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Structural Modifications (structural + modifications)
Selected AbstractsCover Picture: Structural Modifications to Polystyrene via Self-Assembling Molecules (Adv. Funct.ADVANCED FUNCTIONAL MATERIALS, Issue 3 2005Mater. Abstract The cover shows tensile failure of a sample of pure polystyrene (left), and a polystyrene sample with greater impact strength containing 1% by weight of dispersed nanoribbons (right), as reported in work by Stupp and co-workers on p.,487. The nanoribbons are formed by self-assembly of molecules known as dendron rodcoils (DRCs) in styrene monomer, resulting in the formation of a gel. This gel can then be polymerized thermally. We have previously reported that small quantities of self-assembling molecules known as dendron rodcoils (DRCs) can be used as supramolecular additives to modify the properties of polystyrene (PS). These molecules spontaneously assemble into supramolecular nanoribbons that can be incorporated into bulk PS in such a way that the orientation of the polymer is significantly enhanced when mechanically drawn above the glass-transition temperature. In the current study, we more closely evaluate the structural role of the DRC nanoribbons in PS by investigating the mechanical properties and deformation microstructures of polymers modified by self-assembly. In comparision to PS homopolymer, PS containing small amounts (,,1.0,wt.-%) of self-assembling DRC molecules exhibit greater Charpy impact strengths in double-notch four-point bending and significantly greater elongations to failure in uniaxial tension at 250,% prestrain. Although the DRC-modified polymer shows significantly smaller elongations to failure at 1000,% prestrain, both low- and high-prestrain specimens maintain tensile strengths that are comparable to those of the homopolymer. The improved toughness and ductility of DRC-modified PS appears to be related to the increased stress whitening and craze density that was observed near fracture surfaces. However, the mechanism by which the self-assembling DRC molecules toughen PS is different from that of conventional additives. These molecules assemble into supramolecular nanoribbons that enhance polymer orientation, which in turn modifies crazing patterns and improves impact strength and ductility. [source] Role of Sulfate in Structural Modifications of Sodium Barium Borosilicate Glasses Developed for Nuclear Waste ImmobilizationJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2008Raman K. Mishra A sodium barium borosilicate glass matrix with a higher solubility of sulfate has been developed recently at Bhabha Atomic Research Centre for vitrification of sulfate bearing high-level nuclear waste. We report here the studies carried out to understand the influence of sulfate ion on the three-dimensional borosilicate network. Experiments were carried out with sodium barium borosilicate base glass samples loaded with varying amounts of SO42, (0,5 mol%). Phase separation studies on the samples revealed that as much as 3 mol% of SO42, can be loaded within the base glass without any phase separation, however, beyond this limit BaSO4 (barite) crystallizes within the matrix. Thermal analyses of the samples indicated a shift in glass transition temperature from 534° (0 mol% SO42,) to 495°C (3 mol% SO42,) and it remained more or less unaltered afterwards even with high SO42, loading. A similar observation of structure stabilization was obtained from 29Si MAS,NMR studies also, which showed that with 2 mol% of SO42, loading, the Q2:Q3 ratio changed from 59:41 (for samples with 0 mol% SO42, loading) to 62:38 and it remained almost the same afterwards even with higher SO42, loading. 11B MAS NMR patterns of the glass samples, however, remained unchanged with SO42, loading ([BO4]:[BO3]=38:62). Based on 29Si and 11B MAS NMR studies, the authors propose two different ways of interaction of SO42, ions with the borosilicate network: (i) the network modifying action of SO42, ions with -Si,O,Si- linkages, at low SO42, ion concentration (<2 mol%) and (ii) the preferential interaction of SO42, with the Ba2+ ions at high SO42, concentration (>2 mol%). [source] Structural Modifications of (1S,3S)-3-Amino-4-difluoromethylenecyclopentanecarboxylic Acid, a Potent Irreversible Inhibitor of GABA Aminotransferase.CHEMINFORM, Issue 31 2007Hai Yuan Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] Adaptations of amphibious fish for surviving life out of waterFISH AND FISHERIES, Issue 3 2005Martin D J Sayer Abstract There are a small number of fish species, both marine and freshwater, that exhibit a truly amphibious habit that includes periods of aerial exposure. The duration of emersion is reflected in the level of physical and physiological adaptation to an amphibious lifestyle. Fish that are only briefly out of water retain predominantly aquatic attributes whereas there are semi-terrestrial species that are highly adapted to prolonged periods in the aerial habitat. Desiccation is the main stressor for amphibious fish and it cannot be prevented by physiological means. Instead, amphibious fish resist excessive water loss by means of cutaneous modification and behavioural response. The more terrestrially adapted fish species can tolerate considerable water loss and may employ evaporation to aid thermoregulation. The amphibious habit is limited to fish species that can respire aerially. Aerial respiration is usually achieved through modification to existing aquatic pathways. Freshwater air-breathers may respire via the skin or gills but some also have specialized branchial diverticula. Marine species utilize a range of adaptations that may include modified gills, specialized buccopharyngeal epithelia, the intestine and the skin. Areas of enhanced respiratory activity are typified by increased vascularization that permits enhanced perfusion during aerial exposure. As with other adaptations the mode of nitrogenous elimination is related to the typical durations of emersion experienced by the fish. Intertidal species exposed on a regular cycle, and which may retain some contact with water, tend to remain ammoniotelic while reducing excretion rates in order to prevent excessive water loss. Amphibious fish that inhabit environments where emersion is less predictable than the intertidal, can store nitrogen during the state of emersion with some conversion to ureotelism or have been shown to tolerate high ammonia levels in the blood. Finally, the more amphibious fish are more adapted to moving on land and seeing in air. Structural modifications to the pectoral, pelvic, dorsal and anal fins, combined with a well-developed musculature permit effective support and movement on land. For vision in air, there is a general trend for fish to possess close-set, moveable, protruberant eyes set high on the head with various physical adaptations to the structure of the eye to allow for accurate vision in both air and water. [source] Structural modifications resulting from proton transfer in complexes of phenols with pyridine,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 8 2005Irena Majerz Abstract The structural consequences of proton transfer in complexes of phenol (1), 2,6-dichlorophenol (2), 4-nitrophenol (3) and 2,6-dichloro-4-nitrophenol (4) with pyridine were analyzed on the basis of results of B3LYP/6,31G** calculations. Three methods of describing the progress of proton transfer are proposed: the O,H [d(OH)] and C,O [d(CO)] bond lengths and the fraction XPT of the proton transfer form, calculated from the values of the dipole moments. The d(OH) parameter reveals behaviour near to XPT and can be used as a universal measure of the degree of proton transfer. The d(CO) parameter gives nearly linear dependences for various structural parameters, but independent estimation of the specific effects of the substituents is necessary, as separate correlations for each complex are found. A role of resonance interaction in systems containing a p -NO2 substituent is demonstrated. Copyright © 2005 John Wiley & Sons, Ltd. [source] Novel Methyl Helianthrones as Photosensitizers: Synthesis and Biological Evaluation,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 2 2005Shai Rahimipour ABSTRACT A combination of light, oxygen and a photosensitizer is used to induce death of cancer cells by photodynamic therapy. In this study, we have synthesized several new methyl helianthrone derivatives and compared their phototoxicity with that of hypericin. In contrast to hypericin, methyl helianthrones are soluble in aqueous solutions and have a broad range of light absorbance, which allows the use of polychromatic light. Structural modifications of methyl helianthrone demonstrated that substitution of hydrogen atoms of methyl helianthrone at Positions 2 and 5 with Br atoms or methylation of its phenolic hydroxyls, significantly increases the corresponding singlet oxygen quantum yield and their phototoxicity toward ,T3-1, M2R and LNCaP cells. The phototoxicity of some of these compounds was similar to that of hypericin. Methyl helianthrones, like hypericin, accumulated mainly in the perinuclear region as evident by confocal microscopy. Irradiation of cells pretreated with methyl helianthrone derivatives generates intracellular reactive oxygen species and lipid free radicals, as shown by a fluorescentic probe and electron paramagnetic resonance methods, respectively. The phototoxicity of these methyl helianthrones as well as their ability to oxidize membrane lipids were significantly decreased on addition of specific Type-II inhibitors, suggesting the involvement of singlet oxygen as the main oxidant. [source] Influence on antiproliferative activity of structural modification and conjugation of gonadotropin-releasing hormone (GnRH) analoguesCELL PROLIFERATION, Issue 5 2000A. Kálnay Abstract The effect of various GnRH analogues, and their conjugates on proliferation, clonogenicity and cell cycle phase distribution of MCF-7 and Ishikawa human cancer cell lines was studied. GnRH-III, a sea lamprey GnRH analogue reduced cell proliferation by 35% and clonogenicity by 55%. Structural modifications either decreased, or did not alter biological activity. Conjugation of GnRH analogues including MI-1544, MI-1892, and GnRH-III with poly(N-vinylpyrrolidone-co-maleic acid) (P) through a tetrapeptide spacer GFLG(X) substantially increased the inhibitory effect of the GnRH analogues. The conjugate P-X-GnRH-III induced significant accumulation of cells in the G2/M phase; from 8% to 15.6% at 24 h and 9.8% to 15% at 48 h. It was concluded that conjugation of various GnRH analogues substantially enhanced their antiproliferative activity, strongly reduced cell clonogenicity and retarded cell progression through the cell division cycle at the G2/M phase. [source] Carbohydrate-Derived 1,3-Diphosphite Ligands as Chiral Nanoparticle Stabilizers: Promising Catalytic Systems for Asymmetric HydrogenationCHEMSUSCHEM CHEMISTRY AND SUSTAINABILITY, ENERGY & MATERIALS, Issue 8 2009Aitor Gual Dr. Abstract Metallic Ru, Rh, and Ir nanoparticles were prepared by the decomposition of organometallic precursors under H2 pressure in the presence of 1,3-diphosphite ligands, derived from carbohydrates, as stabilizing agents. Structural modifications to the diphosphite backbone were found to influence the nanoparticles, size, dispersion, and catalytic activity. In the hydrogenation of o - and m -methylanisole, the Rh nanoparticles showed higher catalytic activity than the corresponding Ru nanoparticles. The Ir nanoparticles presented the lowest catalytic activity of the series. In all cases, the hydrogenation of o -methylanisole gave total selectivity for the cis -product, however, the ee of the product was always less than 6,%. A maximum of 81,% cis -selectivity was obtained for the hydrogenation of m -methylanisole, however, no asymmetric induction was observed. These results show that the catalytic activity is affected by a combination of influences from the substrate, the diphosphite ligands, and the metallic nanoparticles. [source] Ethanol neurotoxicity and dentate gyrus developmentCONGENITAL ANOMALIES, Issue 3 2008Takanori Miki ABSTRACT Maternal alcohol ingestion during pregnancy adversely affects the developing fetus, often leading to fetal alcohol syndrome (FAS). One of the most severe consequences of FAS is brain damage that is manifested as cognitive, learning, and behavioral deficits. The hippocampus plays a crucial role in such abilities; it is also known as one of the brain regions most vulnerable to ethanol-induced neurotoxicity. Our recent studies using morphometric techniques have further shown that ethanol neurotoxicity appears to affect the development of the dentate gyrus in a region-specific manner; it was found that early postnatal ethanol exposure causes a transitory deficit in the hilus volume of the dentate gyrus. It is strongly speculated that such structural modifications, even transitory ones, appear to result in developmental abnormalities in the brain circuitry and lead to the learning disabilities observed in FAS children. Based on reports on possible factors deciding ethanol neurotoxicity to the brain, we review developmental neurotoxicity to the dentate gyrus of the hippocampal formation. [source] Death Rides the Forest: Perceptions of Fire, Land Use, and Ecological Restoration of Western ForestsCONSERVATION BIOLOGY, Issue 4 2004J. BOONE KAUFFMAN fuego prescrito; incendios catastróficos; incendios en áreas silvestres; incendios no controlados; reducción de riesgo de combustible; restauración de bosques; tala de bosques Abstract:,Large wild fires occurring in forests, grasslands, and chaparral in the last few years have aroused much public concern. Many have described these events as "catastrophes" that must be prevented through aggressive increases in forest thinning. Yet the real catastrophes are not the fires themselves but those land uses, in concert with fire-suppression policies that have resulted in dramatic alterations to ecosystem structure and composition. The first step in the restoration of biological diversity (forest health) of western landscapes must be to implement changes in those factors that have caused degradation or are preventing recovery. This includes changes in policies and practices that have resulted in the current state of wildland ecosystems. Restoration entails much more than simple structural modifications achieved though mechanical means. Restoration should be undertaken at landscape scales and must allow for the occurrence of dominant ecosystem processes, such as the natural fire regimes achieved through natural and/or prescribed fires at appropriate temporal and spatial scales. Resumen:,En años recientes, grandes incendios en bosques, pastizales y chaparrales han causado bastante preocupación en la opinión pública. Muchos han descrito estos eventos como "catástrofes" que deben ser prevenidas mediante incrementos agresivos en la tala de bosques. Pero los incendios mismos no son las verdaderas catástrofes, sino los usos del suelo en conjunto con políticas de supresión de fuego que han resultado en alteraciones dramáticas de la estructura y composición de ecosistemas. El primer paso en la restauración de la diversidad biológica (salud del bosque) en paisajes occidentales debe ser la implementación de cambios en los factores que causaron la degradación o que están impidiendo la recuperación. Esto incluye cambios en políticas y prácticas que han resultado en el estado actual de ecosistemas en áreas silvestres. La restauración implica mucho más que simples modificaciones estructurales obtenidas mediante medios mecánicos. La restauración debe llevarse a cabo a nivel de paisaje y debe permitir que ocurrencia de procesos ecológicos dominantes (por ejemplo, regímenes de incendios naturales logrados mediante incendios naturales y/o prescritos en escalas temporales y espaciales apropiadas). [source] Targeting the development of resveratrol as a chemopreventive agent,DRUG DEVELOPMENT RESEARCH, Issue 6 2010Nian-Guang Li Abstract Tumor development consists of several separate, but closely linked, stages: tumor initiation, promotion, and progression. This long and complex process provides opportunities for intervention both in preventing cancer initiation and in treating the neoplasm during its premalignant stages. Resveratrol, a polyphenolic compound found in many plant species, including grapes, peanuts, and various herbs, has recently been investigated intensely for its cancer chemopreventive property. The present work is an overview of the chemopreventive mechanisms of resveratrol in anti-initiation, anti-promotion, and anti-progression. These, together with the low toxicity of resveratrol, suggest promise for novel chemopreventive agents. However, the low bioavailability and rapid clearance of resveratrol from the circulation require the design of new resveratrol-like chemopreventive agents, the structural modifications and the structure,activity relationship of which are also discussed in this review. Drug Dev Res 71:335,350, 2010. © 2010 Wiley-Liss, Inc. [source] Formation of Metastable Na2CrO4 -Type LiNiPO4 from a Phosphate,Formate PrecursorEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 1 2010Violeta Koleva Abstract High-pressure modification of LiNiPO4 with a Na2CrO4 -type structure was obtained at ambient pressure and low temperature from a mixed LiNi,phosphate,formate precursor, LiNiPO4Hx(HCOO)x·yH2O (where x , 1.2 and y , 2.5). The structural and thermal characterization of the precursor and the LiNiPO4 compositions were carried out by powder XRD analysis, IR spectroscopy, and DSC analysis. Thermal treatment of LiNiPO4Hx(HCOO)x·yH2O precursors between 450 and 650 °C yields a mixture of the two structural modifications of LiNiPO4: the Na2CrO4 type and the olivine type. It was established that the obtained Na2CrO4 -type LiNiPO4 is a metastable phase, which completely transforms at 700 °C into the olivine-type phase. The enthalpy of the phase transition is ,H = ,43.40 kJ,mol,1. The mechanism of formation of the two forms of LiNiPO4 from the LiNi,phosphate,formate precursor is discussed. [source] The Influence of Surface Chemistry and Pore Size on the Adsorption of Proteins on Nanostructured Carbon MaterialsADVANCED FUNCTIONAL MATERIALS, Issue 15 2010Munusami Vijayaraj Abstract Carbon films are synthesized by templating of anodic aluminum oxide films. These carbon materials exhibit nanochannels with controlled diameter and length. Selected chemical treatments are done to tailor the surface chemistry. The adsorption capacities of bovine serum albumin and cytochrome c are measured by temperature-programmed desorption with mass spectrometry (TPD-MS) analysis and with conventional biological assays. The first method allows quantification of the proteins that exhibit strong interactions with the surface, while the second one is used to obtain the total adsorption capacity. Moreover, the TPD-MS profiles, which are related to the structural modifications of the proteins during the adsorption, show that strong interactions take place with hydrophobic surfaces. When oxygenated functions are present, the adsorption capacity increases and the nature of the interactions is modified. The ratio of irreversible to reversible adsorption is significantly different for the two proteins, and is slightly related to the surface chemistry. The influence of nanochannel size is studied: below 50 nm, the coverage ratio shows that access to the porosity is limited by diffusion in the channel and by pore plugging, in agreement with the strong interactions of proteins with the carbon surface. [source] Purification and structure of the major product obtained by reaction of NADPH and NMNH with the myeloperoxidase/hydrogen peroxide/chloride systemFEBS JOURNAL, Issue 10 2001Françoise Auchère The first spectrophotometric study of the reaction of the myeloperoxidase/H2O2/Cl, system with NADPH and NMNH showed that the reaction products were not the corresponding oxidized nucleotides and that modifications would take place on the nicotinamide part of the molecule [Auchère, F. & Capeillère-Blandin, C. (1999) Biochem. J. 343, 603,613]. In this report, in order to obtain more precise information on the structural modifications and mechanism of the reaction, we focus on the purification and isolation of products derived from NADPH and NMNH by RP-HPLC. Electrospray ionization mass spectra indicated that the relative height of the peaks reflected that of the natural isotopic abundance of 35Cl and 37Cl, providing evidence that the products derived from NADPH and NMNH were monochlorinated. Moreover, calculated masses revealed the 1 : 1 addition of HOCl to the molecule. Various 1D and 2D NMR experiments provided data for the assignments of the chemical shifts of protons and carbons and the coupling constants of the protons of the chlorinated nucleotides. Further NOESY experiments allowed the characterization of the spatial structure of the chlorinated product and showed that trans HOCl addition occurred at the C5=C6 carbon double bond of the nicotinamide ring, leading to a chlorohydrin. [source] Design syntheses and mitochondrial complex I inhibitory activity of novel acetogenin mimicsFEBS JOURNAL, Issue 9 2000Kaoru Kuwabara Some natural acetogenins are the most potent inhibitors of mitochondrial complex I. These compounds are characterized by two functional units [i.e. hydroxylated tetrahydrofuran (THF) and ,,,-unsaturated ,-lactone ring moieties] separated by a long alkyl spacer. To elucidate which structural factors of acetogenins, including their active conformation, are crucial for the potent inhibitory activity we synthesized a novel bis-acetogenin and its analogues possessing two ,-lactone rings connected to bis-THF rings by flexible alkyl spacers. The inhibitory potency of the bis-acetogenin with bovine heart mitochondrial complex I was identical to that of bullatacin, one of the most potent natural acetogenins. This result indicated that one molecule of the bis-acetogenin does not work as two reactive inhibitors, suggesting that a ,-lactone and the THF ring moieties act in a cooperative manner on the enzyme. In support of this, either of the two ring moieties synthesized individually showed no or very weak inhibitory effects. Moreover, combined use of the two ring moieties at various molar ratios exhibited no synergistic enhancement of the inhibitory potency. These observations indicate that both functional units work efficiently only when they are directly linked by a flexible alkyl spacer. Therefore, some specific conformation of the spacer must be important for optimal positioning of the two units in the enzyme. Furthermore, the ,,,-unsaturated ,-lactone, the 4-OH group in the spacer region, the long alkyl tail attached to the THF unit and the stereochemistry surrounding the hydroxylated bis-THF rings were not crucial for the activity, although these are the most common structural features of natural acetogenins. The present study provided useful guiding principles not only for simplification of complicated acetogenin structure, but also for further wide structural modifications of these molecules. [source] Influence of Electric Field on Microstructures of Pentacene Thin-Films in Field-Effect Transistors,ADVANCED FUNCTIONAL MATERIALS, Issue 2 2008L. Cheng Abstract We report on electric-field-induced irreversible structural modifications in pentacene thin films after long-term operation of organic field-effect transistor (OFET) devices. Micro-Raman spectroscopy allows for the analysis of the microstructural modifications of pentacene in the small active channel of OFET during device operation. The results suggest that the herringbone packing of pentacene molecules in a solid film is affected by an external electric field, particularly the source-to-drain field that parallels the a,b lattice plane. The analysis of vibrational frequency and Davydov splitting in the Raman spectra reveals a singular behavior suggesting a reduced separation distance between pentacene molecules after long-term operations and, thus, large intermolecular interactions. These results provide evidence for improved OFET performance after long-term operation, related to the microstructures of organic semiconductors. It is known that the application of large electric fields alters the semiconductor properties of the material owing to the generation of defects and the trapping of charges. However, we first suggest that large electric fields may alter the molecular geometry and further induce structural phase transitions in the pentacene films. These results provide a basis for understanding the improved electronic properties in test devices after long-term operations, including enhanced field-effect mobility, improved on/off current ratio, sharp sub-threshold swing, and a slower decay rate in the output drain current. In addition, the effects of source-to-drain electric field, gate electric field, current and charge carriers, and thermal annealing on the pentacene films during OFET operations are discussed. [source] A Novel Synthesis of Highly Substituted Perhydropyrrolizines, Perhydroindolizines, and Pyrrolidines: Inhibition of the Peptidyl-Prolyl cis/trans Isomerase (PPIase) Pin1HELVETICA CHIMICA ACTA, Issue 2 2007Romain Siegrist Abstract In this paper, we describe the synthesis and biological evaluation of highly substituted perhydropyrrolizines that inhibit the peptidyl-prolyl cis/trans isomerase (PPIase) Pin1, an oncogenic target. The enzyme selectively catalyzes the cis/trans isomerization of peptide bonds between a phosphorylated serine or threonine, and proline, thereby inducing a conformational change. Such structural modifications play an important role in many cellular events, such as cell-cycle progression, transcriptional regulation, RNA processing, as well as cell proliferation and differentiation. Based on computer modeling (Fig.,2), the new perhydropyrrolizinone derivatives (,)- 1a,b, decorated with two substituents, were selected and synthesized (Schemes,1,3). While enzymatic assays showed no biological activity, 15N,1H-HSQC-NMR spectroscopy revealed that (,)- 1a,b bind to the WW recognition domain of Pin1, apparently in a mode that does not inhibit PPIase activity. To enforce complexation into the larger active site rather than into the tighter WW domain of Pin1 and to enhance the overall binding affinity, we designed a perhydropyrrolizine scaffold substituted with additional aromatic residues (Fig.,5). A novel, straightforward synthesis towards this class of compounds was developed (Schemes,4 and 5), and the racemic compounds (±)- 22a,22d were found to inhibit Pin1 with Ki values (Ki,=,inhibition constant) in the micromolar range (Table,2). To further enhance the potency of these inhibitors, the optically pure ligands (+)- 22a and (+)- 33b,c were prepared (Schemes,6 and 7) and shown to inhibit Pin1 with Ki values down to the single-digit micromolar range. According to 15N,1H-HSQC-NMR spectroscopy and enzymatic activity assays, binding occurs at both the WW domain and the active site of Pin1. Furthermore, the new synthetic protocol towards perhydropyrrolizines was extended to the preparation of highly substituted perhydroindolizine ((±)- 43; Scheme,8) and pyrrolidine ((±)- 48a,b; Scheme,9) derivatives, illustrating a new, potentially general access to these highly substituted heterocycles. [source] Correlating the positional reactivity of a masked electrophilic center to the topology of the electron densityINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 5 2005Harry L. Price Abstract Masking of electrophilic centers in biological molecules via structural modifications serves to control chemical reactivity and in some cases may affect the regioselectivity of a reaction. In the work presented the regioselective reactivity of the electrophilic cyclopropylpyrrololindole (CPI) center in the DNA alkylator CC-1065, a highly toxic antibiotic that has served as a structural template for the development of a series of novel anticancer drugs containing the CPI reactive center has been examined. The CPI reactive center is an interesting example of chemical masking as it relates to acid-dependent electrophilicity, and regioselective addition of a nucleophile to an electrophilic center. In an effort to better understand the reactivity of the CPI center, calculations using the B3LYP density functional theory (DFT) method, the 6-31G(d) basis set, and the atoms in molecules (AIM) theory were performed on unprotonated and protonated forms of the CPI reactive center. The results of these calculations demonstrate that activation of the CPI group via protonation induces significant changes in the electron density (,), the Laplacian of the density (,2,), and the bond ellipticity (,), and that these changes are linked to the observed reactivity of the CPI reaction center. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [source] The structural design of the bat wing web and its possible role in gas exchangeJOURNAL OF ANATOMY, Issue 6 2007Andrew N. Makanya Abstract The structure of the skin in the epauletted fruit bat (Epomophorus wahlbergi) wing and body trunk was studied with a view to understanding possible adaptations for gas metabolism and thermoregulation. In addition, gas exchange measurements were performed using a respirometer designed for the purpose. The body skin had an epidermis, a dermis with hair follicles and sweat glands and a fat-laden hypodermis. In contrast, the wing web skin was made up of a thin bilayered epidermis separated by a connective tissue core with collagen and elastic fibres and was devoid of hair follicles and sweat glands. The wings spanned 18,24 cm each, with about 753 cm2 of surface exposed to air. The body skin epidermis was thick (61 ± 3 µm, SEM), the stratum corneum alone taking a third of it (21 ± 3 µm). In contrast, the wing web skin epidermis was thinner at 9.8 ± 0.7 µm, with a stratum corneum measuring 4.1 ± 0.3 µm (41%). The wing capillaries in the wing web skin ran in the middle of the connective tissue core, with a resultant surface-capillary diffusion distance of 26.8 ± 3.2 µm. The rate of oxygen consumption (V,O2) of the wings alone and of the whole animal measured under light anaesthesia at ambient temperatures of 24 ºC and 33 ºC, averaged 6% and 10% of the total, respectively. Rate of carbon dioxide production had similar values. The membrane diffusing capacity for the wing web was estimated to be 0.019 ml O2 min,1 mmHg,1. We conclude that in Epomophorus wahlbergi, the wing web has structural modifications that permit a substantial contribution to the total gas exchange. [source] Modifications in the correlation function in poly(vinyl alcohol)/silica hybrid wet gelsJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 1 2009Dimas R. Vollet Small-angle X-ray scattering was used to study structural modifications in tetraethoxysilane-derived poly(vinyl alcohol) (PVA)/silica hybrids. The basic structure of the wet gels can be described as a mass-fractal structure with fractal dimension D equal to 2 and characteristic length ,, which increases with addition of PVA. Wet gels with high PVA content exhibit a positive deviation from the mass-fractal power-law scattering at low q; this deviation is associated with additional scattering due to a second large correlation distance ,, reinforced by the addition of PVA. The fraction of both contributions to the total correlation function was estimated; this is the first time that such a study has been carried out for mass-fractal structures. [source] Morphological variations in a tooth family through ontogeny in Pleurodeles waltl (Lissamphibia, Caudata)JOURNAL OF MORPHOLOGY, Issue 9 2006Tiphaine Davit-Béal Abstract Most nonmammalian species replace their teeth continuously (so-called polyphyodonty), which allows morphological and structural modifications to occur during ontogeny. We have chosen Pleurodeles waltl, a salamander easy to rear in the laboratory, as a model species to establish the morphological foundations necessary for future molecular approaches aiming to understand not only molecular processes involved in tooth development and replacement, but also their changes, notably during metamorphosis, that might usefully inform studies of modifications of tooth morphology during evolution. In order to determine when (in which developmental stage) and how (progressively or suddenly) tooth modifications take place during ontogeny, we concentrated our observations on a single tooth family, located at position I, closest to the symphysis on the left lower jaw. We monitored the development and replacement of the six first teeth in a large growth series ranging from 10-day-old embryos (tooth I1) to adult specimens (tooth I6), using light (LM), scanning (SEM), and transmission electron (TEM) microscopy. A timetable of the developmental and functional period is provided for the six teeth, and tooth development is compared in larvae and young adults. In P. waltl the first functional tooth is not replaced when the second generation tooth forms, in contrast to what occurs for the later generation teeth, leading to the presence of two functional teeth in a single position during the first 2 months of life. Larval tooth I1 shows dramatically different features when compared to adult tooth I6: a dividing zone has appeared between the dentin cone and the pedicel; the pulp cavity has enlarged, allowing accommodation of large blood vessels; the odontoblasts are well organized along the dentin surface; tubules have appeared in the dentin; and teeth have become bicuspidate. Most of these modifications take place progressively from one tooth generation to the next, but the change from monocuspid to bicuspid tooth occurs during the tooth I3 to tooth I4 transition at metamorphosis. J. Morphol. © 2006 Wiley-Liss, Inc. [source] REDOX PROPERTIES ARE CONSERVED IN RUBISCOS FROM DIATOMS AND GREEN ALGAE THROUGH A DIFFERENT PATTERN OF CYSTEINES,JOURNAL OF PHYCOLOGY, Issue 3 2010Julia Marín-Navarro Eukaryotic RUBISCO appears in two sequence-diverging forms, known as red-like (present in nongreen algae) and green-like (of green algae and higher plants) types. Oxidation of cysteines from green-like RUBISCOs is known to result in conformational changes that inactivate the enzyme and render a relaxed structure more prone to proteolytic attack. These changes may have regulatory value for green algae and higher plants, promoting RUBISCO catabolism under stress conditions. We compare here red-like RUBISCOs from several diatoms with a representative green-like RUBISCO from Chlamydomonas reinhardtii, paying special attention to the cysteine-dependent redox properties. Purified diatom RUBISCO preparations displayed a specific carboxylase activity about one order of magnitude lower than that of the C. reinhardtii P. A. Dang. enzyme. Despite having different patterns of cysteine residues in their primary sequence, the red-like enzymes from diatoms inactivated also through oxidation of cysteine sulfhydryls to disulfides with a transition midpoint identical to that of the green-like forms. Cysteine oxidation resulted also in structural modifications of the diatom RUBISCOs, as recognized by a higher sensitivity of the oxidized enzyme to in vitro proteolysis. The coincident redox properties of red- and green-like RUBISCO types suggest that these changes are part of a physiologically significant regulatory mechanism that has been convergently implemented in both groups with a different set of cysteine residues. [source] Fluorene-based liquid crystalline networks with linearly polarized blue emissionJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 21 2007Marta Millaruelo Abstract A series of fluorene-based luminophores containing methacrylic end groups have been prepared and incorporated into uniaxially oriented liquid crystalline films by in situ photopolymerization. Various structural modifications on the 2-(4-cyanophenyl)fluorene core, which include alkyl chains at the 9-position and elongation of the rigid core with one additional phenyl ring, have been investigated to generate emitters with adjusted liquid crystal compatibility, improved luminescence and dichroic properties. Polarized blue-emitting films were produced that had an acceptable photostability, and it was found that the polarization emission was better for samples with low (5%) cross-linker contents. Polarization of the luminescence was favored by the liquid crystalline properties of the luminophore. In addition, the detrimental effect of the alkyl substituent at the fluorene core on the mesomorphism and on the emission polarization can be overcome by lengthening the ,-system. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4804,4817, 2007 [source] Multiscale approach to investigate the radiochemical degradation of epoxy resins under high-energy electron-beam irradiationJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 2 2006N. Longiéras Abstract A multiscale investigation of the degradation mechanism of two epoxy systems exposed to electron-beam irradiation under a helium atmosphere was carried out with a variety of analytical methods, including high-resolution solution- and solid-state NMR spectroscopy, NMR relaxometry, infrared spectroscopy, sterical exclusion chromatography, and differential scanning calorimetry. As a first step, we studied a linear phenoxy polymer, poly(2-hydroxyether of bisphenol A), which provided a basis for the investigation of the degradation of a more complex, insoluble epoxy,amine network, diglycidyl ether of bisphenol A/triethylene tetramine. Among different structural modifications, the main degradation process was shown to produce in both cases a chain scission. For the phenoxy resin, the hydroxypropylidene moiety was identified as the fragile site leading to the formation of two phenolic chain ends and acetone and isopropyl alcohol as low-molecular-weight products. All methods, ranging from molecular to supramolecular scales, were shown to correlate both qualitatively and quantitatively. Experimental results obtained with diglycidyl ether of bisphenol A/triethylene tetramine evidenced a different degradation scheme occurring at the ethylene amine part and producing a dangling vinyl amine as the major degradation product. A selective increase in the molecular mobility at this site was confirmed by a two-dimensional, local-field wide-line separation experiment. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 865,887, 2006 [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] Polarized Raman scattering and phase transition studies of n -diethylenediammonium monohydrogenmonophosphate dihydrateJOURNAL OF RAMAN SPECTROSCOPY, Issue 12 2006S. Elleuch Abstract Polarized Raman scattering studies at room temperature were carried out on N -diethylenediammonium monohydrogenmonophosphate dihydrate single crystal samples, abbreviated N -DDHP. An assignment of the normal modes is proposed based on group theory analysis and correlations with previous data reported for other homologous hydrogen-bonded systems. A careful analysis of the vibrational spectra shows that the assignment of the fundamental vibrational modes can be done based on phosphate, organic and water group vibrations. In addition, differential scanning calorimetry measurement and a Raman spectroscopic study at several temperatures in the range 100,300 K are presented. The occurrence of a low-temperature phase transition near 220 K is well evidenced. From the temperature-dependence behavior of some internal and external modes, this transition is interpreted by an order,disorder transition which involves the structural modifications of both anion and cation groups. Copyright © 2006 John Wiley & Sons, Ltd. [source] Solvent Effects on the NMR Chemical Shifts of Imidazolium-Based Ionic Liquids and Cellulose ThereinMACROMOLECULAR SYMPOSIA, Issue 2 2010Stephanie Hesse-Ertelt Abstract The interactions of ionic liquids (IL) with solvents usually used in liquid-state nuclear magnetic resonance (NMR) spectroscopy are studied. The 1H- and 13C-NMR chemical shift values of 1-n-butyl-3-methyl (BM)- and 1-ethyl-3-methyl (EM)-substituted imidazolium (IM) -chlorides (Cl) and -acetates (Ac) are determined before and after diluting with deuterated solvents (DMSO-d6, D2O, CD3OD, and CDCl3). The dilution offers structural modifications of the IL due to the solvents capacity to ionization. For further investigation of highly viscous cellulose dopes made of imidazolium-based IL, solid-state NMR spectroscopy enables the reproducibility of liquid-state NMR data of pure IL. The correlation of liquid- and solid-state NMR is shown on EMIM-Ac and cellulose/EMIM-Ac dope (10 wt %). [source] Platinum-based anticancer agents: Innovative design strategies and biological perspectivesMEDICINAL RESEARCH REVIEWS, Issue 5 2003Yee-Ping Ho Abstract The impact of cisplatin on cancer chemotherapy cannot be denied. Over the past 20 years, much effort has been dedicated to discover new platinum-based anticancer agents that are superior to cisplatin or its analogue, carboplatin. Most structural modifications are based on changing one or both of the ligand types coordinated to platinum. Altering the leaving group can influence tissue and intracellular distribution of the drug, whereas the carrier ligand usually determines the structure of adducts formed with DNA. DNA,Pt adducts produced by cisplatin and many of its classical analogues are almost identical, and would explain their similar patterns of tumor sensitivity and susceptibility to resistance. Recently some highly innovative design strategies have emerged, aimed at overcoming platinum resistance and/or to introduce novel mechanisms of antitumor action. Platinum compounds bearing the 1,2-diaminocyclohexane carrier ligand; and those of multinuclear Pt complexes giving rise to radically different DNA,Pt adducts, have resulted in novel anticancer agents capable of circumventing cisplatin resistance. Other strategies have focused on integrating biologically active ligands with platinum moieties intended to selectively localizing the anticancer properties. With the rapid advance in molecular biology, combined with innovation, it is possible new Pt-based anticancer agents will materialize in the near future. © 2003 Wiley Periodicals, Inc. Med Res Rev, 23, No. 5, 633,655, 2003 [source] Polymethylene tetraamine backbone as template for the development of biologically active polyaminesMEDICINAL RESEARCH REVIEWS, Issue 2 2003Carlo Melchiorre Abstract The concept that polyamines may represent a universal template in the receptor recognition process is embodied in the design of ligands for different biological targets. As a matter of fact, the insertion of different pharmacophores onto the polymethylene tetraamine backbone can tune both affinity and selectivity for any given receptor. The application of this approach provided a prospect of modifying benextramine (1) structure to achieve specific recognition of muscarinic receptors that led to the discovery of methoctramine (2), which is widely used as a pharmacological tool for muscarinic receptor characterization. In turn, appropriate structural modifications performed on the structure of methoctramine led to the discovery of new polyamines endowed with high affinity and selectivity for (a) muscarinic receptor subtypes, (b) Gi proteins, and (c) muscle-type nicotinic receptors. Thus, polyamines tripitramine (9) and spirotramine (33), among others, were designed, which were shown to be highly selective for muscarinic M2 and M1 receptors, respectively. Several polyamines have been discovered, which inhibit noncompetitively a closed state of the nicotinic receptor. These ligands, such as 66, resulted in important tools for elucidating the mode and site of interaction of polyamines with the ion channel. It was discovered that reducing the flexibility of the diaminohexane spacer of methoctramine led to polyamines, such as 70, which are endowed with a biological profile significantly different from that of the prototype. Most likely, tetraamine (70) is a potent activator of Gi proteins. Finally, the universal template approach formed the basis for modifying benextramine (1) structure to the design of ligands, which display affinity for acetylcholinesterase and muscarinic M2 receptors. Thus, these polyamines, such as caproctamine (78), could have potential in the investigation of Alzheimer disease. © 2002 Wiley Periodicals, Inc. Med Res Rev, 23, No. 2, 200,233, 2003 [source] Role of drug metabolism in drug discovery and developmentMEDICINAL RESEARCH REVIEWS, Issue 5 2001Gondi N. Kumar Abstract Metabolism by the host organism is one of the most important determinants of the pharmacokinetic profile of a drug. High metabolic lability usually leads to poor bioavailability and high clearance. Formation of active or toxic metabolites will have an impact on the pharmacological and toxicological outcomes. There is also potential for drug,drug interactions with coadministered drugs due to inhibition and/or induction of drug metabolism pathways. Hence, optimization of the metabolic liability and drug,drug interaction potential of the new chemical entities are some of the most important steps during the drug discovery process. The rate and site(s) of metabolism of new chemical entities by drug metabolizing enzymes are amenable to modulation by appropriate structural changes. Similarly, the potential for drug,drug interactions can also be minimized by appropriate structural modifications to the drug candidate. However, the optimization of the metabolic stability and drug,drug interaction potential during drug discovery stage has been largely by empirical methods and by trial and error. Recently, a lot of effort has been applied to develop predictive methods to aid the optimization process during drug discovery and development. This article reviews the role of drug metabolism in drug discovery and development. © 2001 John Wiley & Sons, Inc. Med Res Rev, 21, No. 5, 397,411, 2001 [source] | |||||||||||||||||||||||||||||||||||||