ENDOR study of CO2− radicals in hydroxyapatite of γ-irradiated bone

Abstract

ENDOR spectra and ENDOR-induced EPR spectra (dependences of ENDOR signals vs. the value of magnetic field B ) of γ-irradiated bone plates and powders are studied. Typical ENDOR spectrum consisted of two signals centred at the Larmor frequencies of 1 H and 31 P nuclei, respectively. The ENDOR signals were found to be the superposition of the signals of axial (oriented) and orthorhombic (disordered) CO 2 − radicals. Changes of the ratio between the ENDOR signal intensities for g ‖ and g ⊥ at the rotation of the plate in the magnetic field is explained by the orientation-dependent redistribution of the EPR spectral density. EI-EPR spectra satisfactorily coincide with the spectra of conventional EPR absorption. Changes of ENDOR spectrum at the annealing of a sample before γ-irradiation ( T ann = 400 °C) are explained by Larmor ENDOR mechanism. Larmor ENDOR mechanism is associated with diffusive polarization of distant (far from a paramagnetic center) “free” nuclei under the conditions of EPR signal saturation and their depolarization by radio-frequency field in ENDOR.