The Experts below are selected from a list of 267 Experts worldwide ranked by ideXlab platform
Peter Hesemann - One of the best experts on this subject based on the ideXlab platform.
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Synthesis and antimicrobial properties of new chitosan derivatives containing Guanidinium groups.
Carbohydrate polymers, 2020Co-Authors: Ahmed Salama, Mohamed S. Hasanin, Peter HesemannAbstract:Abstract New chitosan derivatives bearing Guanidinium functions were synthesized following different synthesis strategies. N-Guanidinium chitosan acetate and N-Guanidinium chitosan chloride were synthesized by direct reaction between chitosan and cyanamide in the presence of scandium(III) triflate. The synthesis of N-Guanidinium chitosan (N,N′-dicyclohexyl) chloride and N-Guanidinium chitosan (N-(3-dimethylaminopropyl)-N'-ethyl hydrochloride) chloride involved the reaction of chitosan with carbodiimides in ionic liquid. The chitosan derivatives were characterized by analytical techniques including 13C solid state NMR, FT-IR spectroscopies, thermogravimetry and elemental analysis. The antimicrobial properties of chitosan and the new derivatives were investigated using the minimal inhibitory concentration (MIC) technique. All new guanylated chitosan derivatives displayed high antimicrobial activity in comparison with neat chitosan. The N-Guanidinium chitosan acetate reduced the time required for killing to half in comparison with chitosan and recorded MIC values less than 3.125 mg/mL against all assayed microorganisms. This work opens new perspectives for using chitosan derivatives as antimicrobial surfaces.
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Synthesis of N-Guanidinium-Chitosan/Silica Hybrid Composites: Efficient Adsorbents for Anionic Pollutants
Journal of Polymers and the Environment, 2017Co-Authors: Ahmed Salama, Peter HesemannAbstract:A new chitosan derivative, N-Guanidinium chitosan acetate, has been synthesized by direct guanylation of chitosan by cyanamide in presence of scandium(III) triflate under mild acidic condition. Starting from this material, N-Guanidinium chitosan/silica microhybrids were prepared via a sol gel method using 3-glycidoxypropyl trimethoxysilane as silica precursor. Both N-Guanidinium chitosan and the N-Guanidinium chitosan/silica hybrid were characterized by a range of analytical techniques such as 29Si/13C solid state NMR, FT-IR, scanning electron microscopy, thermogravimetry and elemental analysis. The characterization of the chitosan/silica hybrid indicated that this material is a highly hydrophilic nanocomposite material containing an organic core and a highly condensed silica shell. The N-Guanidinium chitosan/silica microhybrids display excellent adsorption properties for anionic dyes such as methyl orange (MO) with very high capacities up to 917 mg/g. The fixation of MO as anionic dye was investigated in detail as a function of contact time, pH and the MO concentration. The adsorption kinetics of MO on N-Guanidinium chitosan/silica microhybrids was more accurately described by pseudo second-order model. Langmuir isotherm model exhibited a better fit with adsorption data than Freundlich isotherm model. This work opens new possibilities for using N-Guanidinium chitosan as a reusable adsorbent for water purification.Graphical Abstract
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Guanidinium vs. ammonium surfactants in soft-templating approaches: Nanostructured silica and zwitterionic i-silica from complementary precursor-surfactant ion-pairs
European Journal of Inorganic Chemistry, 2012Co-Authors: Samir El Hankari, Peter HesemannAbstract:We investigated the behaviour of Guanidinium and ammonium surfactants as structure directing agents in template directed hydrolysis-polycondensation processes. For this purpose, a series of Guanidinium surfactants was synthesized from long chain substituted primary amines. In a first series of experiments, we compared nanostructured silicas obtained via template directed hydrolysis-polycondensation reactions of TEOS in the presence of either Guanidinium or ammonium based surfactants. We observed that typical MCM-41-type silicas displaying 2D hexagonal architecture were obtained in the presence of ammonium surfactants such as cetyl-trimethylammonium bromide (CTAB). In contrast, the use of Guanidinium surfactants led to the formation of hollow silica spheres or spherical silica nanoparticles. In a second time, we investigated the use of ammonium and Guanidinium surfactants in the formation of periodic mesoporous organosilicas containing ionic groups (i-silica). We studied in particular template directed hydrolysis-polycondensation reactions of zwitterionic ammonium sulfonate and ammonium carboxylate precursors. We observed that both reactions involving the ammonium-sulfonate precursor yielded structured i-silicas. In contrast, hydrolysis-polycondensation reactions of the ammonium carboxylate precursor afforded structured materials only in the presence of the Guanidinium surfactant, whereas amorphous materials were obtained in the presence of CTAB. This result reflects specific surfactant-precursor interactions and suggests that the nature of both surfactant and precursor are determinant for the formation of i-silica materials displaying regular architectures on a mesoscopic length scale.
Pavel Jungwirth - One of the best experts on this subject based on the ideXlab platform.
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Orientational Dependence of the Affinity of Guanidinium Ions to the Water Surface
2016Co-Authors: Erik Wernersson, Jan Heyda, Mario Vazdar, Mikael Lund, Philip E. Mason, Pavel JungwirthAbstract:The behavior of Guanidinium chloride at the surface of aqueous solutions is investigated using classical molecular dynamics (MD) simulations. It is found that the population of Guanidinium ions oriented parallel to the interface is greater in the surface region than in bulk. The opposite is true for ions in other orientations. Overall, Guanidinium chloride is depleted in the surface region, in agreement with the fact that the addition of Guanidinium chloride increases the surface tension of water. The orientational dependence of the surface affinity of the Guanidinium cation is related to its anisotropic hydration. To bring the ion to the surface in the parallel orientation does not require hydrogen bonds to be broken, in contrast to other orientations. The surface enrichment of parallel-oriented Guanidinium indicates that its solvation is more favorable near the surface than in bulk solution for this orientation. The dependence of the bulk and surface properties of Guanidinium on the force field parameters is also investigated. Despite significant quantitative differences between the force fields, the surface behavior is qualitatively robust. The implications for the orientations of the Guanidinium groups of arginine side chains on protein surfaces are also outlined
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Orientational Dependence of the Affinity of Guanidinium Ions to the Water Surface
The journal of physical chemistry. B, 2011Co-Authors: Erik Wernersson, Jan Heyda, Mario Vazdar, Mikael Lund, Philip E. Mason, Pavel JungwirthAbstract:The behavior of Guanidinium chloride at the surface of aqueous solutions is investigated using classical molecular dynamics (MD) simulations. It is found that the population of Guanidinium ions oriented parallel to the interface is greater in the surface region than in bulk. The opposite is true for ions in other orientations. Overall, Guanidinium chloride is depleted in the surface region, in agreement with the fact that the addition of Guanidinium chloride increases the surface tension of water. The orientational dependence of the surface affinity of the Guanidinium cation is related to its anisotropic hydration. To bring the ion to the surface in the parallel orientation does not require hydrogen bonds to be broken, in contrast to other orientations. The surface enrichment of parallel-oriented Guanidinium indicates that its solvation is more favorable near the surface than in bulk solution for this orientation. The dependence of the bulk and surface properties of Guanidinium on the force field paramete...
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Like-Charge Guanidinium Pairing from Molecular Dynamics and Ab Initio Calculations
The journal of physical chemistry. A, 2011Co-Authors: Mario Vazdar, Jan Heyda, Jiří Vymětal, Jiří Vondrášek, Pavel JungwirthAbstract:Pairing of Guanidinium moieties in water is explored by molecular dynamics simulations of short arginine-rich peptides and ab initio calculations of a pair of Guanidinium ions in water clusters of increasing size. Molecular dynamics simulations show that, in an aqueous environment, the diarginine Guanidinium like-charged ion pairing is sterically hindered, whereas in the Arg-Ala-Arg tripeptide, this pairing is significant. This result is supported by the survey of protein structure databases, where it is found that stacked arginine pairs in dipeptide fragments exist solely as being imposed by the protein structure. In contrast, when two arginines are separated by a single amino acid, their Guanidinium groups can freely approach each other and they frequently form stacked pairs. Molecular dynamics simulations results are also supported by ab initio calculations, which show stabilization of stacked Guanidinium pairs in sufficiently large water clusters.
Eric V. Anslyn - One of the best experts on this subject based on the ideXlab platform.
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Solid phase synthesis of oligomeric Guanidiniums
Tetrahedron, 1998Co-Authors: Stephen E. Schneider, Patricia A. Bishop, Mary Alice Salazar, Owen A. Bishop, Eric V. AnslynAbstract:Abstract Oligomers containing Guanidinium linkages prepared via solid phase organic synthesis are of interest as possible therapeutic agents and in the assembly of supramolecular architectures. Efficient routes to these oligomers must be developed before their potential may be fully realized. Herein, four routes for their stepwise solid phase synthesis are described. In the first, a resin-bound thiourea was converted to a Guanidinium using 2-chloro-1-methylpyridinium iodide. The second method utilized aza-Wittig couplings to prepare Guanidiniums from resin-bound carbodiimides. Next, highly activated monomers prepared from bis-tert-butyloxycarbonythioureas and 2,4-dinitrofluorobenzene formed Guanidiniums upon reaction with terminal amines. The optimum route, however, relied upon the 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride promoted coupling of a protected thiourea monomer with a resin-bound amine to produce the Guanidinium linkage. The thiourea monomers for this method are easily prepared from mono-protected diamines and benzoyl- or Fmoc-isothiocyanate. The procedure is straightforward, proceeds cleanly in a relatively short period of time, and is compatible with several functional groups.
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Using Guanidinium groups for the recognition of RNA and as catalysts for the hydrolysis of RNA
Bioorganic & medicinal chemistry, 1997Co-Authors: Denise M. Perreault, Larry A. Cabell, Eric V. AnslynAbstract:The Guanidinium functional group is commonly used in nature to recognize and bind anions through ion pairing and hydrogen bonding. Specific hydrogen-bonding patterns can be found in crystal structures of simple Guanidinium salts. Analysis of these simple salts reveals a variety of features which are found in natural systems. These features have been applied to a series of artificial phosphodiesterases for RNA. These receptors incorporate Guanidinium groups positioned to mimic the hydrogen-bonding patterns found in simple Guanidinium salts and natural enzymes. This paper outlines general Guanidinium hydrogen-bonding patterns. Next, the complexation of phosphodiesters with a series of artificial receptors are analyzed in terms of counterions, solvent mixtures, and cavity flexibility. In addition, strategies to enhance catalysis through a pKa analysis of phosphoranes are addressed. Next, we describe how our findings were incorporated into second generation receptors/catalysts. Finally, our future work is discussed.
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The Guanidinium Group: Its Biological Role and Synthetic Analogs
Bioorganic Chemistry Frontiers, 1993Co-Authors: Christine L. Hannon, Eric V. AnslynAbstract:The Guanidinium functional group is commonly used by proteins and enzymes to recognize and bind anions using ion pairing and hydrogen bonding. The specific patterns of hydrogen bonding and the great basicity of the guanidine functional group allow it to play several key roles in recognition and catalysis. This chapter outlines recent findings of Guanidinium groups in natural systems and advances in synthetic receptors that incorporate Guanidinium groups to mimic natural receptors and catalysts. First, general structural considerations that relate to the recognition and binding with Guanidinium will be analyzed. Next, phosphodiester hydrolyzing enzymes and RNA and DNA binding proteins will be analyzed. These systems are discussed in detail as specific examples of the unique recognition properties of Guanidinium and its ability to act as an electrophilic catalyst. Several enzymes and proteins which possess essential arginine groups will be briefly introduced. Finally, several artificial receptors will be analyzed in terms of hydrogen bonding patterns and the ability to use the Guanidinium functional group to specifically recognize, orient, and enhance the cleavage of phosphodiesters.
Irwin Fridovich - One of the best experts on this subject based on the ideXlab platform.
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Competitive Inhibition of Xanthine Oxidase by Guanidinium: Dependence upon Monovalent Anions and Effects on Production of Superoxide
Archives of biochemistry and biophysics, 1993Co-Authors: A. Hausladen, Irwin FridovichAbstract:Guanidinium chloride inhibits xanthine oxidase competitively with respect to xanthine. Although previously attributed solely to the Guanidinium cation, it is now apparent that this inhibition owes much to the counter anion. Thus KCl or KBr, which were not themselves inhibitory, markedly increased the inhibitory potency of Guanidinium sulfate. Weak binding of the Guanidinium cation evidently creates a binding site for a monovalent anion, whose subsequent binding then stabilizes the binding of the Guanidinium. In effect the ion pair is bound to the catalytic center. The proportion of univalent reduction of dioxygen by xanthine oxidase, at fixed concentrations of xanthine and dioxygen and at fixed pH, can be markedly increased by addition of a competitive inhibitor such as Guanidinium bromide.
Chaitali Mukhopadhyay - One of the best experts on this subject based on the ideXlab platform.
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microsecond molecular dynamics simulation of Guanidinium chloride induced unfolding of ubiquitin
Physical Chemistry Chemical Physics, 2014Co-Authors: Manoj Mandal, Chaitali MukhopadhyayAbstract:An all atom molecular dynamics simulation was used to explore the atomic detail mechanism of Guanidinium induced unfolding of the protein ubiquitin. Ubiquitin unfolds through pre-unfolded (intermediate) states, i.e. Guanidinium induced unfolding of ubiquitin appears to be a multi-step process, and loss of hydrophobic contacts of C-terminal residues is crucial for ubiquitin unfolding. Free-energy landscapes show that barrier separation between folded and unfolded basins is ∼5.0 kcal mol−1, and both the basins are of comparable energy. It was observed that Guanidinium ions interact directly with ubiquitin. Favorable electrostatic interaction is the main driving force for such accumulation of Guanidinium ions near protein, but van der Waals energy also contributes. RDF plots show that accumulation of Guanidinium ions near specific residues is the main cause for destabilization of intra-residue interactions crucial to maintain the three-dimensional fold of the protein. One salt-bridge interaction between Lys11 and Glu34 appears to be important to maintain the crystal structure of ubiquitin and this salt-bridge can map the unfolding process of ubiquitin.
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concentration dependent like charge pairing of Guanidinium ions and effect of Guanidinium chloride on the structure and dynamics of water from all atom molecular dynamics simulation
Physical Review E, 2013Co-Authors: Manoj Mandal, Chaitali MukhopadhyayAbstract:An all-atom molecular dynamics simulation shows concentration-dependent like-charge ion pairing of the Guanidinium ion in an aqueous solution of Guanidinium chloride. We have observed two types of like-charge ion pairing for Guanidinium ions, namely, stacked ion pairs and solvent-separated ion pairs. Interestingly, both of these like-charge ion-pair formations are dependent on the concentration of Guanidinium chloride in water. The probability of stacked like-charge ion-pair formation decreases, whereas, the probability of solvent-separated like-charge pairing increases as the concentration of Guanidinium chloride increases, which is shown from radial distribution functions and is confirmed from the energy calculations. Besides like-charge ion-pair formation, we also investigated Guanidinium chloride induced changes in water structure. Hydrogen-bond analysis indicates that Guanidinium chloride does not alter the strict-hydrogen-bonding patterns of water, whereas, it breaks the bend-hydrogen bond and the non-hydrogen-bonding patterns. Tetrahedral order, nearest neighbor orientation, and distance distribution of water molecules around a probe water molecule show the extent of water structure distortion.
