ABSTRACT
Beta-D-fructose single crystals were in situ X-irradiated at 80 K and measured using electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and ENDOR-induced EPR (EIE) techniques at Q-band (34 GHz) microwave frequencies. The measurements revealed the presence of at least four carbon-centered radicals stable at 80 K. By means of ENDOR angular variations in the three principal crystallographic planes, six proton hyperfine coupling tensors could be determined and were assigned to four different radicals by the aid of EIE. Two of the radicals exhibit only beta-proton hyperfine couplings and reveal almost identical EIE spectra. For the other two radicals, the major hyperfine splitting originates from a single alpha-proton hyperfine coupling and their EIE spectra were also quite similar. The similarity of the EIE spectra and hyperfine tensors led to the assumption that there are only two essentially different radical structures. The radical exhibiting only beta-proton hyperfine couplings was assigned to a C3 centered radical arising from H3 abstraction and the other radical suggested to be an open-ring species with a disrupted C2-C3 bond and a double C2-O2 bond. A possible formation mechanism for the latter open-ring radical is presented. By means of cluster density functional theory (DFT) calculations, the structures of the two radicals were determined and a fairly good agreement between the calculated and experimental hyperfine tensors was found.
ABSTRACT
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.
