Anisotropic demineralization and oriented assembly of hydroxyapatite crystals in enamel: smart structures of biominerals

Interaction of Statherin-Derived Peptide with the Surface of Hydroxyapatite: Perspectives Based on Molecular Dynamics Simulations

INTRODUCTION

Statherin-derived peptide (StatpSpS) has shown promise against erosive tooth wear. To elucidate its interaction with the hydroxyapatite (HAP) surface, the mechanism related to adsorption of this peptide with HAP was investigated through nanosecond-long all-atom molecular dynamics simulations.

METHODS

StatpSpS was positioned parallel to the HAP surface in 2 orientations: 1 - neutral and negative residues facing the surface and 2 - positive residues facing the surface. A system containing StatpSpS without HAP was also simulated as control. In the case of systems with HAP, both partially restrained surface and unrestrained surface were constructed. Structural analysis, interaction pattern, and binding-free energy were calculated.

RESULTS

In the peptide system without the HAP, there were some conformational changes during the simulation. In the presence of the surface, only moderate changes were observed. Many residues exhibited short and stable distances to the surface, indicating strong interaction. Specially, the residues ASP1 and SER2 have an important role to anchor the peptide to the surface, with positively charged residues, mainly arginine, playing a major role in the further stabilization of the peptide in an extended conformation, with close contacts to the HAP surface.

CONCLUSION

The interaction between StatpSpS and HAP is strong, and the negative charged residues are important to the anchoring of the peptide in the surface, but after the initial placement the peptide rearranges itself to maximize the interactions between positive charged residues.

X-ray diffraction studies of a partially demineralized oriented cortical bone with the controlled depth of analysis

The in vitro demineralization of bone tissue is used for simulating the osteoporosis related bone loss. This way would be helpful in observations of bone apatite dissolution in microstructural level and may give significant input for understanding crystal-chemistry of bone resorption. In the case of cortical bone, demineralization occurs inhomogeneously, with the formation of a superficial demineralized layer and a transition zone with a gradient of concentration and structural characteristics perpendicular to the reaction advance front. Changes in the microstructural parameters of the bone mineral in this interface zone are of great interest for understanding the resorptive processes in the bone associated with osteoporosis. In this work, the SEM-EDX method was used to estimate the sizes of the demineralized and interface layers in the cortical bone during stepwise demineralization in HCl water solution; the general patterns of changes in the concentrations of Ca, P, and Cl in these layers were established. The calculations of the effective penetration depth of X-rays in diffraction mode for the intact and partially demineralized cortical bone were performed. It is shown that the use of CoKα radiation (instead of the usual CuKα) ensures the depth of probing within the interface zone, which allows to adequately assess the microstructural parameters (crystallite sizes and lattice microdeformations) of altered bioapatite in the zone of its interaction with an acid agent. A nonmonotonic change in the average size of crystallites and microdeformations of the apatite lattice was revealed during acid demineralization of the bone. Using asymmetric XRD geometry, the evidence was obtained that the affected mineral of the transition zone does not contain other crystalline phases except for weakly crystallized apatite. For the first time, the depth-controlled XRD analysis was applied to such a complex (surface-gradient) object as partially demineralized cortical bone. Additionally, we propose a rapid, averaging, and non-destructive method for estimating the depth of the reaction front dividing the demineralized and non-demineralized portions of the bone by XRD. The consistency of XRD and SEM-EDX data on the thickness values of the demineralized layer is shown.

Erosion-Driven Enamel Crystallite Growth Phenomenon at the Tooth Surface In Vitro

The erosion of tooth enamel is a common oral disease. The erosion pattern and location and the effects of nanoscale chemical composition on the erosion susceptibility of enamel have been well documented. However, the enamel remineralization accompanied by erosion and its underlying physicochemical mechanisms still remain poorly understood. Here, using rat molars selected for its good relevancy to human teeth, we investigated the remineralization behavior of the outermost enamel surface at the nanoscale level during erosion in diluted hydrochloric acid solutions. While particles on the outermost enamel surface that represent the termination of crystallites protruding to the surface from the near-surface core eroded by acid-attack, the lateral-growth of the particles (i.e., the main remineralization picture of the surface enamel) occurred concurrently. Ionic analyses indicate that the particle growth is driven by the local increase in pH near the eroding enamel surface as a result of the combination of the PO₄^3- and CO₃^2- released from the enamel surface with H⁺. As the pH increases eventually to the equilibrium pH level (∼5.5), a local supersaturation of solute ions is induced, resulting in particle growth. A simple growth model based on the experimental results together with an assumption that the particle growth is a diffusional process suggests that the particle growth rate is controlled by the degree of supersaturation and accommodation site for solute ions, which are affected by the pH of solution eroding the enamel surface. The remineralization mechanism presented by our study can explain how the enamel on being acid-exposed or tooth decay progress by beverage or food can naturally remineralize in the oral cavity and how remineralization can foster different surface topology at the nanoscale, depending on the pH value of etchant before the dental filling material is applied.

Morphological and porosity changes in primary enamel surface after an in vitro demineralization model

In vitro models are very useful in dentistry, especially to evaluate preventive methods against dental caries. Although they have been used for more than 30 years, specific demineralization models have not been established for primary enamel, which is more prone to demineralization than permanent enamel. This study evaluates porosity changes in primary enamel surface after a demineralization model through a scientifically validated analytical tool. Nine healthy human anterior primary teeth extracted for therapeutic reasons were included in this study, previous informed consent. The samples were randomly assigned to three groups n = 3: G1_2D, G2_4D, and G3_7D. Scanning electron microscopy (SEM) images at ×200 and ×1000 were taken during two stages: before demineralization (BD) and after demineralization (AD). Morphological characterization was observed at ×1000, while porosity (pore count and perimeter) was analyzed by the ImageJ program, using ×200 SEM images previously converted. Several statistical analyses were used to determine differences (p ≤ .05). Morphological characterization AD revealed new pits and cracks on the enamel surface in G1_2D and G2_4D groups. Localized eroded enamel areas were observed in G3_7D. Pore count of enamel surface BD ranged from 64.26 ± 37.62 to 97.93 ± 34.25 and AD ranged from 150.06 ± 64.86 to 256 ± 58.14. AD, G_4D exhibited a decrease in pore perimeter contrary to G_2D and G_7D. Significant differences were observed. Finally, morphological changes were more evident as days of demineralization increased; 7 days of immersion could be employed as an enamel erosive model. The pore count increased after the demineralization model, BD pores perimeter was heterogeneous, and AD varied according to the immersion period.

Evaluating the Abrasivity of a Nanosized Eggshell-Titanium Dioxide on Tooth Enamel Using Atomic Force Microscopy

Objectives  This study aimed to evaluate nanosized eggshell–titanium dioxide (EB@TiO ² ) abrasiveness in comparison with calcium carbonate and hydrated silica-containing toothpaste.

Material and Methods  Thirty-five bovine tooth enamels were prepared, and specimens randomly assigned to a sample group of five ( n = 7), namely, Colgate regular, Colgate pro-relief, Sensodyne rapid relief, Sensodyne repair, and EB@TiO ₂ . Half of the enamel surface was brushed with each respective sample group, while the other half was covered with a tap.

Statistical Analysis  The mean roughness value (Ra) of the brushed and covered halves were measured using an atomic force microscope (AFM). To assess the surface morphology and changes, a scanning electron microscope (SEM) was performed. Using pair sample test and ANOVA, the Ra for the entire specimens were analyzed. A Bonferroni correction was used to identify the mean differences among the five groups (α=.05).

Results  The findings from this analysis show that all the tested toothpaste abraded the enamel significantly ( p < 0.05). The abrasive value contained in toothpaste comprising calcium carbonate was lower than the silica toothpaste hydrated.

Conclusion  Overall, Colgate regular had the lowest toothpaste abrasivity, followed by EB@TiO2, while Sensodyne rapid relief had the most enamel wear. The prominent feature of this study suggests that EB@TiO2 is suitable for oral use, as its abrasivity is comparable with calcium carbonate-containing toothpaste.

Anisotropic aspects of solubility behavior in the demineralization of cortical bone revealed by XRD analysis

Dissolution of cortical bone mineral under demineralization in 0.1 M HCl and 0.1 M EDTA solutions is studied by X-ray diffraction (XRD). The bone specimens (in the form of planar oriented pieces) were cut from a diaphysial fragment of a mature mammal bone so that a cross-section surface and a longitudinal section surface could be analyzed individually. This permitted to compare the dissolution behavior of bone apatite of different morphologies: crystals having the c-axis of the hexagonal unit-cell generally parallel to the long axis of the bone (major morphology) and those having the c-axis almost perpendicular to the bone axis (minor morphology). For these two types of morphology, the crystallite sizes in two mutually perpendicular directions (namely, [002] and [310]) were estimated by Scherrer formula in the initial and the stepwise-demineralized specimens. The data obtained reveal that the crystals belonging to the minor morphology dissolve faster than the crystals of the major morphological type, despite the fact that the crystallites of the minor morphology seem to be only a little smaller than those of the major morphology; the apatite crystallites irrespective of the morphology type are elongated in the c-axis direction. We hypothesize that the revealed difference in solubility may be caused by diverse chemical modifications of apatite of these two morphological types, since the solubility of apatite is strictly regulated by anionic and cationic substitutions in the lattice. The anisotropy effect in solubility of bone mineral seems to be functionally predetermined and this should be a crucial factor in the resorption and remodeling behavior of a bone. Some challenges arising at XRD examination of partially decalcified cortical bone blocks are discussed, as well as the limitations of estimation of bone crystallite size by XRD line-broadening analysis.

Water-Switchable Interfacial Bonding on Tooth Enamel Surface

Tooth enamel is a distinctive nanocomposite with a highly organized hierarchical structure made of nanometer- and micrometer-scale building blocks. This structure has an excellent mechanical function that can last for decades thanks to an effective but underexploited interfacial chemical bonding between the building blocks. In this study, the nanomechanical system test (NST), scanning electron microscope (SEM), X-ray diffraction (XRD, including powder XRD or PXRD, small angle XRD or SAXRD, and grazing incidence small angle XRD or GISAXRD), and atomic force microscope (AFM) have been employed to analyze the water-mediated bonding on the enamel surface. Via the cycling between hydration, dehydration, and rehydration treatments, a reversible change in the interfacial distance (i.e., d-space in the XRD pattern) between hydroxyapatite (HAP) nanocrystallites have been found switchable between the embrittling and toughening on the enamel surface. From the hydrated to the dehydrated conditions, an energy dissipation to deform a unit volume (1 μm³) of biocomposite on the enamel surface and subsurface has decreased by 20%. This finding can help quantify and predict biomineral-surface properties in all humidity and develop new methods to protect tooth enamel of "dry-mouth" patients.

Enamel crystallite strength and wear: nanoscale responses of teeth to chewing loads

The nanoscale responses of teeth to chewing loads are poorly understood. This has contributed to debate concerning the aetiology of enamel wear and resistance to fracture. Here we develop a new model for reactions of individual hydroxyapatite nanofibres to varying loads and directions of force. Hydroxyapatite nanofibres, or crystallites, composed of chains of bonded nanospheres, are the fundamental building blocks of enamel. This study indicates that these nanofibres respond to contact pressure in three distinct ways depending on force magnitude and direction: (i) plucking (nanosphere loss when the strength of the bonding protein 'glue' is exceeded), (ii) plastic deformation (compression to gradually bend nanofibres and squeeze the protein layer), and (iii) fragmentation (nanofibres fracture when the strength of H-bonds that bind smaller nanoparticles into nanospheres is exceeded). Critical contact pressure to initiate plucking is the lowest, followed by plastic deformation, and then fragmentation. Further, lower contact pressures are required for a response with shear forces applied perpendicular to the long axes of crystallites than with crushing forces parallel to them alone. These nanoscale responses are explained as a function of the interfacial nanochemical bonding between and within individual crystallites. In other words, nanochemistry plays a critical role in the responses of enamel to varying chewing loads.

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.

Analysis of the Early Stages and Evolution of Dental Enamel Erosion

Abstract The aim of this study was to evaluate by atomic force microscopy (AFM) the early phases and evolution of dental enamel erosion caused by hydrochloric acid exposure, simulating gastroesophageal reflux episodes. Polished bovine enamel slabs (4x4x2 mm) were selected and exposed to 0.1 mL of 0.01 M hydrochloric acid (pH=2) at 37 ?#61472;?#61616;C using five different exposure intervals (n=1): no acid exposure (control), 10 s, 20 s, 30 s and 40 s. The exposed area was analyzed by AFM in 3 regions to measure the roughness, surface area and morphological surface. The data were analyzed qualitatively. Roughness started as low as that of the control sample, Rrms=3.5 nm, and gradually increased at a rate of 0.3 nm/s, until reaching Rrms=12.5 nm at 30 s. After 40 s, the roughness presented increment of 0.40 nm only. Surface area (SA) increased until 20 s, and for longer exposures, the surface area was constant (at 30 s, SA=4.40 μm2 and at 40 s, SA=4.43 μm2). As regards surface morphology, the control sample presented smaller hydroxyapatite crystals (22 nm) and after 40 s the crystal size was approximately 60 nm. Short periods of exposure were sufficient to produce enamel demineralization in different patterns and the morphological structure was less affected by exposure to hydrochloric acid over 30 s.

The Role of Hydroxyl Channel in Defining Selected Physicochemical Peculiarities Exhibited by Hydroxyapatite

Mysteries surrounding the most important mineral for the vertebrate biology, hydroxyapatite, are many. Perhaps the Greek root of its name, απαταo, meaning 'to deceive' and given to its mineral form by the early gem collectors who confused it with more precious stones, is still applicable today, though in a different connotation, descriptive of a number of physicochemical peculiarities exhibited by it. Comparable to water as the epitome of peculiarities in the realm of liquids, hydroxyapatite can serve as a paradigm for peculiarities in the world of solids. Ten of the peculiar properties of hydroxyapatite are sketched in this review piece, ranging from (i) the crystal lattice flexibility to (ii) notorious surface layer instability to (iii) finite piezoelectricity, pyroelectricity and conductivity to protons to (iv) accelerated growth and improved osteoconductivity in the electromagnetic fields to (v) high nucleation rate at low supersaturations and low crystal growth rate at high supersaturations to (vi) higher bioactivity and resorbability of biological apatite compared to the synthetic ones, and beyond. An attempt has been made to explain this array of curious characteristics by referring to a particular element of the crystal structure of hydroxyapatite: the hydroxyl ion channel extending in the direction of the c-axis, through a crystallographic column created by the overlapping calcium ion triangles.

Dissolution mechanism of calcium apatites in acids: A review of literature

Eight dissolution models of calcium apatites (both fluorapatite and hydroxyapatite) in acids were drawn from the published literature, analyzed and discussed. Major limitations and drawbacks of the models were conversed in details. The models were shown to deal with different aspects of apatite dissolution phenomenon and none of them was able to describe the dissolution process in general. Therefore, an attempt to combine the findings obtained by different researchers was performed which resulted in creation of the general description of apatite dissolution in acids. For this purpose, eight dissolution models were assumed to complement each other and provide the correct description of the specific aspects of apatite dissolution. The general description considers all possible dissolution stages involved and points out to some missing and unclear phenomena to be experimentally studied and verified in future. This creates a new methodological approach to investigate reaction mechanisms based on sets of affine data, obtained by various research groups under dissimilar experimental conditions.

Morphological and structural changes on human dental enamel after Er:YAG laser irradiation: AFM, SEM, and EDS evaluation

OBJECTIVE

The purpose of this study was to evaluate, using atomic force microscopy (AFM), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), the morphological and structural changes of the enamel after irradiation with the Er:YAG laser.

BACKGROUND DATA

A previous study showed that subablative Er:YAG laser irradiation produced undesirable morphological changes on the enamel surface, such as craters and cracks; however, the enamel acid resistance was not increased.

METHODS

Fifty-two samples of human enamel were divided into four groups (n = 13): Group I was the control (no laser irradiation), whereas Groups II, III, and IV were irradiated with the Er:YAG 100 mJ (12.7 J/cm(2)), 100 mJ (7.5 J/cm(2)), and 150 mJ (11 J/cm(2)), respectively, at 10 Hz with water spray. The morphological changes were observed by AFM and SEM. The weight percentages (wt%) of calcium (Ca), phosphorus (P), oxygen (O) and chlorine (Cl) were determined in the resultant craters and their periphery using EDS. Kruskal-Wallis and Mann-Whitney U tests were performed (p ≤ 0.05) to distinguish significant differences among the groups.

RESULTS

The AFM images showed cracks with depths between 250 nm and 750 nm for Groups II and IV, respectively, and the widths of these cracks were 5.37 μm and 2.58 μm. The interior of the cracks showed a rough surface. The SEM micrographs revealed morphological changes. Significant differences were detected in Ca, P, and Cl in the crater and its periphery.

CONCLUSIONS

AFM observations showed triangular-shaped cracks, whereas craters and cracks were evident by SEM in all irradiated samples. It was not possible to establish a characteristic chemical pattern in the craters.

Chemical regeneration of human tooth enamel under near-physiological conditions

Regenerating the microstructure of human tooth enamel under near-physiological conditions (pH 6.0, 37 degrees C, 1 atm) using a simple chemical approach demonstrates a potential application to repair enamel damage in dental clinics.