Kinetics of NMR cross-polarization from protons to phosphorus-31 in bone apatite and its mineral standards

Human bone mineral, brushite (BRU) and synthetic apatites: hydroxyapatite hydrated (HAh), hydroxyapatite calcined (HAc) and carbonatoapatite of type B with 9 wt% of CO3 2(CHA-B), were studied by variable-contact time experiments using conventional crosspolarization (CP) and CP preceded by additional proton spin lock. The CP kinetics, that is the dependence of CP intensity on the contact time, was elucidated for the single 31 P NMR peak from those materials. Proton relaxation time in the rotating frame (T1ρ H ) was additionally measured using a CP-based pulse sequence. In order to avoid an effect of magic angle spinning (MAS) on CP and relaxation, all the experiments were carried out on static samples. The CP kinetic functions for bone mineral and CHA-B were comprised of two components corresponding to different kinetic models [1]. According to the classical model, CP between the concerned spin systems is driven by the difference in their spin temperatures. The non-classical model describes CP in heteronuclear spin clusters, which interact with bulk protons by means of spin diffusion. The two CP components were deconvoluted and characterised by appropriate kinetic parameters. The fast-relaxing component is from the surface of apatite crystals and the slow-relaxing component is from the crystal interior. In the former adsorbed water is involved, while in the latter participate structural hydroxyl groups of apatite. The interpretation was supported by 2D HETCOR experiments. The CP kinetic results for HA and brushite were described by single, non-classical component. It follows that bone and CHA-B have very large crystal surface areas, so the related classical CP components can be easily observed. No HPO4 2ions were detected in the bone mineral. The study shows the similarity between carbonatoapatite of type B and bone mineral as shown by their CP behaviour [2 4]. References: 1. Kolodziejski W, Klinowski J “Kinetics of cross-polarization in solid-state NMR. A guide for chemists” Chem Rev 102 (2002) 613-628 2. A. Kaflak-Hachulska, A. Samoson, W. Kołodziejski „H MAS and H → P CP/MAS NMR Study of Human Bone Mineral” Calcified Tissue International 73 (2003) 476-486 3. W. Kolodziejski “Solid-State NMR Studies of Bone” Topics in Current Chemistry 246 (2005) 235-270 4. A. Kaflak, D. Chmielewski, A. Górecki, A. Slosarczyk, W. Kołodziejski „Efficiency of the 1 H→ 31 P cross-polarization in bone apatite and its mineral standards” Solid State Nuclear Magnetic Resonance 29 (2006) 345-348