ENDOR study of irradiated tooth enamel

Abstract

γand x-irradiated tooth enamel has been studied by EPR and ENDOR. Radiationinduced EPR spectrum of tooth enamel was found to be a superposition of signals with dominant contribution determined by CO 2 radicals. Two types of these radicals were observed: ordered and disordered centers. EPR spectra of both CO 2 centers are described by axial g-tensor with || g = 1.9975 and g ⊥= 2.0021 with c g || ||. The ENDOR spectrum of unannealed enamel powder consists of a singlet at Larmor frequency of 31 P nuclei and doublet at Larmor frequency of 1 H nuclei. Samples annealing at T = 200-250 0 C resulted in the destruction of disordered centers and appearence of superhyperfine structure of ENDOR spectra. Its analysis with advanced the powder ENDOR theory allows to determine the superhyperfine constants and to find for the first time that the ordered CO 2 radical is located in B sites (phosphorous position) of bioapatite lattice. This substitution is accompanied by the shift of the nuclei of the first 31 P shell towards the defects by 0.04 nm and the formation of the OH vacancy in the nearest radical surroundings.

Structure of Biocompatible Coatings Produced from Hydroxyapatite Nanoparticles by Detonation Spraying

“…At present, the generally accepted mechanism for charge compensation at nonisovalent replacement CO 3 2- → PO 4 3- in HAP is the formation of vacancies of calcium and hydroxyl groups in neighboring lattice sites (see, e.g., [ 28 ]). The presence of V OH in carbonated HAP was confirmed experimentally [ 29 ]. Carbonate replacement of B-type in the apatite structure leads to the formation of structural defects located on the sixfold axis (vacancy V OH at OH site) and near this axis (vacancy V Ca at Ca site); as a result, CO 3 2− -HAP becomes less stable as compared to stoichiometric HAP.…”

Influence of Al on the Structure and in Vitro Behavior of Hydroxyapatite Nanopowders

“…The following schemes can be proposed. A decrease in the crystallinity of the samples with the introduction of aluminum (according to XRD data) leads to a complete averaging of the anisotropy of the g -factors of CO 2 – and higher variations of the crystal field parameters. This leads to a broadening of the EPR signal and the impossibility of its detection against the noise level. It is known that carbonate radicals can occupy the position of OH groups and phosphates in the structure of apatites (A and B types of substitutions, respectively , ). When aluminum is introduced, additional introduction of OH groups is possible, leading to the displacement of carbonate groups from OH channels. According to the data of ref , a part of aluminum in apatite structures at high concentrations can replace positions of phosphorus, which in turn can also lead to the reduction of B-type carbonates. The introduction of aluminum causes an increase of the proton-containing groups (HOH, OH, H), which, as is well known, enhances the rate of recombination of radicals.…”

“…It is known that carbonate radicals can occupy the position of OH groups and phosphates in the structure of apatites (A and B types of substitutions, respectively , ). When aluminum is introduced, additional introduction of OH groups is possible, leading to the displacement of carbonate groups from OH channels.…”