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Ligand Effects (ligand + effects)

Distribution by Scientific Domains

Chemistry	88%

Selected Abstracts

Ligand Effects on the Mechanisms of Thermal Bond Activation in the Gas-Phase Reactions NiX+/CH4,Ni(CH3)+/HX (X=H, CH3, OH, F).

HELVETICA CHIMICA ACTA, Issue 12 2008
Short Communication
Abstract The thermal ion-molecule reactions NiX++CH4,Ni(CH3)++HX (X=H, CH3, OH, F) have been studied by mass spectrometric methods, and the experimental data are complemented by density functional theory (DFT)-based computations. With regard to mechanistic aspects, a rather coherent picture emerges such that, for none of the systems studied, oxidative addition/reductive elimination pathways are involved. Rather, the energetically most favored variant corresponds to a , -complex-assisted metathesis (, -CAM). For X=H and CH3, the ligand exchange follows a ,two-state reactivity (TSR)' scenario such that, in the course of the thermal reaction, a twofold spin inversion, i.e., triplet,singlet,triplet, is involved. This TSR feature bypasses the energetically high-lying transition state of the adiabatic ground-state triplet surface. In contrast, for X=F, the exothermic ligand exchange proceeds adiabatically on the triplet ground state, and some arguments are proposed to account for the different behavior of NiX+/Ni(CH3)+ (X=H, CH3) vs. NiF+. While the couple Ni(OH)+/CH4 does not undergo a thermal ligand switch, the DFT computations suggest a potential-energy surface that is mechanistically comparable to the NiF+/CH4 system. Obviously, the ligands X act as a mechanistic distributor to switch between single vs. two-state reactivity patterns. [source]

Significantly Improved Method for the Pd-Catalyzed Coupling of Phenols with Aryl Halides: Understanding Ligand Effects.

CHEMINFORM, Issue 42 2006
Carlos H. Burgos
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]

Significantly Improved Method for the Pd-Catalyzed Coupling of Phenols with Aryl Halides: Understanding Ligand Effects.

Ligand Effects in the Synthesis of N-Heterocycles by Intramolecular Heck Reactions.

CHEMINFORM, Issue 26 2006
Emma L. Cropper
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]

Axial Ligand Effects: Utilization of Chiral Sulfoxide Additives for the Induction of Asymmetry in (Salen)ruthenium(II) Olefin Cyclopropanation Catalysts.

CHEMINFORM, Issue 42 2005
Jason A. Miller
Abstract For Abstract see ChemInform Abstract in Full Text. [source]

Ligand Effects in the Rh(II) Catalyzed Reaction of ,-Diazo Ketoamides.

CHEMINFORM, Issue 12 2005
Jose M. Mejia-Oneto
Abstract For Abstract see ChemInform Abstract in Full Text. [source]

Ligand effects upon deuterium exchange in arenes mediated by [Ir(PR3)2(cod)]+.BF4,

JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS, Issue 1 2004
George J. Ellames
Abstract A series of complexes of general form [Ir(PR3)2(cod)]+ has been prepared and used, without isolation, to mediate deuteration of a range of model substrates. The data suggest that, with many substrates, basicity of the phosphine ligands bound to iridium is an important factor influencing substrate selectivity and the efficiency of deuteration. In addition, the spectrum of activity of iridium complexes bearing pure donor ligands is different in many cases to that of complexes where the ligands are known to be ,-acids. Copyright © 2003 John Wiley & Sons, Ltd. [source]

Kinetic analysis of effector modulation of butyrylcholinesterase-catalysed hydrolysis of acetanilides and homologous esters

FEBS JOURNAL, Issue 10 2008
Patrick Masson
The effects of tyramine, serotonin and benzalkonium on the esterase and aryl acylamidase activities of wild-type human butyrylcholinesterase and its peripheral anionic site mutant, D70G, were investigated. The kinetic study was carried out under steady-state conditions with neutral and positively charged aryl acylamides [o -nitrophenylacetanilide, o -nitrotrifluorophenylacetanilide and m -(acetamido) N,N,N -trimethylanilinium] and homologous esters (o -nitrophenyl acetate and acetylthiocholine). Tyramine was an activator of hydrolysis for neutral substrates and an inhibitor of hydrolysis for positively charged substrates. The affinity of D70G for tyramine was lower than that of the wild-type enzyme. Tyramine activation of hydrolysis for neutral substrates by D70G was linear. Tyramine was found to be a pure competitive inhibitor of hydrolysis for positively charged substrates with both wild-type butyrylcholinesterase and D70G. Serotonin inhibited both esterase and aryl acylamidase activities for both positively charged and neutral substrates. Inhibition of wild-type butyrylcholinesterase was hyperbolic (i.e. partial) with neutral substrates and linear with positively charged substrates. Inhibition of D70G was linear with all substrates. A comparison of the effects of tyramine and serotonin on D70G versus the wild-type enzyme indicated that: (a) the peripheral anionic site is involved in the nonlinear activation and inhibition of the wild-type enzyme; and (b) in the presence of charged substrates, the ligand does not bind to the peripheral anionic site, so that ligand effects are linear, reflecting their sole interaction with the active site binding locus. Benzalkonium acted as an activator at low concentrations with neutral substrates. High concentrations of benzalkonium caused parabolic inhibition of the activity with neutral substrates for both wild-type butyrylcholinesterase and D70G, suggesting multiple binding sites. Benzalkonium caused linear, noncompetitive inhibition of the positively charged aryl acetanilide m -(acetamido) N,N,N -trimethylanilinium for D70G, and an unusual mixed-type inhibition/activation (, > , > 1) for wild-type butyrylcholinesterase with this substrate. No fundamental difference was observed between the effects of ligands on the butyrylcholinesterase-catalysed hydrolysis of esters and amides. Thus, butyrylcholinesterase uses the same machinery, i.e. the catalytic triad S198/H448/E325, for the hydrolysis of both types of substrate. The differences in response to ligand binding depend on whether the substrates are neutral or positively charged, i.e. the differences depend on the function of the peripheral site in wild-type butyrylcholinesterase, or the absence of its function in the D70G mutant. The complex inhibition/activation effects of effectors, depending on the integrity of the peripheral anionic site, reflect the allosteric ,cross-talk' between the peripheral anionic site and the catalytic centre. [source]

Multivalent Drug Design and Inhibition of Cholera Toxin by Specific and Transient Protein,Ligand Interactions

CHEMICAL BIOLOGY & DRUG DESIGN, Issue 5 2008
Jiyun Liu
Multivalent inhibitors of the cholera toxin B pentamer are potential therapeutic drugs for treating cholera and serve as models for demonstrating multivalent ligand effects through a structure-based approach. A crucial yet often overlooked aspect of multivalent drug design is the length, rigidity and chemical composition of the linker used to connect multiple binding moieties. To specifically study the role of chemical linkers in multivalent ligand design, we have synthesized a series of compounds with one and two binding motifs connected by several different linkers. These compounds have affinity for and potency against the cholera toxin B pentamer despite the fact that none can simultaneously bind two toxin receptor sites. Results from saturation transfer difference NMR reveal transient, non-specific interactions between the cholera toxin and linker groups contribute significantly to overall binding affinity of monovalent compounds. However, the same random protein,ligand interactions do not appear to affect binding of bivalent molecules. Moreover, the binding affinities and potencies of these ,non-spanning' bivalent ligands appear to be wholly independent of linker length. Our detailed analysis identifies multiple effects that account for the improved inhibitory potencies of bivalent ligands and suggest approaches to further improve the activity of this class of compounds. [source]