Mechanisms of hydroxyapatite formation on porous gel-silica substrates

The variables affecting the nucleation and crystallization of biological hydroxy carbonate apatite (HCA) on porous gel-silica substrates are examined. Texture is the critical variable with the rate of HCA formation increasing as pore size and pore volume increase, with pore sizes >2 nm required to achieve rapid kinetics (4–6 days) of fully crystallized HCA. The concentration of silanol groups on the silica surface does not control the rate of HCA formation although metastable surface defects, such as trisiloxane rings, may be involved in HCA nucleation.     

Phospholipid bilayer formation on hydroxyapatite sol-gel synthesized films

Lipid bilayers supported by porous biomaterials are being explored as models for cell membranes. Hydroxyapatite is a relevant material currently being used extensively for biomedical applications. In this study, hydroxyapatite films produced via a sol-gel chemistry route have been characterized and explored as a scaffolding material for lipid membranes. The hydroxyapatite has been characterized using XRD, SEM, and AFM, followed by vesicle-fusion of lipids characterized by fluorescence microscopy and fluorescence recovery after photobleaching (FRAP) to determine the diffusion coefficient of this system. The HA films produced in this work were found to produce slow lateral diffusion and, in the two-phase lipid systems, some domains were observed. The low lateral diffusion coefficients were believed to be a result of the large undulations present on the hydroxyapatite film surface.     

Organocatalytic asymmetric fulvene formation

The first catalytic asymmetric formation of a fulvene from a racemic α,α-disubstituted aldehyde has been developed. Dynamic kinetic resolution of rac-2-phenylpropanal by a simple, modularly designed supramolecular organocatalyst consisting of inexpensive components has been established, producing the desired fulvene in good yields and moderate enantioselectivities. Herein we propose a mechanistic interpretation of the observed solvent- and additive-induced stereoselectivity reversal.     

Palladium-catalyzed asymmetric quaternary stereocenter formation

An efficient palladium catalyst is presented for the formation of benzylic quaternary stereocenters by conjugate addition of arylboronic acids to a variety of β,β-disubstituted carbocyclic, heterocyclic, and acyclic enones. The catalyst is readily prepared from PdCl(2), PhBOX, and AgSbF(6), and provides products in up to 99% enantiomeric excess, with good yields. Based on this strategy, (-)-α-cuparenone has been prepared in only two steps.     

Catalytic asymmetric formation of beta-sultams

beta-Sultams, highly strained sulfonyl analogues of beta-lactams, were prepared enantio- and diastereoselectively by tertiary amine catalyzed [2 + 2] cycloaddition reactions. The title compounds are practical precursors of highly enantioenriched biologically interesting beta-aminosulfonyl derivatives.     

Organocatalytic asymmetric allylic carbon-carbon bond formation

Organocatalytic allylic C-C bond-forming addition of activated alkylidenes to alkyl and aryl nitroalkenes has been achieved with high diastereo- and enantioselectivity. Chiral tertiary amine catalysts are used to give allyl intermediates which exhibit gamma-selectivity in the C-C bond forming step. The reactions proceed with up to >99:1 syn:anti ratio for both the alkyl- and aryl nitroalkenes with up 96% and 98% ee, respectively. The products of this conjugate addition are transformed into a range of intermediates, such as optically active conjugated dienes and 1-substituted tetralones, which are difficult to access via alternative methods.     

Effect of Alizarin Red S on the formation of hydroxyapatite crystals

Hydroxyapatite samples were prepared in systems calcium nitrate-diammonium hydrogen phosphate-ammonium hydroxide-organic component with addition of microamounts of Alizarin Red S. The participation of sulfo and hydroxy groups of the organic compounds in the formation of hydroxyapatite crystals was confirmed by X-ray phase analysis, IR spectroscopy, electron microscopy, and elemental analysis.     

2-Chlorotetrafluoroethanesulfinamide induced asymmetric vinylogous Mannich reaction

The generality of (S)-2-chlorotetrafluoroethanesulfinamide (CTFSA) induced asymmetric vinylogous Mannich (AVM) addition reaction has been investigated. The reaction of aldimines derived from (S)-CTFSA with 2-(tert-butyldimethylsilyloxy) furan (TBSOF) took place regioselectively at the γ-site at room temperature in DMSO under air atmosphere to give the desired addition products in syn-configuration with high diastereoselectivities (up to 98:2 dr). However, anti-configuration product as major isomer was obtained in the presence of tetra-butyl ammonium fluoride (TBAF) at −78 °C. Facile removal of the auxiliary group without epimerization demonstrated their synthetic potential for piperidone derivatives.     

Electrokinetics induced asymmetric transport in polymeric nanonozzles

The asymmetric geometry of polymeric nanonozzles provides two different transport directions: a converging direction (from the large opening to the small opening) and a diverging direction (from the small opening to the large opening). Asymmetric transport was observed in such nanochannels for both rigid polystyrene nanoparticles and flexible DNA molecules under a DC electric bias. Small, hard nanoparticles migrate easily in the diverging direction and tend to pack inside the nanochannel in the converging direction. In contrast, large, flexible DNA molecules transport better in the converging direction than in the diverging direction. A high electric field and a high velocity gradient along the tapered region produce different geometric constrictions and vortex-like particle motions for rigid nanoparticles, and also generate various coil-stretching dynamics for DNA molecules. Such nanonozzle arrays are useful in high flux and high sieving efficiency devices for biomolecule delivery or separation, and for loading trace amounts of drugs or genes for controlled drug and gene delivery.     

Alkaloid induced asymmetric electrocarboxylation of 4-methylpropiophenone

The alkaloid-induced electrocarboxylation of 4-methylpropiophenone is examined in mild conditions. Comparative studies with several inductors indicate that the efficient enantiodiscrimination of the electrocarboxylation depends on the nucleophilic quinuclidine nitrogen atom and the OH group of the inductors.     

Defect formation in LT MBE InGaAs and GaAs

Results are obtained from the experimental studies (atomic force and transmission microscopy) of the influence of III/V flux ratio on the topography and internal structure of LT MBE films of InGaAs and GaAs. The layers are shown to contain both the surface growth defects — the volcano-like pits and microdrops of III group elements, and the bulk defects — dislocations, stacking faults, microtwins, and phase microheterogeneities. A high-temperature annealing results in the additional formation of thermal etching pits at the surface and the nanosized arsenic clusters in the bulk. The origin of the defects is discussed.     

Organic thin film formation using asymmetric surface-active viologens

In the present study, asymmetric surface-active viologens were applied successfully for the thin film formation of organic pigments using the immersion plating technique. The influences of the hydrophobicity of the surfactants and the pH of the plating solution on the film formation were investigated. In addition, the interfacial chemistry and electrochemistry of the surfactants were studied, and the mechanism of the film formation has been proposed and discussed.     

Modeling of asymmetric membrane formation by dry-casting method

Many polymeric membranes are produced by phase inversion technique invented by Loeb and Sourirajan in 1962. The dry-casting method is one of the major phase inversion techniques in which a homogeneous polymer solution consisting of solvent(s) and nonsolvent(s) is cast on a support and then evaporation of the casting solution takes place under convective conditions. In this paper, we model membrane formation by the dry-casting method. The model takes into account film shrinkage, evaporative cooling, coupled heat, and mass transfer and incorporates practical and reliable diffusion theory as well as complex boundary conditions especially at the polymer solution/air interface. The predictions from the model provide composition paths, temperature, and thickness of the solution. By plotting the composition paths on the ternary phase diagram, we ascertain the general structural characteristics of the membranes prepared from particular casting conditions. The predictive ability of the model was evaluated by comparing the results with the experimental data obtained from gravimetric measurements for cellulose acetate (CA)–acetone–water system. In an attempt to illustrate the importance of diffusion formalism on the predictions, recently proposed multicomponent diffusion theory and its simplified forms were utilized in the model. The computational results show that the critical factor for capturing the accurate behavior of membrane formation is the diffusion formalism utilized in the model.     

The influence of surfactant mixing ratio on nano-emulsion formation by the pit method

The formation of O/W nano-emulsions by the PIT emulsification method in water/mixed nonionic surfactant/oil systems has been studied. The hydrophilic-lipophilic properties of the surfactant were varied by mixing polyoxyethylene 4-lauryl ether (C12E4) and polyoxyethylene 6-lauryl ether (C12E6). Emulsification was performed in samples with constant oil concentration (20 wt%) by fast cooling from the corresponding HLB temperature to 25 degrees C. Nano-emulsions with droplet radius 60-70 nm and 25-30 nm were obtained at total surfactant concentrations of 4 and 8 wt%, respectively. Moreover, droplet size remained practically unchanged, independent of the surfactant mixing ratio, X(C12E6). At 4 wt% surfactant concentration, the polydispersity and instability of nano-emulsions increased with the increase in X(C12E6). However, at 8 wt% surfactant concentration, nano-emulsions with low polydispersity and high stability were obtained in a wide range of surfactant mixing ratios. Phase behavior studies showed that at 4 wt% surfactant concentration, three-liquid phases (W+D+O) coexist at the starting emulsification temperature. Furthermore, the excess oil phase with respect to the microemulsion D-phase increases with the increase in X(C12E6), which could explain the increase in instability. At 8 wt% surfactant concentration, a microemulsion D-phase is present when emulsification starts. The low droplet size and polydispersity and higher stability of these nano-emulsions have been attributed, in addition to the increase in the surface or interfacial activity, to the spontaneous emulsification produced in the microemulsion D-phase.     

In situ biomimetic formation of nano‐hydroxyapatite crystals on chitosan microspheres

Chitosan (CS) is a biocompatible, noncytotoxic biomaterial used before as base material for composites. On the other hand, nano‐hydroxyapatite (nHA) is one of the main components of human bones, highly used for biomedical applications. In this work, CS microspheres were produced under a W/O emulsion system. CS microspheres with calcium ions were then exposed to Na₃PO₄ solution. In situ biomimetic nHA crystals were formed on CS microspheres to generate 15.14 ± 3.15‐μm composite microspheres. The microspheres were subsequently seeded with MG63 osteoblasts to observe their cell responses. All microspheres were characterized via scanning electron microscopy (SEM), phase‐contrast photomicroscopy, and X‐ray diffraction (XRD) analysis. The results showed flake‐like shape and islet‐like growth of nHA depositions presented on the surface of the CS microspheres. In vitro tests indicated that the CS/nHA microparticles were not only biocompatible but also enhanced cell adhesion and elongation due to the in situ biomimetic synthesis method.     

Acyclic asymmetric induction with carboncarbon bond formation

The acyclic asymmetric induction is described which involves heteroconjugate addition of MeLi to α-hydroxy hetero-olefins and concomitant intramolecular silicon-migration to produce α,α-sulfonyl-dianion of the $\text{threo}$ adduct being of utility for the second CC bond formation in one pot. It also contains the first chemical proof of the regiochemical assignment to the hetero-olefins.     

Mineral formation on dentin induced by nano-bioactive glass

The object of this study was to evaluate the effect of bioactive glass (BG) size on mineral formation on dentin surfaces. Totally demineralized dentin discs were treated using BG suspensions with different particle sizes:i.e., microscale bioactive glass (m-BG), submicroscale bioactive glass (sm-BG) and nanoscale bioactive glass (n-BG). Field-emission scanning electron microscopy and 3D profile measurement laser microscopy were used to observe the surface morphology and roughness. It was found that all BG particles could promoted mineral formation on dentin surfaces, while plug-like depositions were observed on the dentin discs treated by n-BG and they were more acid-resistant. The present results may imply that n-BG has potential clinical application for dentin hypersensitivity treatment.     

Spontaneous formation of asymmetric lipid bilayers by adsorption of vesicles

We have investigated the adsorption of phospholipid mixtures using neutron reflection. Small sonicated unilamellar vesicles (SUV) composed of DOPC and d(62)-DPPC were incubated at 50 degrees C in contact with a silica surface using a method commonly employed to form supported model membranes. The composition of the mixed supported bilayer was found to be substantially different from that of the bulk vesicles in a direction indicating a higher affinity of DPPC for the silica surface. Formation of an asymmetric bilayer arrangement was also discovered in all the cases studied. DPPC tended to dominate the composition of the leaflet next to silica, while the outer leaflet was generally closer to the bulk composition. The supported bilayers also exhibited increasing interfacial roughness in the outer membrane leaflet in the region of the DOPC-DPPC gel-liquid immiscibility region. To our knowledge, this is the first time that both the structure and the absolute composition of a mixed-lipid supported bilayer have been resolved, and the results raise a number of questions regarding the adsorption of vesicles and the properties of supported bilayers, which are discussed in terms of the bulk phase diagram of DOPC and DPPC.