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Full-Text Articles in Engineering
Mechanisms Of Amine-Catalyzed Organosilicate Hydrolysis At Circum-Neutral Ph, Nita Sahai
Mechanisms Of Amine-Catalyzed Organosilicate Hydrolysis At Circum-Neutral Ph, Nita Sahai
Nita Sahai
Mono- and polyamines can catalyze the hydrolysis and condensation of organosilicate starting materials in biomimetic silica synthesis pathways at circum-neutral pHs and room temperature. Our study is focused on understanding the mechanistic role of amines in catalyzing the hydrolysis process that precedes condensation. We have conducted (29)Si NMR experimental studies over a range of temperature and pHs for the hydrolysis rates of trimethylethoxysilane (TMES), a model compound with only one hydrolyzable bond, combined with quantum mechanical hybrid density functional theory calculations of putative intermediate and transition-state structures for TMES and tetramethyl orthosilicate (TMOS). Comparison of calculated energies with experimentally determined …
Effects Of Pseudowollastonite (Casio3) Bioceramic On In Vitro Activity Of Human Mesenchymal Stem Cells, Nita Sahai
Effects Of Pseudowollastonite (Casio3) Bioceramic On In Vitro Activity Of Human Mesenchymal Stem Cells, Nita Sahai
Nita Sahai
We report the effects of two pseudowollastonite (beta-CaSiO(3)) substrates on the attachment, viability, proliferation and osteogenic differentiation of human mesenchymal stem cells (hMSCs), and provide detailed mechanistic links of surface texture, soluble factors and culture media to cell activities. Cell attachment and viability were lower for psWf (fine-grained, roughness 0.74 microm) than for psWc (coarse-grained, roughness 1.25 microm) surface, and were ascribed to the greater specific area of the finer psWf particles resulting in higher release rate of Si, which is cytotoxic at high levels. Interestingly, proliferation was greater on psWf. Osteogenic differentiation occurred on both surfaces, indicated by calcium …
Modeling Apatite Nucleation In The Human Body And In The Geochemical Environment, Nita Sahai
Modeling Apatite Nucleation In The Human Body And In The Geochemical Environment, Nita Sahai
Nita Sahai
No abstract provided.
Role Of Oxide Surface Chemistry And Phospholipid Phase On Adsorption And Self-Assembly: Isotherms And Atomic Force Microscopy, Nita Sahai
Nita Sahai
We have examined the effects of metal oxide surface chemistry and lipid phase on phospholipid adsorption affinity and self-assembly. Adsorption isotherms of ditridecanoylphosphocholine (DTPC) at 40 °C and pH 7.2 on quartz (α-SiO2), rutile (α-TiO2), and corundum (α-Al2O3) particle suspensions indicated oxide-dependent adsorption affinity that decreased as rutile > corundum quartz at low concentrations and corundum > rutile quartz at higher concentrations. Significantly, atomic force microscopy of DTPC and dipalmitoylphosphocholine (DPPC) at high concentration on planar oxide surfaces at 25 °C in liquid-crystal and gel phases, respectively, also showed oxide-specific adsorption. Multiple bilayers formed on corundum (100), indicating greater coverage compared to …
Nanominerals, Mineral Nanoparticles, And Earth Systems, Nita Sahai
Nanominerals, Mineral Nanoparticles, And Earth Systems, Nita Sahai
Nita Sahai
Minerals are more complex than previously thought because of the discovery that their chemical properties vary as a function of particle size when smaller, in at least one dimension, than a few nanometers, to perhaps as much as several tens of nanometers. These variations are most likely due, at least in part, to differences in surface and near-surface atomic structure, as well as crystal shape and surface topography as a function of size in this smallest of size regimes. It has now been established that these variations may make a difference in important geochemical and biogeochemical reactions and kinetics. This …
Oxide-Dependent Adsorption Of A Model Membrane Phospholipid, Dipalmitoylphosphatidylcholine: Bulk Adsorption Isotherms, Nita Sahai
Nita Sahai
The importance of substrate chemistry and structure on supported phospholipid bilayer design and functionality is only recently being recognized. Our goal is to investigate systematically the substrate-dependence of phospholipid adsorption with an emphasis on oxide surface chemistry and to determine the dominant controlling forces. We obtained bulk adsorption isotherms at 55 °C for dipalmitoylphosphatidylcholine (DPPC) at pH values of 5.0, 7.2, and 9.0 and at two ionic strengths with and without Ca2+, on quartz (α-SiO2), rutile (α-TiO2), and corundum (α-Al2O3), which represent a wide a range of points of zero charge (PZC). Adsorption was strongly oxide- and pH-dependent. At pH …
Cyclic Silicate Active Site And Stereochemical Match For Apatite Nucleation On Pseudowollastonite Bioceramic--Bone Interfaces, Nita Sahai
Nita Sahai
Hydroxyapatite (Ca5(PO4)3(OH)) forms on pseudowollastonite (psW) (alpha-CaSiO3) in vitro in simulated body fluid, human parotid saliva and cell-culture medium, and in vivo in implanted rat tibias. We used crystallographic constraints with ab initio molecular orbital calculations to identify the active site and reaction mechanism for heterogeneous nucleation of the earliest calcium phosphate oligomer/phase. The active site is the planar, cyclic, silicate trimer (Si3O9) on the (001) face of psW. The trimer has three silanol groups (>SiOH) arranged at 60 degrees from each other, providing a stereochemical match for O atoms bonded to Ca2+ on the (001) face of hydroxyapatite. …
29 Si Nmr Sensitivity Enhancement Methods For The Quantitative Study Of Organosilicate Hydrolysis And Condensation, Nita Sahai
Nita Sahai
Quantitative kinetic studies of silicon sol–gel chemistry have proven difficult in the past because the analytical methods currently available are often incapable of providing the requisite temporal resolution or structural information. In the case of 29Si NMR spectroscopy, low isotopic abundance, a small, negative magnetogyric ratio, and long relaxation times contribute to experimental difficulty. Typically this can be overcome through the use of paramagnetic relaxation agents, but we find at neutral pH that the relaxation agents Cr(acac)3 and MnCl2 influence reaction rates. In an effort to avoid the use of potentially reactive relaxation agents, we investigate the quantitative accuracy of …
Medical Mineralogy And Geochemistry: An Interfacial Science, Nita Sahai
Medical Mineralogy And Geochemistry: An Interfacial Science, Nita Sahai
Nita Sahai
No abstract provided.
Role Of Fe (Ii) And Phosphate In Arsenic Uptake By Coprecipitation, Nita Sahai
Role Of Fe (Ii) And Phosphate In Arsenic Uptake By Coprecipitation, Nita Sahai
Nita Sahai
No abstract provided.
How Does Bone Sialoprotein Promote The Nucleation Of Hydroxyapatite? A Molecular Dynamics Study Using Model Peptides Of Different Conformations, Nita Sahai
Nita Sahai
Bone sialoprotein (BSP) is a highly phosphorylated, acidic, noncollagenous protein in bone matrix. Although BSP has been proposed to be a nucleator of hydroxyapatite (Ca(5)(PO(4))(3)OH), the major mineral component of bone, no detailed mechanism for the nucleation process has been elucidated at the atomic level to date. In the present work, using a peptide model, we apply molecular dynamics (MD) simulations to study the conformational effect of a proposed nucleating motif of BSP (a phosphorylated, acidic, 10 amino-acid residue sequence) on controlling the distributions of Ca(2+) and inorganic phosphate (Pi) ions in solution, and specifically, we explore whether a nucleating …
Extracellular Polymeric Substances As Armour Against Cytotoxic Minerals: Survival Of Pseudomonas Aeruginosa Cells In Oxide Particle Suspensions, Nita Sahai
Nita Sahai
Our study explored the effects of extracellular polymeric substances (EPS) on the susceptibility to lysis of Pseudomonas aeruginosa in suspensions of oxide particles.
Computational Molecular Basis For Improved Silica Surface Complexation Models, Nita Sahai
Computational Molecular Basis For Improved Silica Surface Complexation Models, Nita Sahai
Nita Sahai
The acidity abd reactivity of surface on amorphous and crystalline polymorphs of silica and other oxides control their thermodynamic stability and kinetic reactivity towards reactants in surface-controlled processes of environmental, industrial, biomedical and technological relevance. Recent advances in computational methodologies such as CPMD and increasing computer power combined with spectroscopic measurements are now making it possible to link, with an impressive degree of accuracy, the molecular-level description of these processes to phenomenological, surface complexion models The future challenge now lies in linking mesoscale properties at the nanometer scale phenomenological models that will afford a more intuitive understanding of the systems …
Silicate Biomaterials For Orthopaedic And Dental Implants, Nita Sahai
Silicate Biomaterials For Orthopaedic And Dental Implants, Nita Sahai
Nita Sahai
No abstract provided.
Interaction Energies Between Oxide Surfaces And Multiple Phosphatidylcholine Bilayers From Extended-Dlvo Theory, Nita Sahai
Nita Sahai
Interaction energies between dipalmitoylphosphatidylcholine (DPPC) bilayers and corundum (α-Al(2)O(3)) or quartz (α-SiO(2)) are calculated according to extended-Derjaguin-Landau-Verwey-Overbeek (DLVO) theory in order to examine oxide-dependent multiple bilayer adsorption. Energies are investigated at two ionic strengths (I=17 or 217 mM) and compared to corresponding DPPC adsorption experiments on corundum and quartz particles. The repulsive hydration interaction diminishes sharply with separation distance, and beyond ∼3 nm, the interaction energy is determined by the electrostatic and van der Waals contributions only. We revise the electrostatic interaction to account for solution exclusion by the bilayers, extending the distance over which this interaction occurs. Calculated oxide-bilayer …
Theoretical Study Of Bone Sialoprotein In Bone Biomineralization, Nita Sahai
Theoretical Study Of Bone Sialoprotein In Bone Biomineralization, Nita Sahai
Nita Sahai
Bone sialoprotein (BSP) is an acidic, non-collagenous protein specific to bone proposed previously to promote hydroxyapatite (HAP) nucleation and modulate HAP nanocrystal growth. Specifically, two phosphorylated acidic amino acid sequences in BSP, highly conserved across several vertebrates, are the proposed active sites. We selected one of these sites, i.e. (Sp)(2)E(8), where Sp represents a phosphoserine as a model peptide to study the role of BSP. We used molecular dynamics simulations to determine whether an α-helix or a random coil peptide conformation promotes templated HAP nucleation. A bioinformatics method helps infer preferential crystal growth directions by predicting the likely peptide conformations …
Exploring The Mechanisms For Highly Reactive Oxygen Species (Hros) Formation In Oxide Particle Suspensions: Implications For The Origin And Evolution Of Life At Mineral Surfaces, Nita Sahai
Nita Sahai
The broad goals of our study were to identify processes by which organic molecules may have been destroyed or altered on early Earth, thus affecting the total inventory of organics available for prebiotic synthesis reactions.
Role Of Fe (Ii), Phosphate, Silicate, Sulfate, And Carbonate In Arsenic Uptake By Coprecipitation In Synthetic And Natural Groundwater, Nita Sahai
Nita Sahai
Competitive effects of phosphate, silicate, sulfate, and carbonate on As(III) and As(V) removal at pH approximately 7.2 have been investigated to test the feasibility of Fe(II)(aq) and hydroxylapatite crystals as inexpensive and potentially efficient agents for remediation of contaminated well-water, using Bangladesh as a type study. Arsenic(III) removal approximately 50-55% is achieved, when Fe(II)(aq) oxidizes to Fe(III) and precipitates as Fe(OH)3 at 25 degrees C and 3h reaction time, in the presence of all the oxyanion. Similar results were obtained for well-water samples from two sites in Bangladesh. Heating at 95 degrees C for 24h results in 70% As(III) uptake …