Wilson Erin E, Awonusi Ayorinde, Morris Michael D, Kohn David H, Tecklenburg Mary Mj, Beck Larry W
Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109, USA.
J Bone Miner Res. 2005 Apr;20(4):625-34. doi: 10.1359/JBMR.041217. Epub 2004 Dec 13.
NMR was used to study the nanostructure of bone tissue. Distance measurements show that the first water layer at the surface of the mineral in cortical bone is structured. This water may serve to couple the mineral to the organic matrix and may play a role in deformation.
The unique mechanical characteristics of bone tissue have not yet been satisfactorily connected to the exact molecular architecture of this complex composite material. Recently developed solid-state nuclear magnetic resonance (NMR) techniques are applied here to the mineral component to provide new structural distance constraints at the subnanometer scale.
NMR dipolar couplings between structural protons (OH(-) and H(2)O) and phosphorus (PO(4)) or carbon (CO(3)) were measured using the 2D Lee-Goldburg Cross-Polarization under Magic-Angle Spinning (2D LG-CPMAS) pulse sequence, which simultaneously suppresses the much stronger proton-proton dipolar interactions. The NMR dipolar couplings measured provide accurate distances between atoms, e.g., OH and PO(4) in apatites. Excised and powdered femoral cortical bone was used for these experiments. Synthetic carbonate ( approximately 2-4 wt%)-substituted hydroxyapatite was also studied for structural comparison.
In synthetic apatite, the hydroxide ions are strongly hydrogen bonded to adjacent carbonate or phosphate ions, with hydrogen bond (O-H) distances of approximately 1.96 A observed. The bone tissue sample, in contrast, shows little evidence of ordered hydroxide. Instead, a very ordered (structural) layer of water molecules is identified, which hydrates the small bioapatite crystallites through very close arrangements. Water protons are approximately 2.3-2.55 A from surface phosphorus atoms.
In synthetic carbonated apatite, strong hydrogen bonds were observed between the hydroxide ions and structural phosphate and carbonate units in the apatite crystal lattice. These hydrogen bonding interactions may contribute to the long-range stability of this mineral structure. The biological apatite in cortical bone tissue shows evidence of hydrogen bonding with an ordered surface water layer at the faces of the mineral particles. This structural water layer has been inferred, but direct spectroscopic evidence of this interstitial water is given here. An ordered structural water layer sandwiched between the mineral and the organic collagen fibers may affect the biomechanical properties of this complex composite material.
核磁共振（NMR）用于研究骨组织的纳米结构。距离测量表明，皮质骨矿物质表面的第一层水是有结构的。这种水可能有助于将矿物质与有机基质连接起来，并可能在变形过程中发挥作用。
骨组织独特的力学特性尚未与这种复杂复合材料的确切分子结构令人满意地联系起来。最近开发的固态核磁共振（NMR）技术在此应用于矿物质成分，以提供亚纳米尺度上新的结构距离限制。
使用二维李-戈德堡交叉极化在魔角旋转（2D LG-CPMAS）脉冲序列下测量结构质子（OH⁻和H₂O）与磷（PO₄）或碳（CO₃）之间的NMR偶极耦合，该序列同时抑制了强得多的质子-质子偶极相互作用。所测量的NMR偶极耦合提供了原子之间的精确距离，例如磷灰石中OH和PO₄之间的距离。切除并磨成粉末的股骨皮质骨用于这些实验。还研究了合成的碳酸盐（约2 - 4 wt%）取代的羟基磷灰石以进行结构比较。
在合成磷灰石中，氢氧根离子与相邻的碳酸盐或磷酸根离子形成强氢键，观察到氢键（O - H）距离约为1.96 Å。相比之下，骨组织样本几乎没有有序氢氧根的证据。相反，鉴定出一层非常有序（有结构）的水分子层，它通过非常紧密的排列使小的生物磷灰石微晶水合。水质子距离表面磷原子约2.3 - 2.55 Å。
在合成碳酸磷灰石中，在磷灰石晶格中的氢氧根离子与结构磷酸根和碳酸根单元之间观察到强氢键。这些氢键相互作用可能有助于这种矿物质结构的长期稳定性。皮质骨组织中的生物磷灰石显示出在矿物质颗粒表面与有序表面水层形成氢键的证据。这种结构水层已被推断出来，但这里给出了这种间隙水的直接光谱证据。夹在矿物质和有机胶原纤维之间的有序结构水层可能会影响这种复杂复合材料的生物力学性能。
