissue ultrastructure is accompanied by development of high mechanical strength. The observed effects are highly sequence-dependent with alternating polar and non-polar groups leading to positive outcomes while diblock sequences have no effect. The observations suggest aromatic groups interact with the collagen while the hydrophilic side chains bind the mineralizing constituents and highlight the potential of synthetic sequence-defined biomimetic polymers to serve as NCP mimics in tissue remineralization.

Amelogenin Affects Brushite Crystal Morphology and Promotes Its Phase Transformation to Monetite

Amelogenin protein is involved in organized apatite crystallization during enamel formation. Brushite (CaHPO4·2H2O), one of the precursors of hydroxyapatite mineralization in vitro, has been used for fabrication of biomaterials for hard tissue repair. In order to explore its potential application in biomimetic material synthesis, we studied the influence of the enamel protein amelogenin on brushite morphology and phase transformation to monetite. Our results show that amelogenin can adsorb onto the surface of brushite, leading to the formation of layered morphology on the (010) face. Amelogenin promoted the phase transformation of brushite into monetite (CaHPO4) in the dry state, presumably by interacting with crystalline water layers in brushite unit cells. Changes to the crystal morphology mediated by amelogenin continued even after the phase transformation from brushite to monetite, leading to the formation of organized platelets with an interlocked structure. This effect of amelogenin on brushite morphology and the phase transformation to monetite could provide a new approach to developing biomimetic materials.

Stability of polymeric separators in lithium metal batteries in a low voltage environment

The separator is an important component in rechargeable lithium (Li) metal batteries; however, less attention has been focused on it so far. In this work, several representative separators based on polyolefins without and with ceramic and polymeric coatings were selected to study the stability of separators against Li metal anode in Li‖Cu and Li‖Li coin cells in a low voltage environment less than 1 V vs. Li/Li+. Moreover, two representative electrolytes of LiPF6 and LiTFSI–LiBOB in a carbonate solvent mixture were also employed to systematically study their interactions with separators in Li metal cells. It was found that separators could largely affect the coulombic efficiency and cycling stability of Li metal cells, especially when using the LiPF6 electrolyte, which is probably due to the effect of the trace amount of HF in the LiPF6 electrolyte. Among these separators, polyethylene separator is the most stable one with Li metal while ceramic and polysiloxy coating layers are not stable with Li metal in the presence of the LiPF6 electrolyte. This work gave more insights into the above phenomena, which could provide references for Li metal battery studies when employing Li‖Cu and Li‖Li cells in a low voltage environment.