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

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 dying until constant weight at 25 degrees C and 400 degrees 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 studies could be determined, corroborating the assignment of A1 to a surface O- ion.

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

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 degrees 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 CO3(3-)-contributions whereas the spectrum of precipitated samples dried at 25 degrees C can mainly be interpreted in terms of CO2-, CO3-, and O- ions. The behavior of these earlier-reported CO2-, CO3-, and O- centers is now studied as a function of drying temperature. In addition, the Spin Hamiltonian parameters of the CO3(3-) centers are determined and some other new paramagnetic radicals are discussed. It is shown that a CO3(2-) ion at a phosphate lattice site (B-type substitution) may give rise to either a CO2-, CO3-, or CO3(3-) radical on X-irradiation, depending on the sample preparation conditions. A surface CO3(2-) ion may cause a surface CO2-, CO3-, or O- radical. From the reported results it is not unambiguously clear whether the CO3(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 CO3(3-) ion increases with increasing carbonate content as was also the case for the earlier reported B-type radicals (isotropic CO2- and CO3-). 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.

The ESR spectrum of X-irradiated carbonated apatites synthesized at low temperature was studied as a function of their carbonate content. Using 13C-enriched samples, four different carbonate-derived radicals and a surface O- ion could be identified. Isotropic CO3- and CO2- ions are present at a B site in the apatite lattice, and anisotropic CO3- and CO2- 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 CO2- 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 CO2- ions. The consequences for phenomenological ESR studies of calcified tissues are discussed.

The contribution of CO3(3-) and CO2- to the ESR spectrum near g = 2 of powdered human tooth enamel.

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.

The microcrystal alignment in human tooth enamel from carious and non-carious teeth in relation to caries susceptibility.

The degree of microcrystal alignment in the intact enamel from carious and non-carious upper incisors, lower incisors and upper canines was determined by means of electron paramagnetic resonance. The results show that a low degree of alignment is not a generally valid indication for caries-susceptibility. Upper canines have a high degree of crystallite alignment whereas lower incisors have a relatively low one. With the exception of the upper central incisors no differences in microcrystal alignment are found between the intact enamel from carious and non-carious lower incisors and upper canines. These results indicate that a possible caries-susceptibility of tooth enamel reflected by low R-values is not solely determined by the degree of microcrystal alignment.

Apatite crystallite alignment in sound human tooth enamel studied by E.S.R.

The degree of microcrystal alignment in enamel of clinically sound upper and lower central incisors, canines and premolars was determined by means of electron paramagnetic resonance. The results show that the degree of alignment depends on the individual properties of the enamel as determined by biological variations, tooth morphology and tooth position. Upper incisors and canines have a much higher degree of crystallite alignment as compared to all other teeth investigated. These results indicate that the caries susceptibility of tooth enamel is not defined only by the degree of microcrystal alignment.

The use of electron spin resonance on human tooth enamel in relation to caries-susceptibility.

By means of the electron spin resonance technique it is possible to detect carbonate-related paramagnetic centers in human tooth enamel. Several arguments indicate that the radical formed after X-ray irradiation is a CO2- rather than a CO(3)3- center. There is experimental evidence that the distribution of the orientation of those centers is related to caries-susceptibility, but care has to be taken with regard to the interpretation of the results formulated in the literature. In our own preliminary research, we have studied the ESR-spectra of both bovine and human tooth enamel. As a model for human tooth enamel, the study of bovine enamel was unsatisfactory, because the signals were visibly different from the ones obtained on human tooth enamel. The reproducibility of the ESR-signals in human tooth enamel was carefully studied and found to be excellent. We also investigated the effect of heating these samples, and noted a drastic change in the spectra. Our numerical values for the R-parameter, as introduced by Cevc et al. (1976, 1980), are considerably lower than the ones they published. Clearly, additional information concerning the sample-preparation-technique and the etiological factors which determine the R-value, should be gathered.