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Fundamental Step (fundamental + step)
Selected AbstractsHigh-Speed Living Polymerization of Polar Vinyl Monomers by Self-Healing Silylium CatalystsCHEMISTRY - A EUROPEAN JOURNAL, Issue 34 2010Dr. Yuetao Zhang Abstract This contribution describes the development and demonstration of the ambient-temperature, high-speed living polymerization of polar vinyl monomers (M) with a low silylium catalyst loading (, 0.05,mol,% relative to M). The catalyst is generated in situ by protonation of a trialkylsilyl ketene acetal (RSKA) initiator (I) with a strong Brønsted acid. The living character of the polymerization system has been demonstrated by several key lines of evidence, including the observed linear growth of the chain length as a function of monomer conversion at a given [M]/[I] ratio, near-precise polymer number-average molecular weight (Mn, controlled by the [M]/[I] ratio) with narrow molecular weight distributions (MWD), absence of an induction period and chain-termination reactions (as revealed by kinetics), readily achievable chain extension, and the successful synthesis of well-defined block copolymers. Fundamental steps of activation, initiation, propagation, and catalyst "self-repair" involved in this living polymerization system have been elucidated, chiefly featuring a propagation "catalysis" cycle consisting of a rate-limiting CC bond formation step and fast release of the silylium catalyst to the incoming monomer. Effects of acid activator, catalyst and monomer structure, and reaction temperature on polymerization characteristics have also been examined. Among the three strong acids incorporating a weakly coordinating borate or a chiral disulfonimide anion, the oxonium acid [H(Et2O)2]+[B(C6F5)4], is the most effective activator, which spontaneously delivers the most active R3Si+, reaching a high catalyst turn-over frequency (TOF) of 6.0×103,h,1 for methyl methacrylate polymerization by Me3Si+ or an exceptionally high TOF of 2.4×105,h,1 for n -butyl acrylate polymerization by iBu3Si+, in addition to its high (>90,%) to quantitative efficiencies and a high degree of control over Mn and MWD (1.07,1.12). An intriguing catalyst "self-repair" feature has also been demonstrated for the current living polymerization system. [source] Real-time Fourier transform infrared study of free-radical UV-induced polymerization of hybrid sol,gel.JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 5 2003Abstract Free-radical photocurable hybrid sol,gel materials have gained special interest. They are becoming more and more widely used for applications in coatings, optics, sensors, catalysis, and so forth. The photochemical step is a fundamental step in the elaboration of this kind of hybrid sol,gel. However, little is known about the specifics of the photochemistry in this material. The relation between the organic and the inorganic part is investigated. Hydrolysis and condensation reactions were characterized by 29Si NMR. A precise description of the material before irradiation is of paramount importance to understand photoinduced phenomena. Real-time Fourier transform infrared spectroscopy was used to examine the photopolymerization of hybrid sol,gel under UV irradiation. UV photopolymerization occurred efficiently in hybrid sol,gel although inhibition of free-radical polymerization by molecular oxygen was pronounced. Important structural modifications during irradiation were also measured. They concern both inorganic and organic parts of the hybrid material. The condensation state of the silicate network was of crucial importance. The presence of the silicate backbone did not limit the final conversion ratio. On the contrary, photopolymerization occurred more efficiently for systems with a higher degree of condensation. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 716,724, 2003 [source] Cyclic acetal hydroxyapatite composites and endogenous osteogenic gene expression of rat marrow stromal cellsJOURNAL OF TISSUE ENGINEERING AND REGENERATIVE MEDICINE, Issue 6 2010Minal Patel Abstract In this study, bone marrow stromal cells (BMSCs) were differentiated on cyclic acetal composites containing hydroxyapatite (HA) particles (110 or 550 nm). These composites were evaluated for their role in influencing osteogenic signalling by encapsulated BMSCs. While a number of factors exert influence on osteogenic signalling during the production of an osteogenic matrix, we hypothesize that HA particles may upregulate bone growth factor expression due to enhanced BMSC adhesion. To this end, fluorescence-activated cell sorting (FACS) analysis was performed for the evaluation of BMSC surface marker expression after culture on two-dimensional (2D) cyclic acetal/HA composites. Three-dimensional (3D) composites were then fabricated by incorporating 110 or 550 nm HA particles at 5, 10 and 50 ng/ml concentrations. Bone growth factor molecules (TGF,1, FGF-2 and PDGFa), bone biomarker molecules (ALP, OC, OPN and OCN) and extracellular matrix-related molecules (FN, MMP-13, Dmp1 and aggrecan) were selected for evaluation of osteogenic signalling mechanisms when in presence of these composites. FACS results at day 0 demonstrated that BMSCs were a heterogeneous population with a small percentage of cells staining positive for CD29, CD90 and CD51/61, while staining negative for CD34 and CD45. At day 3, a significant enrichment of cells staining strongly for CD29, CD90 and CD51/61 was achieved. Gene expression patterns for bone growth factors and extracellular matrix molecules were found to be largely dependent upon the size of HA particles. Bone marker molecules, except OCN, had unaltered expression patterns in response to the varied size of HA particles. Overall, the results indicate that larger-sized HA particles upregulate PDGF and these groups were also associated with the most significant increase in osteodifferentiation markers, particularly ALP. Our results suggest that endogenous signalling is dependent upon material properties. Furthermore, we propose that studying gene expression patterns induced by the surrounding biomaterials environment is a fundamental step in the creation of engineered tissues. Copyright © 2010 John Wiley & Sons, Ltd. [source] Crystallization and preliminary crystallographic characterization of glutamine synthetase from Medicago truncatulaACTA CRYSTALLOGRAPHICA SECTION F (ELECTRONIC), Issue 12 2009Ana Rita Seabra The condensation of ammonium and glutamate into glutamine catalyzed by glutamine synthetase (GS) is a fundamental step in nitrogen metabolism in all kingdoms of life. In plants, this is preceded by the reduction of inorganic nitrogen to an ammonium ion and therefore effectively articulates nitrogen fixation and metabolism. Although the three-dimensional structure of the dodecameric bacterial GS was determined quite some time ago, the quaternary architecture of the plant enzyme has long been assumed to be octameric, mostly on the basis of low-resolution electron-microscopy studies. Recently, the crystallographic structure of a monocotyledonous plant GS was reported that revealed a homodecameric organization. In order to unambiguously establish the quaternary architecture of GS from dicotyledonous plants, GS1a from the model legume Medicago truncatula was overexpressed, purified and crystallized. The collection of synchrotron diffraction data to 2.35,Å resolution allowed the determination of the three-dimensional structure of this enzyme by molecular replacement. [source] Crystallization of IgG1 by mapping its liquid,liquid phase separation curvesBIOTECHNOLOGY & BIOENGINEERING, Issue 5 2006Adam Idu Jion Abstract Monoclonal antibody therapeutics is an important and fast expanding market. While production of these molecules has been a major area of research, much less is known regarding the stabilization of these proteins for delivery as drugs. Crystallization of antibodies is one such promising route for protein stabilization at high titers, and here we took a systematic approach to initiate crystallization through nucleation in a simple PEG (polyethylene glycol), protein in water solution. A ternary mixture of globular proteins, PEG, and water will undergo a liquid,liquid phase separation (LLPS) as shown in a phase diagram or a Binodal curve. Of particular interest within the phase diagram is the position of the critical point, which is where nucleation occurs most rapidly. Detailed LLPS maps were created by increasing concentrations of PEG (from 5% to 11%) and IgG (from 1 to 20 mg/mL). By increasing the molecular weight (MW) of PEG (and hence its radius of gyration) from 1,000 to 6,000 g/mol, the temperatures of the critical point of nucleation were shown to increase. Once these curves were determined, nucleation experiments were conducted close to a chosen critical point (10.5 mg/mL IgG in 11% PEG 1000) and after 3 weeks, crystals of IgG of approximately 100 ,m in size were successfully formed. This is the first example of crystallization of an antibody through systematic mapping of LLPS curves, which is a fundamental step towards the scale-up of antibody crystallization. © 2006 Wiley Periodicals, Inc. [source] Kinetic parameters for step and flash imprint lithography photopolymerizationAICHE JOURNAL, Issue 2 2006Michael D. Dickey Abstract Step and Flash Imprint Lithography (SFIL) is a high-resolution, yet low-cost nanopatterning technique that employs an acrylate-based, free-radical photo-polymerization to replicate a patterned template onto a substrate. Modeling the photo-polymerization requires knowledge of the values of several reaction parameters, which are unique to the acrylate formulation used in SFIL. The values of these parameters were experimentally determined for use in a previously described kinetic model. The rate coefficient for initiation, kI, was determined by measuring the absorbance spectrum of the initiator, Darocur® 1173, and convolving it with the intensity spectrum from the irradiation source. The reaction coefficients kp and kt were measured using the dark polymerization method. The experimental values of both parameters were then mathematically modeled to reflect the changes that occur as a function of conversion. Measuring the kinetic parameters provides insight into the fundamental steps involved in the polymerization. © 2005 American Institute of Chemical Engineers AIChE J, 2006 [source] Action potential initiation and propagation in hippocampal mossy fibre axonsTHE JOURNAL OF PHYSIOLOGY, Issue 7 2008Christoph Schmidt-Hieber Dentate gyrus granule cells transmit action potentials (APs) along their unmyelinated mossy fibre axons to the CA3 region. Although the initiation and propagation of APs are fundamental steps during neural computation, little is known about the site of AP initiation and the speed of propagation in mossy fibre axons. To address these questions, we performed simultaneous somatic and axonal whole-cell recordings from granule cells in acute hippocampal slices of adult mice at ,23°C. Injection of short current pulses or synaptic stimulation evoked axonal and somatic APs with similar amplitudes. By contrast, the time course was significantly different, as axonal APs had a higher maximal rate of rise (464 ± 30 V s,1 in the axon versus 297 ± 12 V s,1 in the soma, mean ±s.e.m.). Furthermore, analysis of latencies between the axonal and somatic signals showed that APs were initiated in the proximal axon at ,20,30 ,m distance from the soma, and propagated orthodromically with a velocity of 0.24 m s,1. Qualitatively similar results were obtained at a recording temperature of ,34°C. Modelling of AP propagation in detailed cable models of granule cells suggested that a ,4 times higher Na+ channel density (,1000 pS ,m,2) in the axon might account for both the higher rate of rise of axonal APs and the robust AP initiation in the proximal mossy fibre axon. This may be of critical importance to separate dendritic integration of thousands of synaptic inputs from the generation and transmission of a common AP output. [source] Anaesthetics and postoperative cognitive dysfunction: a pathological mechanism mimicking Alzheimer's diseaseANAESTHESIA, Issue 4 2010V. Fodale Summary With longevity, postoperative cognitive decline in the elderly has emerged as a major health concern for which several factors have been implicated, one of the most recent being the role of anaesthetics. Interactions of anaesthetic agents and different targets have been studied at the molecular, cellular and structural anatomical levels. Recent in vitro nuclear magnetic resonance spectroscopy studies have shown that several anaesthetics act on the oligomerisation of amyloid , peptide. Uncontrolled production, oligomerisation and deposition of amyloid , peptide, with subsequent development of amyloid plaques, are fundamental steps in the generation of Alzheimer's disease. Amyloid , peptide is naturally present in the central nervous system, and is found at higher tissue concentrations in the elderly. We argue that administering certain general anaesthetics to elderly patients may worsen amyloid , peptide oligomerisation and deposition and thus increase the risk of developing postoperative cognitive dysfunction. The aim of this review is to highlight the clinical aspects of postoperative cognitive dysfunction and to find plausible links between possible anaesthetic effects and the molecular pathological mechanism of Alzheimer's disease. It is hoped that our hypothesis will stimulate further enquiry, especially triggering research into elucidating those anaesthetics that may be more suitable when cognitive dysfunction is a particular concern. [source] | ||||||||||||||||