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

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.

Degradation of hydroxyapatite in vivo and in vitro requires osteoclastic sodium-bicarbonate co-transporter NBCn1

Dissolution of the inorganic bone matrix releases not only calcium and phosphate ions, but also bicarbonate. Electroneutral sodium-bicarbonate co-transporter (NBCn1) is expressed in inactive osteoclasts, but its physiological role in bone resorption has remained unknown. We show here that NBCn1, encoded by the SLC4A7 gene, is directly involved in bone resorption. NBCn1 protein was specifically found at the bone-facing ruffled border areas, and metabolic acidosis increased NBCn1 expression in rats in vivo. In human hematopoietic stem cell cultures, NBCn1 mRNA expression was observed only after formation of resorbing osteoclasts. To further confirm the critical role of NBCn1 during bone resorption, human hematopoietic stem cells were transduced with SLC4A7 shRNA lentiviral particles. Downregulation of NBCn1 both on mRNA and protein level by lentiviral shRNAs significantly inhibited bone resorption and increased intracellular acidification in osteoclasts. The lentiviral particles did not impair osteoclast survival, or differentiation of the hematopoietic or mesenchymal precursor cells into osteoclasts or osteoblasts in vitro. Inhibition of NBCn1 activity may thus provide a new way to regulate osteoclast activity during pathological bone resorption.

Changes in the Paramagnetic Centres in Irradiated and Heated Dental Enamel Studied Using Electron Paramagnetic Resonance

The EPR signals in dental enamel produced by radiation and by heat were studied. The inherent background signal at g = 2.005, and a radiation-produced signal at g = 2.002 have different saturation behaviour with microwave power, and this affords a method of signal optimization. Heating enamel at temperatures from 100 degrees C to 450 degrees C produces a range of radical species from g = 2.002 to g = 2.005, which have been characterized by their g-values, line widths and saturation behaviour. Standard dental drilling produces a range of radicals which appear to be similar to those produced by heat.

Study of dental enamel and synthetic hydroxyapatite irradiated by EPR at K-band

In this preliminary work the EPR spectra of a small dental enamel block and a synthetic B-type hydroxyapatite in powder form, both irradiated with gamma rays, were analyzed in K-band. The spectra of the dental enamel block allow the analysis of independent components with different angular orientations, while the study of the dosimetric properties of the synthetic hydroxyapatites showed good performance of this spectrometer. K-band spectra show better resolution when compared to X-band, while using significantly less sample material.

Comparative X- and Q-band EPR study of radiation-induced radicals in tooth enamel

Human tooth enamel blocks and powders that were either unheated or heated prior to X irradiation at room temperature were investigated by means of Q-band electron paramagnetic resonance (EPR). It was found that the EPR spectra of unheated human tooth enamel consist mainly of two different anisotropic signals, as was suggested previously from an X-band study of analogous samples. In the present study, the two radical contributions could be differentiated convincingly by comparing the anisotropic Q-band spectra of heated and unheated enamel blocks. One type of is probably located in the bulk of the apatitic microcrystallites that constitute the enamel, and it appears in both heated and unheated samples. The other type is presumably located in an intercrystallite position and appears mainly in the unheated samples. Clear differences between g values in the Q-band spectra of heated and unheated enamel suggest that the radicals in the bulk exhibit larger g anisotropy than those in intercrystallite positions. Isotropic signals and contributions that may be from and radicals have also been detected. However, the present work focuses mainly on the signals and discusses potential and/or real difficulties that may be encountered in applications of EPR dosimetry using calcified tissues.

The Chernobyl accident: EPR dosimetry on dental enamel of children

The radiation dose on tooth enamel of children living close to Chernobyl has been evaluated by EPR. The sample preparation was reduced to a minimum of mechanical steps to remove a piece of enamel. A standard X-ray tube at low energy was used for additive irradiation. The filtration effect of facial soft tissue was taken into account. The radiation dose for a group of teeth slightly exceeds the annual dose, whereas for another group the dose very much exceeds the annual dose. Since the higher dose is found in teeth whose enamel have much lower EPR sensitivity to the radiation, it can be suggested that for these teeth the native signal could alter the evaluation of the smaller radiation signal.

X- and Q-band electron paramagnetic resonance of CO2- in hydroxyapatite single crystals

Both X- and Q-band electron paramagnetic resonance (EPR) research has been conducted using slightly carbonated hydroxyapatite (HAp) single crystals after exposure to ionizing radiation. Below a temperature of 90 K, O(-) and CO(2-) radicals were detected, whereas at room temperature only CO(2-) spectra could be observed. The O(-) ion has previously been investigated in high-purity HAp single crystals, whereas EPR spectra of CO(2-) in HAp single crystals have not been reported. Both paramagnetic defects exhibit EPR angular variations in planes containing the c axis of the crystal from which spin Hamiltonian parameters were derived. Arguments are given for the presence of two CO(2-) defects in the irradiated HAp single crystals.

Polarized FT-IR microscopy of calcified turkey leg tendon

Polarized Fourier transform infrared microscopy (FT-IRM) was used to assess the orientation of mineral and matrix components of the normally calcified turkey leg tendon. Two groups of tendon, < 16 weeks of age (young) and > 60 weeks of age (old), were analyzed. Linear sequences from calcified, non-calcified, and transitional regions of the tendons were examined. Spectra collected in the "parallel polarization" mode were acquired with the electric vector of the infrared radiation parallel to the collagen fiber axis whereas spectra collected in the "perpendicular polarization" mode were acquired with the electric vector of the infrared radiation perpendicular to this axis. The v2 carbonate (850-890 cm-1) and v1, v3 phosphate (900-1180 cm-1) contours of the tendon mineral as well as the collagen amide I, II, and III bands of the extracellular matrix all displayed marked dichroism. The CO3(2-) ions substituted for PO4(3-) (878 cm-1, type B substitution) in the tendon mineral displayed parallel dichroism while the CO3(2-) ions substituted for OH (871 cm-1, type A substitution) in the tendon mineral displayed perpendicular dichroism. These orientational effects for both sites of carbonate substitution were greater in the older animals. The polarization properties of the v1, v3 phosphate contour were analyzed by use of an empirical anisotropy parameter (A), the value of which monitors the degree of orientation. This index significantly increased in the older animals indicating that aging produces a more highly oriented mineral. The amide I, II, and III contours of the collagen extracellular matrix also exhibited marked dichroism. The amide I component exhibits perpendicular dichroism while the amide II and III components exhibit parallel dichroism. The current study demonstrates the ability of polarized FT-IRM to assess the orientation of the mineral and matrix components of calcified tissue at the microscopic level.

ESR dating of elephant teeth and radiation dose rate estimation in soil

Chemical analysis of 238U, 232Th and 40K in the dentine as well as enamel of elephant tooth fossil has been carried out in order to estimate the internal absorbed dose rate of the specimens, which was estimated to be (39 +/- 4) mrad/y on the assumption of early uptake model of radionuclides. The external radiation dose rate in the soil including the contribution from cosmic rays was also estimated to be (175 +/- 18) mrad/y with the help of gamma-ray spectroscopic techniques of the soil samples in which the specimens were buried. The 60Co gamma-ray equivalent accumulated dose of (2 +/- 0.2) x 10(4) rad for the tooth enamel gave "ESR age" of (9 +/- 2) x 10(4) y, which falls in the geologically estimated range between 3 x 10(4) and 30 x 10(4) y before the present.

Crystallographic and structural alterations in the mineral phase of human enamel with carious attacks

The present study aims to obtain further information about the crystallographic and structural alterations in the mineral phase of enamel with the onset of caries. For this purpose, X-ray microbeam diffraction analysis was carried out on ground sections prepared from natural white spot lesions. Electron spin resonance (ESR) analysis was also performed on block samples cut from white spot lesions and from undamaged enamel of the same teeth. The results of crystallinity measurements showed that enamel apatite in demineralized lesions was lower in crystallinity than the apatite in the surface layer and surrounding sound enamel. This X-ray diffraction study also revealed the presence of two non-apatitic minerals in the demineralized lesions. One type of the mineral is whitlockite, giving spotty rings. The nature of the other mineral, giving a single ring-like reflection, remains to be elucidated. A comparison of ESR spectra taken from the caries-attacked and undamaged enamel provided evidence that the former samples have a lower degree of alignment of apatite microcrystals. The occurrence of non-apatitic mineral phases and the observed difference in microcrystalline alignment may be the results of a remineralization process.

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+.

Organic-bound chromium in enamel, dentin, and bone studied by electron spin resonance

The present ESR investigation shows clearly that chromium is potentially present in the calcified tissues. With the use of the hydrazine-deproteinated procedure, it is suggested that chromium is associated with the organic constituent of the calcified tissues. The fact that the unheated tissues disclosed no chromium signal indicates either that the potentially present chromium is not paramagnetic, or that it is still paramagnetic, but the chromium signal cannot be detected because of a short relaxation time due to strong interaction with the organic materials.