A resolution-enhanced Fourier Transform Infrared spectroscopic study of the environment of the CO3 2− ion in the mineral phase of enamel during its formation and maturation

Summary A resolution-enhanced Fourier Transform Infrared (FTIR) Spectroscopic study of the CO₃ ²⁻ ion in pig enamel of increasing age and maturity has demonstrated the existence of four different, main carbonate locations. The major CO₃ ²⁻ site arises as a result of the substitution of CO₃ ²⁻ ions in the positions occupied by PO₄ ³⁻ ions in the apatitic lattice. In addition, two minor locations have been identified in positions in which the CO₃ ²⁻ ions substitute for OH⁻ ions. The fourth carbonate group appears to be in an unstable location. Its concentration has been found to decrease with aging and maturation, during which there is a progressive increase in the amount of mineral deposited in the enamel. The distribution of the carbonate ions in the different apatitic sites varies randomly during the formation of the mineral phase in enamel and during its maturation. Although these changes have been shown to be related to changes in the composition of the mineral phase, a comparison of the parameters assessing the degree of crystallinity of the mineral phase from υ₂CO₃ ²⁻ and υ₄PO₃ ²⁻ infrared absorption data reveals a significant discrepancy related to the nonhomogeneous partition of the CO₃ ²⁻ ion in the mineral phase. After maximum mineralization is reached, the composition of the mature mineral phase is decidedly different than that of the initial mineral deposited; the changes affect principally the concentrations of Ca²⁺, OH⁻, and HPO₄ ²⁻ ions, but not the CO₃ ²⁻ ions.     

Carbonate ions in apatites: Infrared investigations in thev 4 CO3 domain

Summary Fourier transform infrared (IR) spectroscopic investigations of precipitated carbonate apatites in thev ₄ CO₃ domain reveal the existence of five bands at 757, 740, 718, 692, 670 cm⁻¹ which can be assigned to several distinct environments of the carbonate ion in the apatite structure. In order to identify these environments precisely, fluoridated and pure type A carbonate apatites (i.e., with carbonate ions in monovalent anionic sites) were examined. The bands at 670 and 757 cm⁻¹ were attributed to type A carbonate and their relative intensity was found to increase when the carbonate content of the apatite diminished or when samples were heated at 400°C. Fluoridated apatites show only two bands, close to 718 and 693 cm⁻¹, corresponding to type B carbonate ions (carbonate in trivalent anionic sites). The band at 740 cm⁻¹ was revealed by heating the samples to 400°C. This is due to OH ions' hydrogen bonded to fluoride and to carbonate ions in an undertermined apatite site. Despite the low intensity of IR bands, investigations in thev ₄ CO₃ domain appear complementary to those in other carbonate vibrational domains and could be useful for a more precise identification of bone mineral.     

Fourier transform infrared spectroscopic study of the carbonate ions in bone mineral during aging

Summary The environment of CO₃ ²⁻ ions in the bone mineral of chickens of different ages and in bone fractions of different density have been investigated by resolution-enhanced Fourier Transform Infrared (FTIR) Spectroscopy. Three carbonate bands appear in thev ₂ CO₃ domain at 878, 871, and 866 cm⁻¹, which may be assigned to three different locations of the ion in the mineral: in monovalent anionic sites of the apatitic structure (878 cm⁻¹), in trivalent anionic sites (871 cm⁻¹), and in unstable location (866 cm⁻¹) probably in perturbed regions of the crystals. The distribution of the carbonate ions among these locations was estimated by comparing the intensities of the corresponding FTIR spectral bands. The intensity ratio of the 878 and 871 cm⁻¹ bands remains remarkably constant in whole bone as well as in the fractions obtained by density centrifugation. On the contrary, the intensity ratio of the 866 cm⁻¹ to the 871 cm⁻¹ band was found to vary directly and decreased with the age of the animal. In bone of the same age, the relative content of the unstable carbonate ion was found to be highest in the most abundant density centrifugation fraction. A resolution factor of the CO₃ ²⁻ band (CO₃ RF) was calculated from the FTIR spectra which was shown to be very sensitive to the degree of crystallinity of the mineral. The crystallinity was found to improve rapidly with the age of the animal. The CO₃ RF in the bone samples obtained by density centrifugation from bone of the same animal was found to be essentially constant. This indicates a fairly homogeneous, crystalline state of the mineral phase. A comparison of the maturation characteristics of synthetic carbonated apatites with bone mineral indicates that a simple, passive, physicochemical maturation process cannot explain the changes observed in the mineral phase of whole bone tissue or in the density centrifugation fractions of bone during aging and maturation.     

Age-related changes in mineral of rat and bovine cortical bone

Summary The mineral of cortical bones has been studied in newborn, growing, and adult rats and in the calf and cow, using X-ray diffraction and infrared spectroscopy during the thermal decomposition of bones and by microassay of carbonate. The mineral of all the bone samples, regardless of species or age, was found to be a calcium-deficient apatite containing both CO₃ ²⁻ and HPO₄ ²⁻ ions in the crystal lattice. The crystal size, Ca/P molar ratio, and CO₃ ²⁻ ion content of cortical bone all increased with increasing age in both the rat and the bovine. The Ca/P ratio varied from 1.51 in newborn rats to 1.69 in adults but remained that of Ca-deficient apatite even though its value was close to that of stoichiometric hydroxyapatite (1.67). Both the carbonate ion and the hydrogenophosphate ion contents varied from one animal species to another and with age within a given species. Maturation was correlated with an increase in carbonate ion content, which replaced the HPO₄ ²⁻ ions. In contrast, the calcium ion number per unit formula did not vary during maturation. Cortical bone mineral, in both species, regardless of age, can therefore be represented by the following formula: Ca_8.3(PO₄)_4.3(CO₃)_x(HPO₄)_y(OH)_0.3; y decreased and x increased with increasing age, (x+y) being constant, equal to 1.7.     

The effect of carbonate content and drying temperature on the ESR-spectrum near g=2 of carbonated calciumapatites synthesized from aqueous media

Summary The ESR spectrum of X-irradiated carbonated apatites precipitated from aqueous solutions was studied as a function of their carbonate content and drying temperature. When the latter increases from 25 to 400°C, the ESR spectrum is gradually modified and becomes similar to the spectrum of carbonated apatites, synthesized at high temperatures by solid state reactions. The latter ESR spectrum is dominated by CO ₃ ³⁻ -contributions whereas the spectrum of precipitated samples dried at 25°C can mainly be interpreted in terms of CO ₂ ⁻ , CO ₃ ⁻ , and O⁻ ions. The behavior of these earlierreported CO ₂ ⁻ , CO ₃ ⁻ , and O⁻ centers is now studied as a function of drying temperature. In addition, the Spin Hamiltonian parameters of the CO ₃ ³⁻ centers are determined and some other new paramagnetic radicals are discussed. It is shown that a CO ₃ ²⁻ ion at a phosphate lattice site (B-type substitution) may give rise to either a CO ₂ ⁻ , CO ₃ ⁻ , or CO ₃ ³⁻ radical on X-irradiation, depending on the sample preparation conditions. A surface CO ₃ ²⁻ ion may cause a surface CO ₂ ⁻ , CO ₃ ⁻ , or O⁻ radical. From the reported results it is not unambiguously clear whether the CO ₃ ³⁻ ion detected in the samples with the relatively lowest carbonate content should be located on the surface or on a hydroxyl lattice site (A-type substitution). An important result is that the absolute concentration of the B-type CO ₃ ³⁻ ion increases with increasing carbonate content as was also the case for the earlier reported B-type radicals (isotropic CO ₂ ⁻ and CO ₃ ⁻ ). On the other hand, the absolute concentration of the surface radicals decreases with increasing carbonate content. The reported results show that similar deconvolution techniques can be applied in the future for the study of ESR spectra of calcified tissues. This will allow a more efficient phenomenological investigation of the latter.     

Effect of carbonate content on the ESR spectrum near g=2 of carbonated calciumapatites synthesized from aqueous media

Summary The ESR spectrum of X-irradiated carbonated apatites synthesized at low temperature was studied as a function of their carbonate content. Using¹³C-enriched samples, four different carbonate-derived radicals and a surface O⁻ ion could be identified. Isotropic CO ₃ ⁻ and CO ₂ ⁻ ions are present at a B site in the apatite lattice, and anisotropic CO ₃ ⁻ and CO ₂ ⁻ radicals are located at the surface of the crystallites. Only the isotropic ESR signals increase with increasing carbonate content. The anisotropic signal ascribed to a surface CO ₂ ⁻ radical is mainly responsible for the so-called asymmetric ESR signal near g=2. It is argued that this surface signal may still be composite and caused by several very similar CO ₂ ⁻ ions. The consequences for phenomenological ESR studies of calcified tissues are discussed.     

MicroRaman Spectral Study of the PO4 and CO3 Vibrational Modes in Synthetic and Biological Apatites

The carbonate and phosphate vibrational modes of different synthetic and biological carbonated apatites were investigated by Raman microspectroscopy, and compared with those of hydroxyapatite. The ν₁ phosphate band at 960 cm⁻¹ shifts slightly due to carbonate substitution in both A and B sites. The spectrum of type A carbonated apatite exhibits two ν₁ PO₄ ³⁻ bands at 947 and 957 cm⁻¹. No significant change was observed in the ν₂ and ν₄ phosphate mode regions in any carbonated samples. The ν₃ PO₄ ³⁻ region seems to be more affected by carbonation: two main bands were observed, as in the hydroxyapatite spectrum, but at lower wave numbers. The phosphate spectra of all biominerals apatite were consistent with type AB carbonated apatite. In the enamel spectrum, bands were observed at 3513 and at 3573 cm⁻¹ presumably due to two different hydroxyl environments. Two different bands due to the carbonate ν₁ mode were identified depending on the carbonate substitution site A or B, at 1107 and 1070 cm⁻¹, respectively. Our results, compared with the infrared data already reported, suggest that even low levels of carbonate substitution induce modifications of the hydroxyapatite spectrum. Increasing substitution ratios, however, do not bring about any further alteration. The spectra of dentine and bone showed a strong similarity at a micrometric level. This study demonstrates the existence of acidic phosphate, observable by Raman microspectrometry, in mature biominerals. The HPO₄ ²⁻ and CO₃ ²⁻ contents increase from enamel to dentine and bone, however, these two phenomena do not seem to be correlated. Received: 5 January 1998 / Accepted: 12 May 1998     

The carbonate environment in bone mineral: A resolution-enhanced fourier transform infrared spectroscopy study

Summary The environment of carbonate ions in bones of different species (rat, rabbit, chicken, cow, human) was investigated by Fourier Transform Infrared Spectroscopy (FTIR) associated with a self-deconvolution technique. The carbonate bands in thev ₂ CO ₃ ²⁻ domain show three components which were identified by using synthetic standards and different properties of the apatitic structure (ionic affinity for crystallographic locations, ionic exchange). The major component at 871 cm⁻¹ is due to carbonate ions located in PO ₄ ³⁻ sites (type B carbonate). A band at 878 cm⁻¹ was exclusively assigned to carbonate ions substituting for OH⁻ ions in the apatitic structure (type A carbonate). A band at 866 cm⁻¹ not previously observed was shown to correspond to a labile carbonate environment. The intensity ratio of type A to type B carbonate appears remarkably constant in all bone samples. The 866 cm⁻¹ carbonate band varies in its relative intensity in different species.     

Carbonate Assignment and Calibration in the Raman Spectrum of Apatite

A series of apatites with varying carbonate levels was prepared in order to assign the carbonate bands and calibrate for Raman analysis of natural materials. Overlap of carbonate bands with phosphate peaks was resolved by curve fitting. A peak at 1,071 cm⁻¹ was assigned to a combination of the carbonate ν₁ mode at 1,070 cm⁻¹ with a phosphate ν₃ mode at 1,076 cm⁻¹. In addition, the carbonate ν₄ mode was identified in apatite samples with >4% carbonate. The carbonate ν₄ bands at 715 and 689 cm⁻¹ identify the samples as B-type carbonated apatite. The carbonate content of apatite was calibrated to a carbonate Raman band, and the method was used to determine the carbonate content of a sample of bovine cortical bone, 7.7 ± 0.4%.     

Occurrence of nitrogenous species in precipitated B-type carbonated hydroxyapatites

Summary B-type carbonated hydroxyapatites, prepared in aqueous media free of alkali ions, fix ammonium ions present in the reaction medium. A small portion of the carbonate ions introduced into the apatite structure enter by the substitution mechanism (CO ₃ ²⁻ , NH ₄ ⁺ )→(PO ₄ ³⁻ , Ca²⁺). With these results for the structural incorporation of ammonium ions, differences in lattice parameters observed among specimens with the same degree of carbonation were attributed to some substitution of NH ₄ ⁺ for Ca²⁺. The fixed ammonium ions were shown to be the source of the cyanamide and cyanate ions that develop on heating. Above 500°C these apatites lost both the carbonate and the cyanate and cyanamide ions.     

Calcium phosphate saturation levels in ultrafiltered serum

Summary Calcifications occurring in arteriosclerotic plaque and other pathological deposits are important health concerns, and the nature of these deposits and their mechanisms of formation warrant investigation. Crystals of the relevant calcium phosphates were equilibrated with the undiluted ultrafiltered human serum (u.f.s.) at 37°C by constant stirring and periodically removing samples for calcium and phosphate analysis and for pH measurement. The solubility measurements were carried out both with and without a 5.5% CO₂ atmosphere, the physiological partial pressure of CO₂. The apparent ion activity products of well-crystallized dicalcium phosphate dihydrate (DCPD), octacalcium phosphate (OCP), and hydroxyapatite (OHAp) equilibrated, in u.f.s. were calculated from the calcium and phosphate concentrations and pH in each case for comparison with their known, solubility products. In this way the well-crystallized calcium phosphates serve as fiducial solubility standards, thereby minimizing errors due to complexing of calcium and phosphate ions by u.f.s. constituents. Under 5.5%, CO₂ native u.f.s. was found to be substantially undersaturated with respect to DCPD, slightly supersaturated with respect to OCP, and highly supersaturated with respect to OHAp. The ion activity product of DCPD in DCPD-saturated u.f.s. was 2.4×10⁻⁷, and the ion activity product of OCP in OCP-saturated u.f.s. was 4×10⁻⁴⁹, slightly above their solubility products (K_sp(DCPD)=2.3×10⁻⁷, K_sp(OCP)=2.5×10⁻⁴⁹). The ion activity products of DCPD and OCP in u.f.s. under CO₂ indicate that the concentrations of calcium and phosphate complexing agents (except bicarbonate) are quite low. The u.f.s. remained supersaturated with respect to OHAp even after 2 months of equilibration. This is attributed to the presence of crystal growth inhibitors in u.f.s.     

A Comparative Infrared Spectroscopic Study of Hydroxide and Carbonate Absorption Bands in Spectra of Shark Enameloid, Shark Dentin, and a Geological Apatite

The purpose of the present work was to investigate the infrared (IR) spectrum of shark enameloid, especially with regard to hydroxide and carbonate bands. With thin sections placed directly in the IR beam it was possible to get high concentrations of ions without interfering effects from a dispersion medium (e.g., alkali halides). For comparison, spectra of shark dentin and a geo-apatite were also recorded. In spectra of shark enameloid and geo-apatite medium strong hydroxide absorption bands were found around 3535 cm⁻¹, and in shark dentin and geo-apatite spectra weak shoulders were observed at about 3570 cm⁻¹. Hydroxide libration bands at about 740 cm⁻¹ were found in shark enameloid and geo-apatite spectra; in the latter, also a band at 680 cm⁻¹. Carbonate bands were found in shark enameloid spectra at 1480 (weak shoulder), 1453, 1423, and 868 cm⁻¹. In shark dentin spectra there were carbonate bands at 1452, 1417, and 875 cm⁻¹, and probably also a carbonate band at about 1530 cm⁻¹ overlapped by an amide II band. Weak carbonate bands were also found in the spectra of the geo-apatite at 1452 cm⁻¹, and at about 1425 and 880 cm⁻¹. The relative intensities of the bands at 1453 cm⁻¹ (contributed from A and B sites) and around 1420 cm⁻¹ (B sites) changed from shark enameloid to shark dentin, and also from shark enameloid to the geo-apatite. More A sites seem to be occupied by carbonate in shark dentin than in shark enameloid, supposedly owing to fluoride occupation of A sites in shark enameloid. In geo-apatite and shark enameloid there are hydroxide ions hydrogen bonded to fluoride. Both shark enameloid and the geo-apatite are fluoride rich, and geo-apatite seems to have the highest fluoride concentration. There are, however, indications that the hydroxide concentration is also higher in the geo-apatite than in shark enameloid. This can be explained by the much higher carbonate content, and partly also by the higher water content in shark enameloid. There are A sites in geo-apatite and probably also in shark enameloid which are occupied by carbonate, but the proportion of occupied A sites relative to occupied B sites is greater in geo-apatite than in shark enameloid. This difference can be explained by the preference of A sites when the carbonate concentration is very low. On the other hand, for greater amounts of carbonate such as we have in shark enameloid, B sites are preferred. Received: 1 June 1998 / Accepted: 12 March 1999     

F−-CO 3 2− interaction in IR spectra of fluoridated CO3-apatites

Summary Spectroscopic properties of fluoridated CO₃-apatites were studied by means of infrared (IR) absorption analysis. The peak of the IR band caused by CO ₃ ²⁻ ions at 875 cm⁻¹ shifted to a lower frequency with the degree of fluoridation. This result suggests that there is a significant interaction between F⁻ and CO ₃ ²⁻ ions in the apatite crystals. The interaction of both these ions is discussed with regard to the solubility behavior of fluoridated CO₃-apatites in the acetate buffer solution at pH 4.0 and at 37°C. Solubility of these fluoridated CO₃-apatites approached that of fluoridated hydroxyapatites at high fluoride content.     

Resolution-enhanced fourier transform infrared spectroscopy study of the environment of phosphate ions in the early deposits of a solid phase of calcium-phosphate in bone and enamel,and their evolution with age. I: Investigations in thev 4 PO4 domain

Summary In order to investigate the possible existence in biological and poorly crystalline synthetic apatites of local atomic organizations different from that of apatite, resolution-enhanced, Fourier transform infrared spectroscopy studies were carried out on chicken bone, pig enamel, and poorly crystalline synthetic apatites containing carbonate and HPO₄ ²⁻ groups. The spectra obtained were compared to those of synthetic well crystallized apatites (stoichiometric hydroxyapatite, HPO₄ ²⁻-containing apatite, type B carbonate apatite) and nonapatitic calcium phosphates which have been suggested as precursors of the apatitic phase [octacalcium phosphate (OCP), brushite, and β tricalcium phosphate and whitlockite]. The spectra of bone and enamel, as well as poorly crystalline, synthetic apatite in thev ₄ PO₄ domain, exhibit, in addition to the three apatitic bands, three absorption bands that were shown to be independent of the organic matrix. Two low-wave number bands at 520–530 and 540–550 cm⁻¹ are assigned to HPO₄ ²⁻. Reference to known calcium phosphates shows that bands in this domain also exist in HPO₄ ²⁻-containing apatite, brushite, and OCP. However, the lack of specific absorption bands prevents a clear identification of these HPO₄ ²⁻ environments. The third absorption band (610–615 cm⁻¹) is not related to HPO₄ ²⁻ or OH⁻ ions. It appears to be due to a labile PO₄ ³⁻ environment which could not be identified with any phosphate environment existing in our reference samples, and thus seems specific of poorly crystalline apatites. Correlation of the variations in band intensities show that 610–615 cm⁻¹ band is related to an absorption band at 560 cm⁻¹ superimposed on an apatite band. All the nonapatitic phosphate environments were shown to decrease during aging of enamel, bone, and synthetic apatites. Moreover, EDTA etching show that the labile PO₄ ³⁻ environment exhibited a heterogeneous distribution in the insoluble precipitate.     

Thermal decomposition of human tooth enamel

Summary Further insight into human tooth enamel, dense fraction (TE), has been obtained by following the change and loss of CO₃ ²⁻, OH⁻, structurally incorporated H₂O, Cl⁻, and, indirectly, HPO₄ ²⁻ after TE had been heated in N₂ or vacuum in the range 25–1000°C. Quantitative infrared spectroscopic, lattice parameter, and thermogravimetric measures were used. Loss of the CO₃ ²⁻ components begins at much lower temperature (e.g., 100°C) than previously recognized, which has implications for treatments in vitro and possibly in vivo. CO₃ ²⁻ in B sites is lost continuously from the outset; the amount in A sites first decreases and then increases above 200° to a maximum at ∼800°C (>10% of the possible A sites filled), where it is responsible for an increase ina lattice parameter. A substantial fraction of the CO₃ ²⁻ in B sites moves to A sites before being evolved, apparently via a CO₂ intermediary. This implies an interconnectedness of the A and B sites which may be significant in vivo. No loss of Cl⁻ was observed at temperatures below 700–800°C. Structural OH⁻ content increases ∼70% to a maximum near 400°C. Structurally incorporated water is lost continuously up to ∼800°C with a sharp loss at 250–300°C. The “sudden”a lattice parameter contraction, ∼0.014?, occurs at a kinetics-dependent temperature in the 250–300°C range and is accompanied by reordering and the “sharp” loss of ∼1/3 of the structurally incorporated H₂O. The hypothesis that structurally incorporated H₂O is the principal cause of the enlargement of thea lattice parameter of TE compared to hydroxyapatite (9.44 vs 9.42?) is thus allowed by these experimental results.     

ESR study of 13C-enriched carbonated calciumapatites precipitated from aqueous solutions

Summary The ESR spectrum of X-irradiated carbonated apatites precipitated from aqueous solutions was studied at carbonate contents ranging from approximately 5.4 up to 12.4 wt%. ¹²C- as well as ¹³C-enriched samples were prepared and examined with ESR after drying until constant weight at 25°C and 400°C. In these carbonated apatites, two CO₃ ^3- radicals were detected, one of which is derived from B-type carbonate situated at a phosphate lattice site. The other CO₃ ^3- most probably arises from CO₃ ^2- ions situated at OH^- lattice sites. In contradistinction to the B-type contribution, the A-type contribution to the overall ESR signal decreases in absolute terms with increasing carbonate concentration. In addition, the line shape and the complete set of principal g-values of the A1-signal found in previous studles could be determined, corroborating the assignment of A1 to a surface O^- ion.Research Associate of the N.F.S.R. (Belgium)     

Composition of apatites in human urinary calculi

Summary Calcium-deficient carbonate apatites in urinary calculi from 17 patients have been studied. Magnesium ions in each case were detected as a component of apatite. In most of the cases, CO ₃ ²⁻ ions were included in the apatitic lattice. Using a calcination procedure, we have determined the most probable average value for the CO ₃ ²⁻ content, degree of deficiency, x, and the Mg/Ca ratio in each sample.     

In vitro effects of aluminum on bone phosphatases: A possible interaction with bPTH and vitamin D3 metabolites

Summary The present study was undertaken to test the in vitro action of aluminum on bone phosphatase activities and the possible interaction of this metal with parathyroid hormone (bPTH) or vitamin D₃ dihydroxymetabolites [1,25- and 24,25(OH)₂D₃). Three-day-old rat calvaria were incubated for 24 h with one of the following: bPTH at 5×10⁻⁸M, 1,25-or 24,25(OH)₂D₃ at 2.5×10⁻⁹M, Al at concentrations ranging from 3×10⁻¹¹M to 6×10⁻⁶M, or their corresponding solvents. Al effects were also investigated when the medium phosphate or calcium concentrations were modified. In some experiments, Al was added simultaneously with bPTH or one of the vitamin D₃ metabolites at the beginning of the 24 h incubation. At the end of all incubations, acid and alkaline phosphatase activities were measured in bone cytoplasmic extract. The results show that: (a) When compared to the value found in half calvaria incubated in a control medium, the bone acid and alkaline phosphatase content is significantly higher in paired halves incubated with Al (3×10⁻¹¹M to 1.5×10⁻⁶M) as well as with bPTH, 1,25-, or 24,25(OH)₂D₃ and sharply decreased with higher Al concentrations (6×10⁻⁶M). (b) The Al effect on phosphatase activities is modified in a free phosphate or a free calcium medium. (c) The presence of Al at 1.5×10⁻⁶M or 6×10⁻⁶M significantly decreases the bPTH or 1,25(OH)₂D₃-induced stimulation of bone phosphatase activities. (d) A similar interaction could not be found between Al and 24,25-(OH)₂D₃.     

Precipitation of calcium phosphates from electrolyte solutions V. The influence of citrate ions

Summary The influence of citrate ions on the precipitation of crystalline apatitic precipitates with low Ca/P molar ratios [octacalcium phosphate (OCP) and calcium-deficient apatites (DA) (system A)] and of the intercrystalline mixtures of calcium hydrogen phosphate dihydrate (DCPD) and DA (system B) was investigated. Samples were prepared by direct mixing of calcium chloride solutions (A, 6·10⁻³ mol dm⁻³; B, 1·10⁻¹ mol dm⁻³) and sodium phosphate solutions (A, 6·10⁻³ mol dm⁻³; B, 2·10⁻² mol dm⁻³) containing citrate (0–2·10⁻³ mol dm⁻³) and preadjusted to pH 7.4. In the presence of citrate ions: (a) crystal growth of OCP and DA was slowed down; (b) habit modification of DCPD crystals occurred; and (c) equilibration in intercrystalline mixtures of DCPD and DA's was slowed down. All phenomena were caused by surface adsorption of negatively charged ions, most probably CaC₆H₅O₇-, which is the prevalent calcium citrate species under the given experimental conditions. Habit modification of DCPD was induced by preferential adsorption at the (001) crystal plane.     

The contribution of CO3 3− and CO2 − to the ESR spectrum near g=2 of powdered human tooth enamel

Summary The ESR spectrum near g=2 of powdered human tooth enamel from upper central incisors and lower canines was studied as a function of microwave power, irradiation, and storage time. The results clearly demonstrate that the ESR spectrum is composite with at least five paramagnetic species contributing to the signal. The main stable component is assigned to CO₂ ⁻. Two other components arise from CO₃ ³⁻ radicals, one of which is demonstrated to be the same center as is present on a phosphate site in sodium- and carbonate-containing calciumapatite.