Study on adsorption mechanism of proteins onto synthetic calcium hydroxyapatites through ionic concentration measurements.

To clarify the adsorption mechanism of proteins onto calcium hydroxyapatite (Hap), the kinetic studies of dissolution and ion-exchange properties of synthetic Hap particles in the absence and presence of proteins were examined at 15 degrees C. In the absence of proteins, Hap particles slightly dissolved to give low amounts of calcium ([Ca(2+)] = 0.09-0.14 micromol m(-2)) and phosphate [PO(4) (3-)] = 0.01-0.08 micromol m(-2)) ions in KCl, CaCl(2), BaCl(2) and AlCl(3) solutions. The [Ca(2+)] increased with increase in the Ca/P ratio of Hap, while the [PO(4) (3-)] decreased. The[ Ca(2+)] and [ PO(4) (3-)] were independent of the ionic strength. Ba(2+) and AI(3+) ions were completely ion-exchanged with calcium ions in Hap lattice within 2 hr. The solution pH was increased by 1.1-1.8 after the dissolution of OH(-) ions on the Hap surface. In the presence of bovine serum albumin (BSA), the Hap particles dissolved slightly faster than the systems without protein. This fact was explained by a complexation of dissolved ions to functional groups of BSA. The adsorption of BSA induced a reduction of [Ca(2+)] and [ PO(4) (3-)] in the aqueous medium and minima appeared on [Ca(2+)] and [PO(4) (3-)] profiles before the BSA adsorption reached a saturation. This result suggests that the adsorption of BSA onto Hap is governed by [Ca(2+)] ions complexing to BSA molecules (binding effect) together with the operation of [Ca(2+)] ions exposing on the Hap surfaces by dissolution of OH(-) ions, so-called "C-sites". The addition of BaCl(2) and AlCl(3 )steeply increased the amounts of adsorbed BSA (n(BSA)) at the initial adsorption step by the strong binding effect of these di- and tri-valent cations between BSA and Hap. However, after eliminating these cations from the Hap surface by the ion-exchange reaction, the binding effects disappeared and n(BSA) decreased. Since the number of functional groups is small, the binding effect of the counter ions was only slightly detected for the systems with di- and trivalent ions on the adsorption systems of lysozyme (LSZ).

Preparation and microstructural studies on hydrothermally prepared hematite.

A 0.1 M FeCl3 aqueous solution was hydrothermally treated in a 1.5-dm3 stainless steel autoclave at varied temperatures (Th: 110-200 degrees C). Highly ordered cubic particles with uniform micropores 0.8 nm in diameter are produced at Th = 115 degrees C, although the cubic particles become less uniform with wide pore size distribution by raising the Th.

TPD-TG-MS study of carbonate calcium hydroxyapatite particles.

Simultaneous temperature-programmed desorption, mass spectroscopy, and thermogravimetry (TPD-MS-TG) were carried out to explore the evolution process of H2O and CO2 from calcium hydroxyapatite particles containing carbonate ions of 0.1-10.1 mass%. The TPD-MS curves of both H2O and CO2 exhibited two peaks: desorption of H2O and CO2 adsorbed on the particles below 600 degrees C, and liberation of H2O and CO2 in the crystals above 700 degrees C. The increase of carbonate ion content in the particles increased the weight loss in the TPD-TG curves.

TPD, FTIR, and Molecular Adsorption Studies of Calcium Hydroxyapatite Surface Modified with Hexanoic and Decanoic Acids.

The surface of synthetic calcium hydroxyapatite Ca10(PO4)6 (OH)2 (CaHAP) particles was modified with hexanoic CH3(CH2)4 COOH (HA) and decanoic CH3(CH2)8COOH (DA) acids in hexane and the resulting materials were characterized by various means. XRD pattern, specific surface area, and morphology of the CaHAP particles were essentially not altered by the modification. FTIR and TPD results indicated that HA and DA molecules are hydrogen-bonded to the surface P-OH groups of CaHAP in three adsorption types. The surface of modified particles became hydrophobic. When the modified samples were outgassed at 500 degrees C, HA and DA were removed and the surface P-OH groups were revived. The modified material adsorbed much less H2O and CO2 than the unmodified one. Copyright 1999 Academic Press.

Preparation and Characterization of Carbonated Barium Hydroxyapatites.

Carbonated barium hydroxyapatite (Ba10(PO4)6(OH)2-2x(CO3)x, X = 0.30-0.57, BaHAP) particles with different Ba/P molar ratios were prepared by a wet method. CO2-3 ions were incorporated into OH- sites of a BaHAP lattice during the preparation at high solution pH. The obtained BaHAP particles were well crystallized and showed a high thermostability. On elevating the mixing temperature of H3PO4 and Ba(OH)2 solutions, the mean particle size of BaHAP particles decreased and their specific surface area increased. The amount of CO2 adsorbed irreversibly on BaHAP particles increased with an increase of their Ba/P molar ratio. Copyright 1999 Academic Press.

Adsorption of Bovine Serum Albumin and Lysozyme on Hydrophobic Calcium Hydroxyapatites.

The adsorption of bovine serum albumin (BSA) and lysozyme (LSZ) to oleyl phosphate(OP)-grafted calcium hydroxyapatite (OP-CaHAP) with different degrees of hydrophobicity, ranging the number of surface oleyl group per unit nm2 (nO) from 0 to 2.60, was investigated. The pronounced effects of the hydrophobic moiety of adsorbent on protein adsorption were observed. The saturated amount of adsorbed BSA (ns) was increased up to nO = 0.6 by an enlargement of hydrophobic interaction between hydrophobic CaHAP particle and proteins. However, ns decreased at nO >/= 1.3 by increasing the electrostatic repulsive force between negatively charged BSA and OP-CaHAP particles. On the other hand, the ns value of LSZ was continuously increased up to nO = 2.0 and saturated by increasing either the hydrophobic interaction or the electrostatic attraction of positively charged LSZ and negatively charged OP-grafted CaHAPs. The BSA adsorption experiment revealed that the effect of positively charged adsorption sites on the exposed ac or bc crystal faces (C-sites) of the CaHAPs is screened by the OP-groups grafted on their particle surfaces. Copyright 1999 Academic Press.

Surface Structure and Properties of Calcium Hydroxyapatite Modified by Hexamethyldisilazane.

The surface of synthetic calcium hydroxyapatite Ca10(PO4)6(OH)2 (CaHAP) particles was treated by repeated modification with hexamethyldisilazane [(CH3)3Si]2NH (HMDS) in hexane and thermal treatment and the surface of the modified CaHAP was characterized by various means. No remarkable change in XRD patterns or in particle shape by the modification was observed. The width of the CaHAP particles gradually increased with repeating the modification. FTIR results indicated that HMDS reacted with surface P-OH groups of CaHAP to yield surface Si-(CH3)3 groups. The surface of the modified CaHAP was hydrophobic. The surface Si-(CH3)3 groups turned to three kinds of surface Si-OH groups by treating the modified materials at 500 degreesC in air. These formed surface Si-OH groups and the remaining surface P-OH groups reacted with HMDS by repeating the modification, resulted in the increase of the surface Si atoms. The modified material having surface Si-(CH3)3 or Si-OH groups adsorbed much less CO2 than the unmodified one. Copyright 1998 Academic Press.

Preparation and Characterization of Barium-Strontium Hydroxyapatites

Barium-strontium hydroxyapatite solid solutions with different molar ratio Ba/(Ba + Sr) were synthesized by a wet method and characterized by various means. The solid solution particles could be prepared at molar ratios ranging from 0 to 1; however, Ba2+ ions were more difficult to be incorporated into hydroxyapatite crystals compared to Sr2+ ions. With increasing Ba2+ content, the particles grew and finally turned into pure rod-shaped barium hydroxyapatite particles with a size of ca. 0.2 x 2 &mgr;m. The resulting particles were agglomerates consisted of primary fine particles except for strontium hydroxyapatite. The molar ratios (Ba + Sr)/P of all the particles were larger than the stoichiometric ratio of 1.67, suggesting that CO32- ions, OH- ions, and/or H2O molecules substitute for PO43- ions in the crystal lattices. The amount of CO2 adsorbed irreversibly on the particles increased with increasing (Ba + Sr)/P except for strontium hydroxyapatite and fitted a curve with a minimum at a cation/P ratio of ca. 1.56 as well as other HAPs.