磷酸鈣相轉變機制之研究及其在生物礦化的意義

Abstract

We have assigned the 31P high-resolution spectrum of octacalcium phosphate by 31P double quantum and HETCOR spectroscopy. The finite pulse RFDR sequence was used effectively for 31P double quantum NMR spectroscopy at a spinning frequency of 10 kHz. The 31P NMR data measured for hydroxyapatite and octacalcium phosphate show that sizable double-quantum excitation efficiency can be obtained with the ratio of the recoupling field to spinning frequency set equal to 1.67. The 31P peaks at -0.2, 2.0, 3.3 and 3.7 ppm are assigned to P5/P6, P3, P2/P4 and P1 sites in OCP, respectively. Our data reveal that substantial amount of the PO4 3- groups at the P2 and P4 sites have been transformed to HPO42- in our octacalcium phosphate sample. (Chapter 3) Based on the chemical shift data of OCP, we were able to study the molecular mechanism of OCP to hydroxyapatite (HAp) transformation in vitro by several physical techniques, with particular emphasis on solid-state 31P homonuclear double-quantum (DQ) NMR spectroscopy. The in vitro system is prepared by mixing urea, sodium phosphate monobasic dehydrate, and calcium nitrate tetrahydrate at 100 °C. The images obtained by scanning electron microscopy and transmission electron microscopy show that the blade like OCP crystals will transform into hexagonal rod-shaped HAp crystals as the pH of the reaction mixture increases slowly from 4.35 to 6.69 in 12 h. Together with computer-assisted lattice matching, our DQ NMR data reveal that OCP crystals transform to HAp topotaxially, where the [ ]HAp and [ ]HAp axes are along the same directions as the [001]OCP and [010]OCP axes, respectively. On the basis of our in vitro results, the formation of the central dark line commonly found in biological hard tissues could be explained by the inherent lattice mismatch between OCP and HAp. (Chapter 4) In addition to synthetic crystals, the structure and composition of the biological calcified tissues (rat dentine) were also analyzed by several advanced solid-state NMR techniques without any chemical pretreatment. The measurements of OH- content of calcium phosphates in the rat dentine were made by solid-state NMR spectroscopy under magic-angle spinning. The 31P{1H} (HETCOR) and the Lee-Goldburg homonuclear decoupling technique were combined to provide an efficient suppression of 1H-1H spin diffusion during polarization transfer. The analyses were carried out and repeated for different contact times in order to extrapolate the amount of OH- for each sample. Dentine samples of rats at different ages were studied, viz. 3 weeks, 5 months and 24 months. The OH- content of the rat dentine was found to decrease as the teeth become mature. The teeth aging also results in different cp values for HPO42- site in the biological samples. To enhance the spectral resolution, we combine the HETCOR experiment and the DQ experiment to extract the 31P homonuclear second moment of 31P signals with different proton environments. Based on the DQ results, the density of phosphate groups in the dentine was found to increase as the rat dentine become mature. (Chapter 5)