Recent progress in the chemistry, crystal chemistry and structure of the apatites

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.

Confusion between Carbonate Apatite and Biological Apatite (Carbonated Hydroxyapatite) in Bone and Teeth

Biological apatite in enamel, dentin, cementum, and bone is highly individualized hydroxyapatite with high tissue dependency. Often, standard and average textbook values for biological apatite do not apply to actual subjects, and the reported results of analyses differ among investigators. In particular, the term biological apatite is often confusingly and incorrectly used to describe carbonate apatite. The purpose of this review is to prevent further confusion. We believe that apatite should be well understood across disciplines and the terminology clearly defined. According to a definition by the International Mineralogical Association’s Commission on New Minerals Nomenclature and Classification, biological apatite formed by living organisms is a type of hydroxyapatite. More specifically, it is carbonated hydroxyapatite, which is quite different from frequently misnamed carbonate apatite. We hope that this definition will be widely adopted to remove confusion around the naming of apatite in many research and applied fields.

Synthesis, luminescence and application of novel europium, cerium and terbium-doped apatite phosphors

Novel La₈Ba₂(Si₄P₂O₂₂N₂)O₂:Eu²⁺/Ce³⁺/Tb³⁺ apatite phosphors with tuneable light-emission have been developed.

EPR dosimetry with tooth enamel: A review

When tooth enamel is exposed to ionizing radiation, radicals are formed, which can be detected using electron paramagnetic resonance (EPR) techniques. EPR dosimetry using tooth enamel is based on the (presumed) correlation between the intensity or amplitude of some of the radiation-induced signals with the dose absorbed in the enamel. In the present paper a critical review is given of this widely applied dosimetric method. The first part of the paper is fairly fundamental and deals with the main properties of tooth enamel and some of its model systems (e.g., synthetic apatites). Considerable attention is also paid to the numerous radiation-induced and native EPR signals and the radicals responsible for them. The relevant methods for EPR detection, identification and spectrum analyzing are reviewed from a general point of view. Finally, the needs for solid-state modelling and studies of the linearity of the dose response are investigated. The second part is devoted to the practical implementation of EPR dosimetry using enamel. It concerns specific problems of preparation of samples, their irradiation and spectrum acquisition. It also describes how the dosimetric signal intensity and dose can be retrieved from the EPR spectra. Special attention is paid to the energy dependence of the EPR response and to sources of uncertainties. Results of and problems encountered in international intercomparisons and epidemiological studies are also dealt with. In the final section the future of EPR dosimetry with tooth enamel is analyzed.

Physical properties of root cementum: Part 9. Effect of systemic fluoride intake on root resorption in rats

INTRODUCTION

Orthodontically induced inflammatory root resorption is a common complication in orthodontic treatment. Fluoride has been reported to have a beneficial effect against root resorption in dental traumatology. The effect of fluoride on orthodontically induced inflammatory root resorption has not been investigated. This study was undertaken to investigate the effect of fluoride on the incidence of root resorption.

METHODS

Thirty-two female 8-week-old Wistar rats were separated into 4 groups. Two groups (6 rats per group) were controls; they did not undergo orthodontic tooth movement. The other 2 groups (10 rats per group) had orthodontic tooth movement consisting of activated 100-g closing nickel-titanium coils (NiTi 10-000-06, GAC International, Bohemia, NY) connecting the mandibular first molar to the incisors. Fluoridated water (100 ppm) was given ad libitum to 1 control and 1 experimental group. The other 2 groups received deionized water. After 2 weeks, the animals were killed, and the samples were harvested. Resorption craters were scanned with a Micro CT (SkyScan 1072, Aartselaar, Belgium). Software analysis of the scanned samples provided a volumetric measurement of the resorption craters on the mandibular molar cementum surface.

RESULTS

Resorption sites were found in the control samples, especially on the distal surfaces; this could be attributed to normal physiological tooth drift. Resorption sites were significantly (P <.05) increased in the groups receiving orthodontic tooth movement.

CONCLUSIONS

Fluoride reduces the size of resorption craters, but the effect is variable and not statistically significant (P >.05).

Derivation of elastic stiffness from site-matched mineral density and acoustic impedance maps

200 MHz acoustic impedance maps and site-matched synchrotron radiation micro computed tomography (SR-muCT) maps of tissue degree of mineralization of bone (DMB) were used to derive the elastic coefficient c(33) in cross sections of human cortical bone. To accomplish this goal, a model was developed to relate the DMB accessible with SR-muCT to mass density. The formulation incorporates the volume fractions and densities of the major bone tissue components (collagen, mineral and water), and accounts for tissue porosity. We found that the mass density can be well modelled by a second-order polynomial fit to DMB (R(2) = 0.999) and appears to be consistent with measurements of many different types of mineralized tissues. The derived elastic coefficient c(33) correlated more strongly with the acoustic impedance (R(2) = 0.996) than with mass density (R(2) = 0.310). This finding suggests that estimates of c(33) made from measurements of the acoustic impedance are more reliable than those made from density measurements. Mass density and elastic coefficient were in the range between 1.66 and 2.00 g cm(-3) and 14.8 and 75.4 GPa, respectively. Although SAM inspection is limited to the evaluation of carefully prepared sample surfaces, it provides a two-dimensional quantitative estimate of elastic tissue properties at the tissue level.

pH and carbonate levels in developing enamel

Our earlier studies showed that the surface of developing and calcifying enamel changes its pH alternatively along the tooth axis when stained with pH indicating dyes. Based on the pH conditions, the enamel at this stage was distinguished as neutral zone (N1 and N2) and acid zone (A1 and A2). The aim of the present study was to correlate changes of pH with proteolytic activity and crystal size of the calcifying bovine enamel. Specimens of developing bovine enamel were separated into four maturing stages using pH staining methods. Crystal chemistry of the developing enamel was investigated using thermogravimetry (TGA), ICP emission spectrometry, X-ray diffractometry (XRD) and infrared spectroscopy (IR). Previous biochemical analysis of proteolytic enzyme activity from enamel indicated that the optimal pH of the major protease was approximately pH 6.0, coinciding with the pH of the A1 zone. IR, TGA and XRD analyses showed that most of the organic components of the enamel decomposed at 580 degrees C. Higher levels of carbonate were observed in the secretory stages than in mature enamel. The Ca/P molar ratio of the enamel apatite was lower than the stoichiometric value of 1.67. These results suggest that growth and maturation of enamel apatite crystals is related to a decrease in the carbonate level and appear to be related to the alternative calcification and decomposition of enamel proteins.

TPD, FTIR, and Molecular Adsorption Studies of Calcium Hydroxyapatite Surface Modified with Hexanoic and Decanoic Acids

The surface of synthetic calcium hydroxyapatite Ca10(PO4)6 (OH)2 (CaHAP) particles was modified with hexanoic CH3(CH2)4 COOH (HA) and decanoic CH3(CH2)8COOH (DA) acids in hexane and the resulting materials were characterized by various means. XRD pattern, specific surface area, and morphology of the CaHAP particles were essentially not altered by the modification. FTIR and TPD results indicated that HA and DA molecules are hydrogen-bonded to the surface P-OH groups of CaHAP in three adsorption types. The surface of modified particles became hydrophobic. When the modified samples were outgassed at 500 degrees C, HA and DA were removed and the surface P-OH groups were revived. The modified material adsorbed much less H2O and CO2 than the unmodified one. Copyright 1999 Academic Press.

Surface Structure and Properties of Calcium Hydroxyapatite Modified by Hexamethyldisilazane

The surface of synthetic calcium hydroxyapatite Ca10(PO4)6(OH)2 (CaHAP) particles was treated by repeated modification with hexamethyldisilazane [(CH3)3Si]2NH (HMDS) in hexane and thermal treatment and the surface of the modified CaHAP was characterized by various means. No remarkable change in XRD patterns or in particle shape by the modification was observed. The width of the CaHAP particles gradually increased with repeating the modification. FTIR results indicated that HMDS reacted with surface P-OH groups of CaHAP to yield surface Si-(CH3)3 groups. The surface of the modified CaHAP was hydrophobic. The surface Si-(CH3)3 groups turned to three kinds of surface Si-OH groups by treating the modified materials at 500 degreesC in air. These formed surface Si-OH groups and the remaining surface P-OH groups reacted with HMDS by repeating the modification, resulted in the increase of the surface Si atoms. The modified material having surface Si-(CH3)3 or Si-OH groups adsorbed much less CO2 than the unmodified one. Copyright 1998 Academic Press.

The petrology of oolitic phosphorites from Esprit (Aldabra), western Indian Ocean

The island of Esprit, at the western end of Aldabra lagoon, is capped by two groups of phosphorites. Near the summit, bedded deposits rest on and in cavities within the subaerially eroded surface of the limestones forming the island. The limestones themselves have not been phosphatized. On the lower slopes, and derived from the phosphorites above, are small outcrops of coarse, phosphate-cemented, bioclastic sediments and large, irregular fans of phosphoritic conglomerates. The phosphorites can be divided into five petrographic groups. Oolitic phosphorites are the most common and are apparently primary. Associated with them are lithoclast-bearing rocks, fine-grained phosphorites, bioclastic deposits and internal sediments, all of which are also wholly phosphatic. There have been numerous reworking episodes in the history of these rocks, such that large volumes now consist only of phosphatic cement sequences (the linings of former cavities) and internal sediments. Up to fourteen changes in the depositional milieu can be seen. The distribution of both cements and internal sediments is restricted, recording the paths of particular transport streams in groundwaters. The minerals identified are carbonatian hydroxyfluorapatites. Most cements are multilayer colloform crusts with a radially fibrous structure, but, in addition, crystals show a range of morphologies, including hexagonal, monoclinic and cubic forms. Some cements are carbonate, but other minerals that may have been present have been pseudomorphed by phosphates. The primary source of the phosphate in these rocks is thought to have been avian guano, deposited on a limestone surface at a time when sealevel was 7-8 m above its present position. Phosphate-rich derivatives from this were carried downwards by surface-wash processes and precipitated in a series of caves in the limestones excavated at the water table and drained as sealevel fell. Solution pipes were formed when sealevel was at least 1-2 m below its present position, but marine coquinal sediments deposited within these suggest that it was subsequently higher. Continued erosion of the host limestones, destroying the caves, released both phosphorite detritus, redeposited as the low-level sediments and transported into beach calcarenites, and phosphatic solutions for precipitation as cements. The phosphorites are tentatively dated as deposited 170-230 ka B.P .

FT-IR photoacoustic depth profiling spectroscopy of enamel

Photoacoustic Fourier transform infrared (PA-FT-IR) depth profiling spectra of the enamel of an intact human tooth are obtained in a completely nondestructive fashion. The compositional and structural changes in the tissue are probed from the enamel surface to a depth of about 200 microns. These changes reflect the state of tissue development. The subsurface carbonate gradient in the enamel could be observed over the range of about 10-100 microns. The carbonate-to-phosphate ratio increases in the depth profile. The depth profile also reveals changes in the substitutional distribution of carbonate ions. Type A carbonates (hydroxyl substituted) increase relative to type B carbonates (phosphate substituted) with increasing thermal diffusion length. In addition to the changes in the carbonate ion distribution and content, the PA-FT-IR depth profile clearly indicates a dramatic increase in the protein content relative to the phosphate content with increased depth. The changes in the carbonate content and distribution, along with the changes in the protein content, may be responsible for the changes observed in the apatitic structure in the depth profile of the enamel.

A glass ionomer for reconstructive surgery. Ionogran--an ionomeric micro implant. A biological evaluation

A porous glass-ionomer (Ionogran) was evaluated and compared to hydroxyapatite Interpore 200. In an in-vivo screening test procedure, microfilter diffusion chambers embedded on the tibial surfaces of adult baboons were used to evaluate cell survival, cell regeneration and cell differentiation. In a bulk testing procedure a standardized bone core was removed from the tibia through the knee joint and the material exposed to the defense mechanisms of the system. Both materials were found to be biocompatible, non-toxic and did not inhibit cell proliferation in the enclosed environment of diffusion chambers. Histological response and cell arrangement within the chambers containing Ionogran were similar to that of the controls with cancellous bone only. Highly specialized haemopoietic tissues were in direct contact with the Ionogran. In the presence of hydroxyapatite new bone was actively formed. Haemopoietic elements were never found in contact with the hydroxyapatite. In the bulk testing procedure, the general histological appearance was that of a reactive tissue response to a low grade persistent stimulation coupled to surface degradation products. The Ionogran was found to be stable, osteo-conductive and promoted osteoid formation when in contact with bone. From a clinical point of view the material could serve as a permanent scaffold, maintaining form, and will be of value in bone reconstructive procedures.

Enhanced caries inhibition by certain NaF-surfactant combinations in animal models

The purpose of this animal investigation was to determine whether the cariostatic effect of sodium fluoride can be enhanced by combining it with certain surfactants. The surfactants, Zonyl FSC and Lodyne S-110, were selected because they had been found to accelerate the rate of deposition of firmly-bound fluoride in vitro in enamel during enamel-fluoride interaction. Sixty male hamsters and 60 female rats, 21 days old, were placed on the high-sucrose diet 2000 and inoculated with Streptococcus mutans (strain #6715). Each set of animals was divided into six groups of 10 each. The groups received either distilled water, NaF, NaF with Lodyne S-110, NaF with Zonyl FSC, Lodyne S-110, or Zonyl FSC. The animals were scored for caries at the termination of the study. The caries findings in the hamster and the rat series closely paralleled each other. Animals receiving NaF with Zonyl FSC experienced the least decay (about 90% inhibition compared with about 65% for the NaF-alone group). A statistical evaluation showed significant interaction between Zonyl FSC and NaF. No effect on caries experience was seen from administration of Lodyne S-110.

The composition of recrystallized bone mineral

Bone mineral and synthetic calcium-deficient carbonate apatite (CDCA), when defined in terms of their respective thermal stabilities and ignition products, are homologous. When heated to 550 degrees, they may have a structure similar to that of the mineral dahllite. When heated to temperatures greater than 550 degrees, CDCA (like bone mineral) losses its structural CO3 and is recrystallized to whitlockite and/or OH-apatite, depending on its stoichiometry.

Carbonate apatites from aqueous and non-aqueous media studied by ESR, IR, and X-ray diffraction: effect of NH4+ ions on crystallographic parameters

Carbonate-containing apatites prepared from aqueous and non-aqueous media were investigated by ESR, IR, and X-ray diffraction. A typical asymmetric signal at g = 2.0, which many workers have observed in irradiated calcified tissues, is interpreted in terms of CO3(3)- and Co-2 and the trapped electron center, The CO3(3)- and CO2- ions originated from the carbonate which substitutes for the phosphate ion in the apatite crystalline structure. From the comparison of the data between the carbonate apatites prepared under the presence of Na+ and NH4+ ions, it is suggested that NH4+ ions contribute to the crystallographic parameters, either by substituting in the structure or by suppressing the CO3 incorporation into the structure, or by both. The production of the CO2- radical ion could be accounted for based on the concept of hydrogen bond between the oxygen atom of CO2(3)- and the hydrogen atom of NH4+.

The laser-Raman spectroscopic studies on human enamel and precipitated carbonate-containing apatites

Laser-Raman spectra of synthetic apatites containing from zero to 12.4 wt% carbonate ions and enamel apatite are reported. Special attention is paid to the correlation between the intensities of the carbonate, phosphate, and hydroxyl modes and the carbonate content in the apatites. The data presented here indicate that most of the CO2-3 incorporated in the apatite lattice substitutes for PO3-4 and suggest that a small amount of CO2-3 might be located in the OH- position.

Orientations of carbonate ions in human tooth enamel studied with use of the CO3-3 radical ions as probes

With the use of CO3-3 radical ions as probes, it is concluded that the carbonate ions substitute in the apatite lattice of human tooth enamel in two different ways. Most of the carbonate ions in the apatite lattice are aligned with their symmetry axis parallel to the c axis, whereas the remaining carbonate ions are aligned with their symmetry axis perpendicular to the c axis.

Reactions of powdered human enamel and fluoride solutions with and without intermediate CaHPO4 x 2H2O formation

Powdered human enamel treated for three minutes each with a CaHPO4 x 2H2O forming solution and a fluoride solution, and then suspended in "synthetic saliva" for 72 hours acquired a large amount of apatitic fluoride. This apatitic fluoride appears to form after the fluoride treatment.

The caries resistance of human teeth is determined by the spatial arrangement of hydroxyapatite microcrystals in the enamel

Teeth from different people differ in their susceptibility to caries. The beginning and the development of tooth decay depend on the presence of bacterial plaque and on the structure of the tooth enamel. But it has not been established whether the organic or the inorganic component of the enamel (which is comprised principally of hydroxyapatite microcrystals organized in a firm matrix), is responsible for most of the resistance of teeth to caries. We report here that the electron spin resonance (ESR) lines of the (CO3)3 defect of samples of enamel prepared from caries-resistant teeth differ significantly from the corresponding spectra of the caries-sensitive samples. We have traced these differences to the hydroxyapatite microcrystalline alignment in the tooth enamel, and found that this alignment is specific to individuals and is therefore probably nutritionally or genetically determined.

Analysis of paramagnetic centers in X-ray-irradiated enamel, bone, and carbonate-containing hydroxyapatite by electron spin resonance spectroscopy

Carbonate-containing hydroxyapatite, enamel, and bone were irradiated by an X-ray and investigated between 77 degrees and 350 degrees K by means of electron spin resonance (ESR) spectroscopy. The ESR spectrum of enamel irradiated at 77 degrees K in vacuum and observed at the same temperature was almost the same as that of the carbonate-containing hydroxyapatite. The temperature dependence of signal intensities confirms a spin-energy exchange between the mineral and organic constituents in bone, but in enamel no or very little spin-energy exchange between the mineral and organic constituents. Considerable similarity among the ESR spectra of enamel, bone, and carbonate-containing apatite was obtained after X-ray irradiation in air at 300 degrees K with both an X-band and a Q-band ESR spectrometer. The Q-band spectrum can be interpreted in terms of two paramagnetic species. One is identified as a CO3(3-) anion radical which has an axial symmetry with g factors of 2.0029 and 1.9972. The other species is found to be centered at g = 2.0019.

Reaction of acidulated phosphate-fluoride solutions with human apatite

Finely pulverized human enamel was reacted with acidulated phosphate-fluoride solutions. The products were analyzed for fluoride content and examined by X-ray difjraction and infrared absorption. Fluoride content of the products and the amount of calcium fluoride formed increased with increasing reaction time, temperature, or both. Misleading results may be obtained if they are based only on fluoride uptake analyses.

Monoclinic Hydroxyapatite

The existence of a monoclinic phase of hydroxyapatite, Ca(2)(PO(4))(4)OH, has been confirmed, by single-crystal structure analysis (weighted "reliability" factor = 3.9 percent on |F|(2)). The structure has space group P21/b, a = 9.4214(8) angstroms, b = 2a, c = 6.8814(7) angstroms, and gamma = 120 degrees , and is analogous to that of chlorapatite. The distortions from the hexagonal structure with which the monoclinic structure is pseudosymmetric are similar to those in chlorapatite, including enlargement of that triangular array of oxygen atoms in which the chlorine ion or, in hydroxyapatite, the hydroxyl hydrogen ion is approximately centered. The hydroxyapatite specimen was prepared by the conversion of a single crystal of chlorapatite in steam at 1200 degrees C, was mimetically twinned, and was approximately 37 percent monoclinic.

Incorporation of fluoride by human enamel. I. Solid-state diffusion process

The incorporation of fluoride by human enamel from topical fluoride solutions involves the process of di~usion and is partially governed by Fick's laws of diffusion. Further, the law of mass action applies, and the fluoride within the enamel is partitioned into substitutional and interstitial species. Thus, the phenomenon must be considered from the viewpoint of physical solid-state chemistry to understand it on an atomistic level.