Nanocrystals of apatitic calcium phosphate impart the organicinorganic nanocomposite in bone with favorable mechanical properties. So far, the factors preventing crystal growth beyond the favorable thickness of ca. 3 nm have not been identified. Here we show that the apatite surfaces are studded with strongly bound citrate molecules, whose signals have been identified unambiguously by multinuclear magnetic resonance (NMR) analysis. NMR reveals that bound citrate accounts for 5.5 wt% of the organic matter in bone and covers apatite at a density of about 1 molecule per ð2 nmÞ 2 , with its three carboxylate groups at distances of 0.3 to 0.45 nm from the apatite surface. Bound citrate is highly conserved, being found in fish, avian, and mammalian bone, which indicates its critical role in interfering with crystal thickening and stabilizing the apatite nanocrystals in bone.T he load-bearing material in bone is a fascinating organicinorganic nanocomposite whose stiffness is provided by thin nanocrystals of carbonated apatite, a calcium phosphate, imbedded in an organic matrix consisting mostly of collagen, a fibrous protein (1-5). The small (ca. 3-nm) thickness of the apatite nanocrystals is favorable for mechanical properties, likely preventing crack propagation (6). While the size and shape of the nanocrystals have been studied extensively (4, 5), the mechanism stabilizing them at a thickness corresponding to only about four unit cells has not been elucidated. A better understanding of the factors controlling the nanocrystals in bone is desirable for prevention and treatment of bone diseases such as osteoporosis, which causes millions of fractures each year (7), and for more efficient synthesis of biomimetic nanocomposites (8, 9). In vitro experiments have shown that carboxylate-rich proteins such as osteocalcin and osteopontin (7) can affect hydroxyapatite crystal formation and growth (10, 11). These observations might suggest that such proteins limit nanocrystal thickening (12); however, these proteins are not sufficiently abundant in vivo to bind to all the nanocrystal surfaces at high enough area concentration; possibly, they control the length of the nanocrystals (7).Here we show instead that the surfaces of the apatite crystals in bone are studded with strongly bound citrate molecules, at a density of ca. 1∕ð2 nmÞ 2 , using advanced solid-state nuclear magnetic resonance (NMR) as a unique tool for probing buried interfaces. Citrate is quite abundant in bone (ca. 1 wt%, or 5 wt% of the organic components) (13,14). Before 1975, citrate in bone was studied by simple wet-chemical methods and thought to regulate bone demineralization (14). However, citrate is no longer even mentioned in most of the prominent literature on the bone nanocomposite published during the last thirty years (1)(2)(3)(4)(5)(15)(16)(17)(18). We now highlight the importance of citrate in bone by demonstrating that it is not a dissolved calcium-solubilizing agent but a strongly bound, integral part of the nanocomposite. Structurally, citrate s...

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We demonstrate the presence of a symbiotic stability reinforcement effect between bioentities and crystalline ZIFs, where the ZIF protects biomolecules from denaturation and the biomolecules improve the acid resistance of the ZIF framework. The strategy provides a potential route for stabilizing MOFs for diverse technological and industrial applications.

Sugar molecules adsorbed at hydrated inorganic oxide surfaces occur ubiquitously in nature and in technologically important materials and processes, including marine biomineralization, cement hydration, corrosion inhibition, bioadhesion, and bone resorption. Among these examples, surprisingly diverse hydration behaviors are observed for oxides in the presence of saccharides with closely related compositions and structures. Glucose, sucrose, and maltodextrin, for example, exhibit significant differences in their adsorption selectivities and alkaline reaction properties on hydrating aluminate, silicate, and aluminosilicate surfaces that are shown to be due to the molecular architectures of the saccharides. Solid-state 1 H, 13 C, 29 Si, and 27 Al nuclear magnetic resonance (NMR) spectroscopy measurements, including at very high magnetic fields (19 T), distinguish and quantify the different molecular species, their chemical transformations, and their site-specific adsorption on different aluminate and silicate moieties. Two-dimensional NMR results establish nonselective adsorption of glucose degradation products containing carboxylic acids on both hydrated silicates and aluminates. In contrast, sucrose adsorbs intact at hydrated silicate sites and selectively at anhydrous, but not hydrated, aluminate moieties. Quantitative surface force measurements establish that sucrose adsorbs strongly as multilayers on hydrated aluminosilicate surfaces. The molecular structures and physicochemical properties of the saccharides and their degradation species correlate well with their adsorption behaviors. The results explain the dramatically different effects that small amounts of different types of sugars have on the rates at which aluminate, silicate, and aluminosilicate species hydrate, with important implications for diverse materials and applications. S accharide molecules and their interactions with inorganic oxide surfaces play crucial roles in a variety of natural and synthetic processes, including biomineralization, biomolecule synthesis, bone resorption, heterogeneous catalysis, corrosion inhibition, and cement hydration. For example, mono-and oligosaccharides are thought to control the morphologies and structures of carbonate skeletons in marine organisms through sitespecific binding to the mineral phases (1, 2). Interactions of simple organic molecules with aluminosilicate surfaces and exchangeable cations in clays have been hypothesized to be key factors in abiotic syntheses of organic molecules (3). For example, sugar-silicate complexes have been shown to stabilize the abiotic formation of biologically important sugars, such as ribose (4). Similar interactions are thought to promote the adhesion of marine organisms at hydrated inorganic surfaces (5). Biofuels can be produced when polysaccharides are converted to monosaccharides and lower molecular weight alkenes at aluminosilicate zeolite surfaces by heterogeneous reactions in the presence of water (6). Saccharides have also been found to inhibit the corrosion of me...