Experimental evidence and structural modeling of nonstoichiometric (010) surfaces coexisting in hydroxyapatite nano-crystals.

High-resolution transmission electron microscopy (HRTEM) and ab initio quantum-mechanical calculations of electronic structure were combined to investigate the structure of the hydroxyapatite (HA) (010) surface, which plays an important role in HA interactions with biological media. HA was synthesized by in vitro precipitation at 37°C. HRTEM images revealed thin elongated rod nanoparticles with preferential growth along the [001] direction and terminations parallel to the (010) plane. The focal series reconstruction (FSR) technique was applied to develop an atomic-scale structural model of the high-resolution images. The HRTEM simulations identified the coexistence of two structurally distinct terminations for (010) surfaces: a rather flat Ca(II)-terminated surface and a zig-zag structure with open OH channels. Density functional theory (DFT) was applied in a periodic slab plane-wave pseudopotential approach to refine details of atomic coordination and bond lengths of Ca(I) and Ca(II) sites in hydrated HA (010) surfaces, starting from the HRTEM model.

Theoretical and experimental studies of substitution of cadmium into hydroxyapatite.

Substitution of cadmium into bulk hydroxyapatite Ca((10-x))Cd(x)(PO(4))(6)(OH)(2) (CdHA: x = 0.12, 1.3, 2.5) is studied by combining X-ray diffraction data from synchrotron radiation, Fourier transform infra-red spectroscopy (FTIR) and density functional theory (DFT) calculations. Energetic and electronic analyses are carried out for several configurations of Cd substitution for Ca at both cationic sites. Rietveld analysis shows preferential occupation of the Ca2 site by cadmium. FTIR data suggest a non-negligible covalent character of Cd-OH. The much-discussed cation site preference for substitution is determined on the basis of relaxed-lattice energetics, and interpreted in terms of chemical concepts; theory indicates that the Ca2 site is clearly favored and this preference is related to the more covalent character of this site compared to that of site 1.

Mechanism of Zn stabilization in hydroxyapatite and hydrated (0 0 1) surfaces of hydroxyapatite.

A basic understanding of Zn incorporation on bulk and hydrated (0 0 1) surfaces of hydroxyapatite (HA) is attained through electronic structure calculations which use a combined first principles density functional (DFT) and extended Hückel tight binding (EHTB) methodology. A Zn substituted hydroxyapatite relaxed structure is obtained through a periodic cell DFT geometry optimization method. Electronic structure properties are calculated by using both cluster DFT and periodic cell EHTB methods. Bond order calculations show that Zn preference for the Ca2 vacancy, near the OH channel and with greater structural flexibility, is associated with the formation of a four-fold (bulk) and nearly four-fold (surface) coordination, as in ZnO. When occupying the octahedral Ca1 vacancy, Zn remains six-fold in the bulk, but coordination decreases to five-fold in the surface. In the bulk and surface, Zn2 is found to be more covalent than Zn1, due to a decrease in bond lengths at the four-fold site, which approach the 1.99 Å ZnO value. Zn is however considerably less bound in the biomaterial than in the oxide, where calculated bond orders are twice as large as in HA. Surface phosphate groups (PO(4)) and hydroxide ions behave as compact individual units as in the bulk; no evidence is found for the presence of HPO(4). Ca-O bond orders decrease at the surface, with a consequent increase in ionicity. Comparison between DFT and EHTB results show that the latter method gives a good qualitative account of charge and bonding in these systems.

Radial-template approach for accurate density representation in computational quantum theory.

A method for improved representation of electronic charge and spin densities for molecular and solid state systems is presented, based upon extensions of least squares fits to quantum mechanical "true" densities using basis functions of limited support. Attention is given to optimization of radial degrees of fit freedom, and the design of fit functions permitting rapid analytic manipulation and calculation of properties, such as Coulomb potentials. The method is demonstrated for covalent CO and for a large metal-organic crystalline structure.

The electronic properties of an oxygen vacancy at ZrO(2)-terminated (001) surfaces of a cubic PbZrO(3): computer simulations from the first principles.

Combining B3PW hybrid exchange-correlation functional within the density functional theory (DFT) and a supercell model, we calculated from the first principles the electronic structure of both ideal PbZrO(3) (001) surface (with ZrO(2)- and PbO-terminations) and a neutral oxygen vacancy also called the F center. The atomic relaxation and electronic density redistributions are discussed. Thermodynamic analysis of pure surfaces indicates that ZrO(2) termination is energetically more favorable than PbO-termination. The O vacancy on the ZrO(2)-surface attracts approximately 0.3 e (0.7 e in the bulk PbZrO(3)), while the remaining electron density from the missing O(2-) ion is localized mostly on atoms nearest to a vacancy. The calculated defect formation energy is smaller than in the bulk which should lead to the vacancy segregation to the surface. Unlike Ti-based perovskites, the vacancy-induced (deep) energy level lies in PbZrO(3) in the middle of the band gap.

First-principles thermodynamics of coherent interfaces in samarium-doped ceria nanoscale superlattices.

Nanoscale superlattices of samarium-doped ceria layers with varying doping levels have been recently proposed as a novel fuel cell electrolyte. We calculate the equilibrium composition profile across the coherent {100} interfaces present in this system using lattice-gas Monte Carlo simulations with long-range interactions determined from electrostatics and short-range interactions obtained from ab initio calculations. These simulations reveal the formation of a diffuse, nonmonotonic, and surprisingly wide (11 nm at 400 K) interface composition profile, despite the absence of space charge regions.

Osteoblast proliferation on hydroxyapatite thin coatings produced by right angle magnetron sputtering.

Right angle magnetron sputtering (RAMS) was used to produce hydroxyapatite (HA) film coatings on pure titanium substrates and oriented silicon wafer (Si(0 0 1)) substrates with flat surfaces as well as engineered surfaces having different forms. Analyses using synchrotron XRD, AFM, XPS, FTIR and SEM with EDS showed that as-sputtered thin coatings consist of highly crystalline hydroxyapatite. The HA coatings induced calcium phosphate precipitation when immersed in simulated body fluid, suggesting in vivo bioactive behavior. In vitro experiments, using murine osteoblasts, showed that cells rapidly adhere, spread and proliferate over the thin coating surface, while simultaneously generating strong in-plane stresses, as observed on SEM images. Human osteoblasts were seeded at a density of 2500 cells cm(-2) on silicon and titanium HA coated substrates by RAMS. Uncoated glass was used as a reference substrate for further counting of cells. The highest proliferation of human osteoblasts was achieved on HA RAMS-coated titanium substrates. These experiments demonstrate that RAMS is a promising coating technique for biomedical applications.

A theoretical and experimental study of lead substitution in calcium hydroxyapatite.

Characterization of lead substitution for calcium in hydroxyapatite (CaHA) is carried out, using experimental techniques and Density Functional theoretical (DFT) analyses. Theoretical modeling is used to obtain information of the Pb chemical environment for occupancy at either Ca(I) or Ca(II) sites of CaHA. Effects of the larger ionic radius of Pb(+2) compared to Ca(+2) are apparent in embedded cluster calculations of local chemical bonding properties. DFT periodic planewave pseudopotential studies are used to provide first-principles predictions of local structural relaxation and site preference for Pb(x)Ca(10-x)HA over the composition range x< or = 6. General characteristics of the polycrystalline material are verified by X-ray diffraction and FTIR analysis, showing the presence of a single phase of CaHA structure. A short range structure around lead is proposed in order to interpret the Pb L-edge EXAFS spectrum of the solid solution Ca(6.6)Pb(3.4)HA. In this concentration we observe that lead mainly occupies Ca(II) sites; the EXAFS fit slightly favors Pb clustering, while theory indicates the importance of Pb-Pb avoidance on site (II).

Direct observation of charge transfer at a MgO(111) surface.

Transmission electron diffraction (TED) combined with direct methods have been used to study the sqrt[3]xsqrt[3]R30 degrees reconstruction on the polar (111) surface of MgO and refine the valence charge distribution. The surface is nonstoichiometric and is terminated by a single magnesium atom. A charge-compensating electron hole is localized in the next oxygen layer and there is a nominal charge transfer from the oxygen atoms to the top magnesium atom. The partial charges that we obtain for the surface atoms are in reasonable agreement with empirical bond-valence estimations.

Preparation and structure of (Cu(pc))(3)(ReO(4))(2) and theoretical investigation of Cu(pc)(ReO(4)), Cu(pc)(ReO(4))(2), and (Cu(pc))(3)(ReO(4))(2).

The compound (Cu(pc))(3)(ReO(4))(2) (pc = phthalocyaninato) has been prepared by electrocrystallization from a p-dichlorobenzene solution of Cu(pc) and [N(n-Bu)(4)][ReO(4)]. (Cu(pc))(3)(ReO(4))(2) crystallizes with one formula unit in the triclinic space group P1 with cell constants at T = 153 K of a = 12.6202(6) A, b = 13.7596(7) A, c = 14.0294(7) A, alpha = 64.6376(9) degrees, beta = 64.2570(9) degrees, and gamma = 66.0549(9) degrees. The molecular structure comprises a stack of three Cu(pc) rings; the Cu center in each peripheral ring has an attached ReO(4) ligand, the Cu-O distance being 2.374(5) A. The inter-ring distance is 3.1595(11) A. Density functional theory calculations for Cu(pc)(ReO(4)) and Cu(pc)(ReO(4))(2) suggest that the ring-pi states are the source of the electron donation to the ligands. Calculations for (Cu(pc))(3)(ReO(4))(2) suggest that the charges on the central and peripheral rings are nearly equal. For all three compounds the basic spin distributions are similar to each other and to those of other Cu(pc) systems. For (Cu(pc))(3)(ReO(4))(2) a localized, rather than an itinerant, description of the magnetism is indicated.

Optical, magnetic, and electronic properties of peripherally fused macrocycles: molybdocene porphyrazines.

Metal-free and copper porphyrazines, [H(2)pz] and [Cu pz], have been fused at the periphery with molybdocene dithiolene, [Cp(2)Mo]. The optical, magnetic, and electronic properties of the resulting neutral and cationic complexes are studied, using first-principles density functional theory implemented by the discrete variational method. Analysis of the charge and spin distribution shows that the porphyrazine core is strongly coupled with the peripheral complex. The calculated optical absorption is found to be in reasonable agreement with experimental spectra, lending support to our theoretical model. Under appropriate circumstances one observes interaction of unpaired spins localized in the vicinity of both metal sites. The calculated spin distribution shows that [Cp(2)Mo][Cu pz] and [Cp(2)Mo][H(2)pz](+) have a magnetic moment of 1 micro(B) while [Cp(2)Mo][Cu pz](+) and [Cp(2)Mo][H(2)pz] have no moment, in good agreement with the results of X-band EPR spectra. The Cu-Mo magnetic interaction is antiferromagnetic, being mediated by pyrrol nitrogens, meso nitrogens, carbons, and sulfurs.

Sorbitol permease: an apical membrane transporter in cultured renal papillary epithelial cells.

The efflux of sorbitol from the rabbit papillary epithelial cell line PAP-HT25 occurs through a specific transport pathway, which we denote the "sorbitol permease." The permease was studied by measuring cell volume changes that accompanied osmotic swelling and by determination of the sorbitol efflux from plasma membrane vesicles. The cell volume studies showed that sorbitol efflux in response to hypotonicity occurred only across the apical membrane of the cells and that loss of sorbitol was the primary mechanism for regulatory volume decrease (RVD) by these cells. Quinidine, a permeant inhibitor of the sorbitol permease, was shown to prevent RVD when added to either apical or basolateral bathing solution. Cell volume experiments also showed that the permease was present only on the apical membrane of cells that had been grown in isotonic medium and did not accumulate sorbitol. The permease could be demonstrated in membrane vesicles obtained from cells exposed to a hypotonic environment before being homogenized. Quinidine blocked the sorbitol efflux from vesicles indicating that it either directly inhibited the permease or a membrane-associated activation step.

Laboratory investigations on rotavirus in infantile gastroenteritis in Jamaica.

Between March 1982 and March 1983 rotaviruses were detected in faecal specimens from 193 (19%) of 1020 Jamaican children under 5 years old with acute gastroenteritis. The excretion of rotaviruses in a control group of non-diarrhoeal children was only 1.4%. Rotaviruses were detected more frequently in the cooler months of the year; the prevalence ranged from 1.7% in August to 46% in January. In a comparison of the ELISA technique and electron microscopy for investigating rotavirus infections, the ELISA was demonstrated to be more sensitive especially for weakly positive cases. Because of its relative simplicity, low cost and rapidity the ELISA appears to be more suitable for use in developing countries.

Effects of dolichol monophosphate on galactose incorporation into glycoconjugates of cell cultures.

Cultured-cell homogenates catalysed the incorporation of galactose from UDP-galactose into protein and sphingolipid acceptors. Dolichol monophosphate stimulated the incorporation of galactose into glycoproteins, but it did not affect the rate of glycosylation of either exogenous or endogenous glycosphingolipids. It is proposed that, under certain conditions, galactose may be incorporated into glycoproteins via polyisoprenol intermediates, as is the case with N-acetylglucosamine and mannose.

The effects of detergents and phospholipids on a glycolipid galactosyltransferase.

1. Phospholipids activated the enzyme, lactosylceramide: UDP-galactose alpha-galactosyltransferase in hamster cells (NIL 2E clone 8) when assayed in the presence of neutral detergents. 2. Phosphatidylserine and phosphatidylinositol were the most effective phospholipids in activating the enzyme. Other phospholipids were also effective, but sphingomyelin and lysophosphatidylcholine were inhibitory. 3. Considerable enzyme activity was obtained in the absence of any detergent. Most of this activity was due to glycosylation of endogenous acceptors. 4. There was a complex effect of detergents on the enzyme activity. Very low concentrations were sharply inhibitory, but higher concentrations, above the critical micelle concentration for detergent, caused regeneration of activity. 5. The phospholipids, in the absence of a detergent, are required to maintain the lipid substrate, lactosylceramide, in a suitable dispersion where it can be acted upon by the enzyme. In the presence of detergents, it is proposed that the phospholipids also act by affecting the state of the lipid substrate.