HAP Surface Research Articles

X-ray absorption spectroscopy shining (synchrotron) light onto the insertion of Zn2+ in calcium phosphate ceramics and its influence on their behaviour under biological conditions.

The present study gives a fine description of the Zn2+ location in Zn-doped Biphasic Calcium Phosphate (BCP) samples heat treated between 500 °C and 1100 °C. Structural considerations were used to explain the sample interactions with biological fluid (DMEM). X-ray Absorption Spectroscopy (XAS) experiments were used to characterize the powdered samples. The main results (1) indicate the presence of Zn2+ complexes physisorbed at the HAp surface for a sintering temperature of 500 °C, (2) confirm the insertion of Zn2+ into the β-TCP phase using a substitution mechanism for a sintering temperature around 700 °C, and (3) fully describe the insertion of Zn2+ into the HAp phase by an interstitial mechanism for heat treatment above 900 °C (composition Ca10Znx(PO4)6(OH)2-2xO2x; xmax∼ 0.25). The formation of the linear O-Zn-O entity with dZn-O = 1.72(2) Å has been clearly evidenced by Fourier transform amplitude fitting in the R-space (Zn two-fold coordination unambiguously described for the first time). The mineralisation stimulatory effect of Zn2+ incorporated into BCP has been explained by two different mechanisms. For samples heat treated between 500 °C and 800 °C, the stimulatory effect is attributed to the presence of soluble Zn2+ species: Zn2+ physisorbed at the HAp surface for sintering treatment around 500 °C and Zn2+ incorporated into about 20 wt% (weight percent) of the soluble Zn-doped β-TCP phase for sintering treatment around 800 °C. A sintering temperature above 900 °C led to the formation of an extremely weakly soluble and well-crystallized Zn-doped HAp phase which acts by facilitating the nucleation of a calcium phosphate phase at its surface.

Fabrication and characterization of nanocomposites with high-impact polystyrene and hydroxyapatite with well-defined polystyrene via ATRP

An efficient technique for the SI-ATRP of PS at the HAP surfaces was achieved through immobilization of the macroinitiator (BIBB). FTIR, TGA, NMR, and TEM studies confirmed that the modified individual nanoparticles composed of a HAP core and well-defined, densely grafted outer PS, which demonstrated that the surface of HAP was grafted with polymer chains successfully. Then, HAP/HIPS nanocomposites with different loadings of pristine HAP and modified HAP were prepared by solution mixing. The mechanical properties of the nanocomposites increased remarkably by introducing surface-coated nanoparticles due to the strong interfacial interaction and good compatibility between the two components, which was further confirmed by the SEM images of the brittle fracture face of the nanocomposites. Compared with the corresponding HIPS samples, DSC results demonstrated that the HAP/HIPS nanocomposites exhibited higher glass transition temperature ( Tg).

Surface chemical analysis and chromatographic characterization of polyethylenimine-coated hydroxyapatite with various amount of polyethylenimine

Polyethylenimine (PEI) has been widely used as a coating material to produce stationary phase for ion-exchange chromatography of biomolecules. However, a precise study of the PEI coating fraction has been lacking, despite such quantification being very important for fundamental research as well as identifying further industrial applications.In this study, we produced four types of PEI-coated hydroxyapatite (PEI-HAp) with various fractions of PEI (0.16%, 0.5%, 1.0%, 1.5%) using a spray-drying system to evaluate correlations between coating fractions and the thermochemical or chromatographic behaviors of theses products. The thermal analyses of these matrices showed two exothermic peaks when the PEI coating fraction exceeded 1.0%. The one peak indicated a PEI decomposition peak and the other would indicate bond dissociation of PEI layers formed over the HAp surface as the PEI concentration increased. Furthermore, the chromatographic analysis for the surface chemical characteristics showed the correlation between coating fraction and the retention time of protein or nucleotide. Acidic or phosphorylated proteins were more strongly adsorbed as the PEI coating fraction increased when the initial coating fraction was low, but at fraction exceeding 0.5%, constant retention was observed. The retention time of nucleotides increased in proportion to the fraction of PEI added. The good selectivity of PEI-HAp may be attributable to multifunctional interactions of electrostatic and bare Ca sites on HAp, not just the amino sites of PEI. These precise studies of PEI coating fraction are our original novel contributions, which could be achieved by quantitative consideration using thermal analysis and chromatography.

Surface modification of hydroxyapatite for hydrogen generation

Hydrogen provides a safe and clean alternative to carbon-based fuels. Having the proper catalytic support for production of hydrogen is a valuable technology. We report on the surface modification of hydroxyapatite as a novel catalytic support material for hydrogen generation. Aside from being inexpensive and durable, we reveal that Ru ion exchange on the HAP surface provides a highly active support for sodium borohydride hydrolysis, exemplifying a high total turnover number on the order of 24,000 mol H 2/mol Ru. Moreover, we observe that the RuHAP support exhibits a long catalytic lifetime of approximately 1 month upon repeated exposure to NaBH 4 solutions. We identified the ability of complex surface morphology to enhance hydrolysis by the catalytic transition metal covered surface. Particularly, we found that the complex morphology of polycrystalline RuHAP catalytic supports exhibits shorter induction times for hydrogen generation as well as improved reaction rates as compared with single crystal supports possessing the same Ru content. By decreasing induction time and enhancing catalytic activity, we find it feasible to further explore this catalyst support in the construction of a practical hydrogen generation system.

Bone cell–material interactions on metal-ion doped polarized hydroxyapatite

The objective of this work is to study the influence of Mg 2+ and Sr 2+ dopants on in vitro bone cell–material interactions of electrically polarized hydroxyapatite [HAp, Ca 10(PO 4) 6(OH) 2] ceramics with an aim to achieve additional advantage of matching bone chemistry along with the original benefits of electrical polarization treatment relevant to biomedical applications. To achieve our research objective, commercial phase pure HAp has been doped with MgO, and SrO in single, and binary compositions. All samples have been sintered at 1200 °C for 2 h and subsequently polarized using an external d.c. field (2.0 kV/cm) at 400 °C for 1 h. Combined addition of 1 wt.% MgO/1 wt.% SrO in HAp has been most beneficial in enhancing the polarizability in which stored charge was 4.19 μC/cm 2 compared to pure HAp of 2.23 μC/cm 2. Bone cell–material interaction has been studied by culturing with human fetal osteoblast cells (hFOB) for a maximum of 7 days. Scanning electron microscope (SEM) images of cell morphology reveal that favorable surface properties and dopant chemistry lead to good cellular adherence and spreading on negatively charged surfaces of both Sr 2+ and Mg 2+ doped HAp samples over undoped HAp. MTT assay results at 7 days show the highest viable cell densities on the negatively charged surfaces of binary doped HAp samples, while positive charged doped HAp surfaces exhibit limited cellular growth in comparison to neutral surfaces.

Influence of MgO, SrO, and ZnO Dopants on Electro-Thermal Polarization Behavior and In Vitro Biological Properties of Hydroxyapatite Ceramics

The objective of this work was to investigate the influence of trace element (Mg2+, Zn2+, and Sr2+) doping on polarization behavior of sintered hydroxyapatite [HAp, Ca10(PO4)6(OH)2] pertinent to biomedical applications. For this purpose, commercially procured phase pure HAp powder was doped with MgO, SrO, and ZnO dopants in different single, binary, and ternary compositions. All samples were sintered at 1200°C for 2 h and subsequently electro-thermally polarized via application of an external dc field (2.0 kV/cm) at 400°C. Combined addition of 1 wt% MgO/1 wt% SrO in HAp was found to be the most beneficial in enhancing the polarizability (stored charge ∼4.19 μC/cm2) of pure HAp (stored charge ∼2.23 μC/cm2) by inhibiting high-temperature HAp phase decomposition. Furthermore, in vitro bone cell–material interaction has been studied for polarized binary doped (1 wt% MgO+1 wt% SrO in HAp) HAp samples by culturing with human fetal osteoblast cells for a maximum of 7 days to establish the significance of dopants on polarized HAp for bone graft applications. Scanning electron microscope images of cell morphology revealed that favorable surface properties and dopants chemistry led to good cellular adherence and extracellular matrix formation on negatively charged surfaces of binary doped HAp samples in comparison with undoped HAp surfaces. MTT assay results at 7 days showed the highest cell proliferation on negatively charged surfaces of binary doped HAp samples.

Effects of Modification of Calcium Hydroxyapatites by Trivalent Metal Ions on the Protein Adsorption Behavior

The effects of modification of calcium hydroxyapatites (Hap; Ca10(PO4)6(OH)2) by trivalent metal ions (Al(III), La(III), and Fe(III)) on protein adsorption behavior were examined using bovine serum albumin (BSA; isoelectric point (iep) = 4.7 and molecular mass (M(s)) = 67,200 Da). The Al(III)-, La(III)-, and Fe(III)-substituted Hap particles were prepared by the coprecipitation method with different atomic ratios, metal/(Ca + metal), abbreviated as X(metal). The particles precipitated at X(metal) = 0 (original-Hap) were rod-like and 10 x 36 nm2 in size. The short, rod-like original-Hap particles were elongated upon adding metal ions up to X(metal) = 0.10, and the extent of the particle growth was in the order of La(III) < Al(III) << Fe(III). The crystallinity of the materials was slightly lowered by increasing X(metal) for all systems. The adsorption isotherms of BSA onto the Al(III)-, La(III)-, and Fe(III)-substituted Hap particles showed the Langmuirian type. The saturated amounts of adsorbed BSA (n(s)(BSA)) values were strongly dependent on X(metal) in each system. The n(s)(BSA) values for the Fe(III)-substituted Hap system were increased with an increase in X(Fe) (X(metal) value of Hap particles substituted with Fe(III)); the n(s)(BSA) value obtained at X(Fe) = 0.10 was 2.7-fold more than that for the original-Hap particle, though those for the La(III) system were decreased to ca. 1/5. On the other hand, the n(s)(BSA) values for the Al(III) system were decreased with substitution of small amounts of Al(III), showing a minimum point at X(Al) = 0.01, but they were increased again at X(Al) over 0.03. Since the concentrations of hetero metal ions dissolved from the particles exhibited extremely low values, the possibility of binder effects of trivalent cations dissolved from the particle surface for adsorbing BSA to trivalent-ion-substituted Hap particles was excluded. The increase of n(s)(BSA) by an increase in X(Fe) was explained by elongation of mean particle length along with the production of surface hydroxo ions, such as Fe(OH)2+ or Fe(OH)2+, to induce the hydrogen bond between the Fe(III)-substituted Hap surface and BSA molecules, though the number of original C sites established by Ca(II) atoms was reduced. In the case of La(III)-substituted Hap particles, the number of original C sites established by Ca(II) atoms was reduced by La(III) substitution but the mean particle length remained almost constant. Furthermore, surface hydroxo La(III) groups were absent. Therefore, the reduction of n(s)(BSA) was explained by both the unaltered mean particle length and their low surface hydrophilicity. The change of n(s)(BSA) values by X(Al) resembled that of the mean particle length. These results implied that both the mean particle length and surface hydrophilicity of Al(III)-, La(III)-, and Fe(III)-substituted Hap particles are determining factors of the adsorption amounts of BSA.

Mechanics of Collagen in the Human Bone: Role of Collagen-Hydroxyapatite Interactions

AbstractHere, we report results of our simulations studies on modeling the collagen-hydroxyapatite (HAP) interface in bone and influence of these interactions on mechanical behavior of collagen through molecular dynamics and steered molecular dynamics (SMD). Models of hexagonal HAP (10-10) and (0001) surface, and collagen with and without telopeptides were built to investigate the mechanical response of collagen in the proximity of mineral. The collagen molecule was pulled normal and parallel to the (0001) surface of hydroxyapatite. Water molecules were found have an important impact on deformation behavior of collagen in the proximity of HAP due to their large interaction energy with both collagen and HAP. Collagen appears stiffer at small displacement when pulled normal to HAP surface. At large displacement, collagen pulled parallel to HAP surface is stiffer. This difference in mechanical response of collagen pulled in parallel and perpendicular direction results from a difference in deformation mechanism of collagen. Further, the collagen molecule pulled in the proximity of HAP, parallel to surface, showed marked improvement in stiffness compared to absence of HAP. Furthermore, the deformation behavior of collagen not only depends on the presence or absence of HAP and direction of pulling, but also on the type of mineral surface in the proximity. The collagen pulled parallel to (10-10) and (0001) surfaces showed characteristically different type of load-displacement response. In addition, here we also report simulations on 300 nm length of collagen molecule indicating the role of length of model on the observed response in terms of both the magnitude of modulus obtained as well as the mechanisms of response of collagen to loading.

Photografting of methacrylic acid onto hydroxyapatite particles surfaces

The unsaturated functional groups (CC) were introduced onto hydroxypatite (HAP) particles surfaces by esterification reaction between hydroxyl groups on the HAP surface and methacrylic acid (MAA). Then, graft polymerization of MAA onto the esterified HAP particles surfaces was carried out by ultraviolet light (UV) initiation. The structure of the surface-esterified and grafted HAP was studied by ESCA, FTIR, Scanning Electron microscopy, laser particle sizes analyze, esterification, and graft degrees tests. The results show that poly(methyl acrylic acid) (PMAA) is successfully inserted onto HAP particles surfaces by covalent bond linkage. The esterification degree between MAA and HAP particles increases with reaction temperature and time, and tends to level off at about 6 h under 90°C. The graft degree of MAA onto HAP particles with 1.14% esterification degree and its average particle sizes increase with UV initiation polymerization time, and to reach a maximum at 25 min under 40°C, then decrease as irradiation time further increases. The surfaces of the grafted HAP particles become rough and loose. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Effects of Heat Treatment of Calcium Hydroxyapatite Particles on the Protein Adsorption Behavior

The effects of heat treatment of calcium hydroxyapatite (Hap) on the protein adsorption behavior were examined using typical proteins of bovine serum albumin (BSA: isoelectric point (iep) = 4.7, molecular mass (Ms) = 67,200 Da, acidic protein), myoglobin (MGB: iep = 7.0, Ms = 17,800 Da, neutral protein), and lysozyme (LSZ: iep = 11.1, Ms = 14,600 Da, basic protein). The TEM, XRD, and gas adsorption measurements ascertained that all of the Hap particles examined were highly crystallized and nonporous. The Hap single phase was continued up to the heat treatment temperature of 600 degrees C. However, after treatment above 800 degrees C in air, the beta-Ca3(PO4)2 (beta-TCP) phase slightly appeared. TG and ICP-AES measurements suggested that all of the Hap particles are Ca2+-deficient. Also, it was indicated from FTIR and XPS measurements that a partially dehydrated oxyhydroxyapatite (pd-OHap) was formed after treatment at high temperature. The saturated amounts of adsorbed BSA (nsBSA) did not vary on the Hap particles after heat treatment at 200 and 400 degrees C. However, nsBSA values were increased by raising the heat treatment temperature above 600 degrees C. The adsorption coverage of BSA was increased up to ca. 1.4. This adsorption coverage of BSA (thetaBSA) over unity suggests that the BSA molecules densely adsorbed and a part of BSA molecules adsorbed as end-on type on the Hap particle surface or BSA molecules became contracted. Similar adsorption behavior was observed on the LSZ system, but the adsorption coverage of LSZ (thetaLSZ) values are much less than thetaBSA. On the other hand, no effect of the heat treatment of Hap particles was observed on the adsorption of MGB. The increases of nsBSA and nsLSZ were explained by the increase of calcium and phosphate ions in the solutions dissolved from beta-TCP formed after heat treatment of Hap, especially treated at high temperature. The dissolved Ca2+ and PO(4)3 - ions may act as binders between proteins and Hap surfaces; the adsorption of Ca2+ ions on the Hap surface offers an adsorption site for BSA owing to its positive charge. In the case of adsorption of positively charged LSZ molecules, PO43- ions act as a binder in an opposite way. Since the MGB molecules are neutral, no binding effect of either ion was observed.

Role of surface charge and wettability on early stage mineralization and bone cell–materials interactions of polarized hydroxyapatite

Our objective was to determine the role of surface charge and wettability on early stage mineralization as well as bone cell adhesion and proliferation on polarized HAp surface. To estimate the surface wettability, contact angles were measured in water, simulated body fluid (SBF) and Dulbecco’s modified Eagle’s medium/nutrient mixture F-12 Ham (DMEM). Experimental results show that HAp surface wettability and surface energy can be tailored by inducing surface charge without introducing any volumetric effects in the material. Increasing the surface charge increased the wettability and also the energy of HAp surfaces in all tested media. A maximum surface energy of 49.47 ± 3.76 mJ/m 2 was estimated for positively charged HAp surfaces polarized at 400 oC. The in vitro bioactivity of polarized HAp samples was evaluated by soaking in SBF and DMEM (cell media). Cell–materials interaction was studied by culturing with human fetal osteoblast cells (hFOB). In vitro results show that tailoring the combined effect of wettability and charge polarity on the HAp surface enable differential binding of inorganic ions (e.g., Ca 2+, Cl −, Na +, HCO 3 − etc) and organic cell adhesive proteins (e.g., fibronectin, vitronectin etc) with different surface properties, which results in accelerated or decelerated mineralization as well as cell adhesion and proliferation on polarized HAp surface.

A solubility and surface complexation study of a non-stoichiometric hydroxyapatite

The dissolution and surface complexation of a non-stoichiometric hydroxyapatite (Ca 8.4(HPO 4) 1.6(PO 4) 4.4(OH) 0.4), (HAP) was studied in the pH range 3.5–10.5, at 25 °C in 0.1 M Na(Cl). The results from well-equilibrated batch experiments, potentiometric titrations, and zeta-potential measurements were combined with information provided by Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy and X-ray Photoelectron Spectroscopy (XPS). The information from the analyses was used to design an equilibration model that takes into account dissolution, surface potential, solution and surface complexation, as well as possible phase transformations. The results from the XPS measurements clearly show that the surface of the mineral has a different composition than the bulk and that the Ca/P ratio of the surface layer is 1.4 ± 0.1. This ratio was also found in solution in the batches equilibrated at low pH where the dominating reaction is dissolution. In the batches equilibrated at near neutral pH values, however, the Ca/P ratio in solution attains values as high as 25, which is due to re-adsorption of phosphate ions to the HAP surface. The total concentration of protons as well as the total concentration of dissolved calcium and phosphate in solution were used to calculate a model for the dissolution and surface complexation of HAP. The constant capacitance model was applied in designing the following surface complexation model: CaOH + H + ⇋ CaOH 2 + log β ( intr . ) = 8.41 ± 0.16 OPO 3 H 2 ⇋ OPO 3 H - + H + log β ( intr . ) = - 1.11 ± 0.13 CaOH + HPO 4 2 - + H + ⇋ CaOPO 3 H - + H 2 O log β ( intr . ) = 11.63 ± 0.15 OPO 3 H 2 + Na + ⇋ OPO 3 Na - + 2 H + log β ( intr . ) = - 11.08 ± 0.12 In addition a solubility product with log β = −23.27 ± 0.29 was obtained: Ca 8.4 ( HPO 4 ) 1.6 ( PO 4 ) 4.4 ( OH ) 0.4 ( s ) + 4.8 H + ⇋ 8.4 Ca 2 + + 6 HPO 4 2 - + 0.4 H 2 O Furthermore, this model predicts a pH zpc = 7.9, which is in agreement with pH iep = 8.1 obtained from zeta potential measurements. This model can be used as a helpful tool to predict the reactivity of HAP in different aquatic environments.

Adsorption of Proteins Derived from Fetal Bovine Serum onto Hydroxyapatite Nanocrystals with Quartz Crystal Microbalance Technique

The adsorption of multiple proteins derived from fetal bovine serum (FBS) in phosphate buffer saline (PBS) and alpha minimum essential (aMEM) was in situ analyzed with a quartz crystal microbalance with dissipation technique on gold, titanium and HAp sensors. The adsorption behaviors of FBS proteins were varied depending on the sensors. The DD/Df value of the HAp sensor were clearly different in PBS and aMEM, and others were not changed. The viscoelastic properties of the protein films adsorbed on the HAp sensor in PBS were flexible in comparison with those on the gold and titanium sensors. The D-f plots incidated that the proteins adsorbed on HAp in PBS would lead to competitive adsorption and conformational change and those in aMEM could form a monolayer. The adsorption behavior on the HAp in carbonate buffer saline was found to be similar to that in aMEM. These differential adsorption behaviors on the HAp surface were attributed to the pre-adsorptive ion, such PO43- or CO32- in the solvent.

Fabrication of platy apatite nanocrystals loaded with TiO 2 nanoparticles by two-step emulsion method and their photocatalytic activity

Nanometer-sized TiO 2 island structure on the platy hydroxyapatite nanocrystals (HAp) has been accomplished by two-step emulsion process. At the first step, platy HAp nanocrystals, of which size was in the range of 70–200 nm after heat-treatment at 1078 K for 1 h, were prepared using the W/O emulsion system. Before the second step, HAp nanocrystals were immersed in NaH 2PO 4 solution for the formation of hydroxyl group on their surface. In the following, titanium tetraisopropoxide reacted with the hydroxyl group of HAp surface to form TiO 2 nanoparticles on the surface of HAp nanocrystals, which were dispersed in the micrometer-sized methanol droplets of polyethylene cetylether–cyclohexane mixture (methanol/oil emulsion). The resulting hydroxyapatite nanocrystals loaded with TiO 2 nanoparticles showed the high photocatalytic activity comparing to the commercial TiO 2 catalyst.

Solid‐state P‐31 NMR study of the formation of hydroxyapatite in the presence of glutaric acid

We demonstrate that glutaric acid can be used to prepare nanorods of hydroxyapatite under hydrothermal condition at 100 degrees C with a Ca(2+):glutaric acid molar ratio of 1:4. Frequency-switched Lee-Goldburg irradiation is employed to obtain high-resolution (31)P{(1)H} correlation spectra of the reaction mixture at two different reaction periods, from which it is shown that octacalcium phosphate is the precursor phase of the final hydroxyapatite product. In addition, the spectra show that a substantial amount of water molecules is trapped between the glutaric acid and the hydroxyapatite surface, indicating that water molecules may play a prominent role in the noncovalent interaction of the glutaric acid and the HAp surface.

Effects of Pyrophosphate Ions on Protein Adsorption onto Calcium Hydroxyapatite

The effects of pyrophosphate ions (PP: P2O7(4-)) on the adsorption of proteins onto calcium hydroxyapatite (Hap) were examined using typical proteins of bovine serum albumin (BSA: isoelectric point (iep) = 4.7, molecular mass (M(s)) = 67 200 Da, acidic protein), myoglobin (MGB: iep = 7.0, M(s) = 17 800 Da, neutral protein), and lysozyme (LSZ: iep = 11.1, M(s) = 14,600 Da, basic protein). The UV and CD measurements determined that both the secondary and the tertiary structures of protein molecules do not vary in the presence of PP. The adsorption of BSA was strongly depressed by the addition of PP in all the methods with changing the order of PP addition. Even if BSA was pre-adsorbed on the Hap surface, PP replaced BSA molecules by strong preferential adsorption onto Hap to reduce the amounts of adsorbed BSA. A similar effect was observed with the adsorption of MGB. On the other hand, the amount of adsorbed LSZ (n(LSZ)) was increased with an increase in the concentration of PP, and the n(LSZ) value showed a maximum point in each adsorption isotherm. This fact was explained by a compression of the electric double layer (EDL) around each LSZ molecule by PP. This compression of the EDL induced the reduction of lateral electrostatic repulsions between charged LSZ molecules on the Hap surface and enhanced the formation of closed-packed monolayers to raise the n(LSZ) value. However, since the number of PPs around a LSZ molecule is decreased by an increase in the LSZ concentration in each system, the thickness of the EDL may be increased. Hence, n(LSZ) was reduced again after the maximum point in each system. Tripolyphosphate (TPP: P3O10(5-)) ions exhibited similar effects on the adsorption behaviors of all proteins, but a much more pronounced effect was observed on the LSZ system. TPP with a higher eletronegativity shielded the EDL more highly than PP to increase the n(LSZ) value. The results of the zeta potential for all the protein systems supported the modes of protein adsorption discussed.

Thermodynamic Roles of Basic Amino Acids in Statherin Recognition of Hydroxyapatite

Salivary statherin is a highly acidic, 43 amino acid residue protein that functions as an inhibitor of primary and secondary crystallization of the biomineral hydroxyapatite. The acidic domain at the N-terminus was previously shown to be important in the binding of statherin to hydroxyapatite surfaces. This acidic segment is followed by a basic segment whose role is unclear. In this study, the role of the basic amino acids in the hydroxyapatite adsorption thermodynamics has been determined using isothermal titration calorimetry and equilibrium adsorption isotherm analysis. Single point mutations of the basic side chains to alanine lowered the binding affinity to the surface but did not perturb the maximal surface coverage and the adsorption enthalpy. The structural and dynamic properties of the single point mutants as characterized by solid-state NMR techniques were not altered either. Simultaneous replacement of all four basic amino acids with alanine lowered the adsorption equilibrium constant by 5-fold and the maximal surface coverage by nearly 2-fold. The initial exothermic phase of adsorption exhibited by native statherin is preserved in this mutant, along with the alpha-helical structure and the dynamic properties of the N-terminal domain. These results help to refine the two binding site model of statherin adsorption proposed earlier in our study of wild-type statherin (Goobes, R., Goobes, G., Campbell, C.T., and Stayton, P.S. (2006) Biochemistry 45, 5576-5586). The basic charges function to reduce protein-protein charge repulsion on the HAP surface, and in their absence, there is a considerable decrease in statherin packing density on the surface at binding saturation.

Protein adsorption characteristics of calcium hydroxyapatites modified with pyrophosphoric acids

Protein adsorption characteristics of calcium hydroxyapatite (Hap) modified with pyrophosphoric acids (PP a) were examined. The PP a modified Hap particles (abbreviated as PP-Hap) possessed anchored polyphosphate (PP: P–{O–PO(OH)} n –OH) branches on their surfaces. The proteins of bovine serum albumin (BSA: isoelectric point (iep) = 4.7, molecular mass ( M s) = 67,200 Da, acidic protein), myoglobin (MGB: iep = 7.0, M s = 17,800 Da, neutral protein), and lysozyme (LSZ: iep = 11.1, M s = 14,600 Da, basic protein) were examined. The ζ potential (zp) of PP-Hap particles as a function of pH overlapped; zp-pH curves were independent of the concentration of pyrophosphoric acids (abbreviated as [PP a]) used for modifying Hap surface. The saturated amounts of adsorbed BSA ( Δ n ads BSA ) were increased three-fold by the surface modification with PP a though they were independent of the [PP a]. Furthermore, the fraction of BSA desorption was independent of the [PP a]. This enhancement of BSA adsorption onto the PP-Hap is due to the hydrogen bonding between oxygen and OH groups of the PP-branches and functional groups of BSA molecules. In the case of LSZ, a more higher adsorption enhancement was observed; the saturated amount of adsorbed LSZ ( Δ n ads LSZ ) for Hap modified at [PP a] = 6 mmol/dm 3 was nine-fold than that for Hap unmodified. This remarkable adsorption enhancement was explained by a three-dimensional binding mechanism; LSZ molecules were trapped inside of the PP-branches. Hence, a fraction of LSZ desorption was decreased with an increase in the [PP a]; as more PP-branches are presented on the surface the higher retardation of LSZ desorption was induced. It was expected from their small size that MGB adsorb between the PP-branches as well as LSZ. However, the amounts of adsorbed MGB ( Δ n ads MGB ) did not vary and were independent of the [PP a] due to the small numbers of functional groups of MGB. In addition, no dependence of the fraction of MGB desorption on the [PP a] was observed. The results of zp for all the protein systems supported the mode of protein adsorption discussed. The anchored structure of the PP-branches developed on the Hap surface to provide three-dimensional protein adsorption spaces was proved by a comparative experiment that was elucidating the effect of pyrophosphate ions for BSA adsorption onto Hap.

Development of Ruthenium−Hydroxyapatite-Encapsulated Superparamagnetic γ-Fe2O3 Nanocrystallites as an Efficient Oxidation Catalyst by Molecular Oxygen

Magnetic γ-Fe2O3 nanocrystallites dispersed in a hydroxyapatite matrix (HAP-γ-Fe2O3) have been synthesized for the first time as a new catalyst support. The cation-exchange ability of the external HAP surface enables an equimolar substitution of Ru for Ca to form a catalytically active center (RuHAP-γ-Fe2O3). Characterization by several spectroscopic methods demonstrated the formation of γ-Fe2O3 nanocrystallites within the HAP matrix having a mean diameter of 8.0 nm (σ = 18.4 A, σ/d = 21.7%). The Ru species was proven to exist in the monomeric form in a highly dispersed manner. The RuHAP-γ-Fe2O3 exhibits superior catalytic activity for the oxidation of various alcohols to the corresponding carbonyl compounds using molecular oxygen as a primary oxidant. The magnetic properties of the RuHAP-γ-Fe2O3 provided a convenient route for separation of the catalyst from the reaction mixture by application of an external permanent magnet. No metal leaching was detected, and the spent catalyst could be recycled withou...

Mechanical improvement of hydroxyapatite by TiO x nanoparticles deposition

Deposition of Ti was carried out by laser ablation onto hydroxyapatite porous discs in an Ar atmosphere. Ti nanoparticles were deposited onto HAp surface in order to modify its roughness and morphology as it is observed by scanning electron microscopy (SEM) and scanning probe microscopy (SPM). A homogeneous distribution of Ti over the disc surface was corroborated by elemental mapping. A comparison of the hydroxyapatite hardness before and after deposition was performed using SPM nanoindentation. Transmission Electron Microscopy (TEM) showed that the Ti nanoparticles obtained were covered by an oxygen shell. It is shown that surface modifications of the covered HAp by Ti result in better mechanical properties, reducing the possible damage to the HAp ceramic by friction or impacts as it often happens in meniscus, bone junctions and the inclusion of prosthesis for human treatments.