Types of Ligand

Surface heterogeneity may merely be a reflection of different types of chemisorption and chemisorption sites, as in the examples of Figs. XVIII-9 and XVIII-10. The presence of various crystal planes, as in powders, leads to heterogeneous adsorption behavior the effect may vary with particle size, as in the case of O2 on Pd [107]. Heterogeneity may be deliberate many catalysts consist of combinations of active surfaces, such as bimetallic alloys. In this last case, the surface properties may be intermediate between those of the pure metals (but one component may be in surface excess as with any solution) or they may be distinctly different. In this last case, one speaks of various effects ensemble, dilution, ligand, and kinetic (see Ref. 108 for details).  [c.700]

Automated, miniaturized, and parallelized synthesis and testing (combinatorial chemistry/high-throughput screening) are accelerating the development of a complex of methods for data mining and computer screening (virtual screening) of object libraries. Clusters of objects are recognized (duster analysis) on the basis of the estimation of the distances in the descriptor space (dissimilarities). In the case of object selection, dasses that are as diverse as possible are selected so that all the different types of properties (e.g., bioactivities) within a larger collection are sampled using as few objects as possible (diversity analysis). Key chemical features and the spatial relationships among them that are considered to be responsible for a desired biological activity may be identified (pharmacophore recognition) using local similarity, e.g., via common substructures in sets of active molecules pharmacophore searching in 3D databases (see below) may be carried out using a pharmacophore as the query. Shape similarity of ligands to a receptor site (ligand docking) may be used for finding structures that fit into proteins.  [c.313]

Research on ligand effects in aqueous solution has mainly focused on two types of organic reactions  [c.76]

The most important types of reactions are precipitation reactions, acid-base reactions, metal-ligand complexation reactions, and redox reactions. In a precipitation reaction two or more soluble species combine to produce an insoluble product called a precipitate. The equilibrium properties of a precipitation reaction are described by a solubility product.  [c.175]

Activation of the Stationary-Phase Surface. Activation of polysaccharide, siHca, or polyacrylamide stationary phases involve the formation of a reactive intermediate, covalendy attached to the surface, to which a difimctional alkyl-, aryl-, or glycol spacer is subsequently joined. The other end of the spacer is subsequently reacted with the ligand. Cyanogen bromide, CNBr, has been widely used to activate agarose and dextran gels (49). The attachment of ligands, and sometimes activation of supports, is generally carried out in the laboratories of the chromatography process developers because fully prepared affinity stationary phases are not as widely available as stationary phases for the other types of chromatography.  [c.57]

Color from Charge Transfer. This mechanism is best approached from MO theory, although ligand field theory can also be used. There are several types of color-producing charge-transfer (CT) processes.  [c.419]

The influence of flow rates of eluent and sample solution, amount of ligand, types and least amount of eluent for elution of Cobalt ion from column were investigated.  [c.284]

The most important types of reactions are precipitation reactions, acid-base reactions, metal-ligand complexation reactions, and redox reactions. In a precipitation reaction two or more soluble species combine to produce an insoluble product called a precipitate. The equilibrium properties of a precipitation reaction are described by a solubility product.  [c.175]

One can see that at this point only two types of experimental data are used for the analysis of a ligand-receptor system ligand chemical structure (from which one generates the 3D pharmacophore conformation) and the primary receptor sequence (which is used to generate the 3D receptor model). The most important property of the receptors, however, is their ability to discriminate ligands on the basis of their chemical structure we quantify this as ligand binding constants. The ability to reproduce binding constants, or at least their relative order, is the most sensitive test of any putative receptor model. The technique that, in principle, can provide such data is the free energy simulations/thermodynamic  [c.353]

The stability of the phosphine adducts is notable as is the fact that thioethers readily form such adducts whereas ethers do not. Bis-ligand adducts of moderate stability play an important role in activating decaborane for several types of reaction to be considered in moie detail in subsequent paragraphs, e.g.  [c.163]

In addition to acting as an rf ligand, CgHg can coordinate in other modes, " some of which are illustrated in Fig. 19.33. Many of these complexes show fluxional behaviour in solution (p. 935) and the distinction between the various types of bonding is not as clear-cut as implied by the limiting structures in Fig. 19.33.  [c.943]

The Sharpless dihydroxylation reaction has great potential for the enantioselec-tive synthesis of natural products. Which of the possible enantiomers/diastereomers is formed, is determined by the actual structure of the chiral catalyst complex formed from osmium tetroxide and the enantiomerically pure dihydroquinine or dihydroquinidine-type ligand. By switching to the respective other alkaloid ligand, the opposite enantiomer/diastereomer can be obtained as the major or even single product. 7ra -alkenes are better substrates and react with higher enantioselectivity than c -alkenes. The latter alkene geometry is less favourable for the binding site of the chiral catalyst complex. Many types of functional groups are tolerated as substituents at the double bond or in its vicinity, e.g. ketones, esters, amides, carbamates, halogenes, ethers and silyls. The reaction conditions are mild and the yields are generally good.  [c.258]

The power of combinatorial chemistry resides in both the large numbers of compounds that can be prepared within a very short period of time and the rapid assay and deconvolution techniques that may be used for testing to discover the optimal or near-optimal selector within the library. This availability of libraries encompassing a broad diversity of ligand types enables rapid identification of suitable selector families, their comparative screening, and the rapid preparation of custom-made separation media for the resolution of specific racemates [99]. As an additional benefit, studies carried out with broad arrays of structurally related families of selectors can further improve the general understanding of chiral recognition.  [c.90]

The ligand MOs are of two types a MOs, which are cylindrically symmetrical about the metal-ligand bond, and n MOs which are not. The a type of metal-ligand bonding is usually  [c.273]

In this section we describe the behavior of a ligand subjected to three types of external forces a constant force, forces exerted by a moving stiff harmonic spring, and forces exerted by a soft harmonic spring. We then present a method of reconstruction of the potential of mean force from SMD force measurements employing a stiff spring (Izrailev et al., 1997 Balsera ct al., 1997).  [c.55]

Different types of other coal liquefaction processes have been also developed to convert coals to liqnid hydrocarbon fnels. These include high-temperature solvent extraction processes in which no catalyst is added. The solvent is usually a hydroaromatic hydrogen donor, whereas molecnlar hydrogen is added as a secondary source of hydrogen. Similar but catalytic liquefaction processes use zinc chloride and other catalysts, usually under forceful conditions (375-425°C, 100-200 atm). In our own research, superacidic HF-BFo-induced hydroliquefaction of coals, which involves depolymerization-ionic hydrogenation, was found to be highly effective at relatively modest temperatnres (150-170°C).  [c.132]

Among the eadiest reports of chiral separations by Hquid chromatography were based on work done by Davankov using ligand exchange (66). These types of columns are available from Phenomenex, J. T. Baker, and Regis Technologies, Inc. As noted previously in the discussion regarding ligand exchange in tic, chiral separations by ligand exchange in hplc is accompHshed using bidentate amino acid ligands, immobilized on a chromatographic substrate, and a divalent metal cation which participates in the formation of a diastereomeric complex with a bidentate chiral analyte and the ligand. Although almost any amino acid can form the basis for the chiral selector, proline and hydroxyproHne exhibit the most widespread utiHty. Also, although other metals can be used, copper(II) is usually the metal of choice and is added to the aqueous buffer mobile phase.  [c.63]

In the presence of piperidine, iron(II) sulfate or chloride reacts with 2,4-pentanedione in degassed water under nitrogen to form hydrated bis(2,4-pentanedionato)iron(II). Drying under high vacuum affords the anhydrous compound bis(2,4-pentanedionato)iron(II) [14024-17-0], Fe(C H202)2 or Fe(acac)2 (2). Although the stoichiometry suggests the compound is simple, it is coordinatively unsaturated and has an unusual tetrameric stmcture which consists of two asymmetric Fe2 linked by long Fe—C bonds. The acac-ligand which contains the methylene carbon bound to iron in the second asymmetric unit also has an oxygen atom that bridges the two iron atoms of the first asymmetric unit. Fe(acac)2 reacts with numerous bases to form six-coordinate adducts. AH of the iron(II) acac complexes are air sensitive. Fe(acac)2 is used as a catalyst in several types of reactions.  [c.438]

NMD A LGICs exhibit slow activation kinetics but are highly permeable to calcium relative to other glutamate receptors. They are known for their involvement in calcium-dependent phenomena such as the formation of long-term potentiation (LTP) and long-term depression (LTD), that are thought to underlie learning and memory. However, excess activation of NMD A receptors may contribute to pathologic processes such as excess excitabiUty and calcium-dependent cell death following the hypoxia or ischemia associated with stroke. While MK 801 encountered problems in clinical trials for this indication, CGS 19755 may represent the first NMD A receptor ligand for the treatment of stroke. NMD A receptor modulators may also have potential as anxiolytics, antipsychotics, cognition enhancers, and in the treatment of certain types of pain.  [c.553]

The original formaUsm of dmg—receptor interactions assumed only two types of dmgs, the agonists and the antagonists. However, this was an oversimplification. In many homologous series, such as /V-a1ky1trimethy1 ammonium salts active at cholinergic receptors or Ai-alkylcatecholarnines active at adrenergic receptors (Fig. 6), there are clearly ligands that even at saturating concentration produce only partial response (1,10,35). Additionally, the use of irreversibly acting receptor antagonists, eg, 2-halogeneothylamines such as phenoxybenzamine, which react covalentiy and eliminate receptors from response generation, did not cause the anticipated reduction of maximum response. Rather, the observed response was frequendy a parallel rightward shift of the dose—response curve prior to a depression of response such as that shown in Figure 8b (1,10). These observations indicate that response may not be proportional to receptor occupancy and that spare receptors, ie, receptors in excess of those necessary to generate maximum response, exist. These conclusions have been demonstrated by studies of ligand binding to receptors using radioligands (see Radioactive tracers) and studies of the coupling of receptors to the effector units.  [c.275]

After it was found that the rhodium carbonyl—triphenylphospine—complex, HRh(CO)[P(CgH 2]2, is very effective in increasing the yield of hydroformylated product, many studies were done in greater detail with rhodium catalysts (13). The reactions are generally performed under the following conditions 60—100°C, 0.69—3.4 MPa (7—35 kg/cm ), H2 to CO molar ratio more than one, and excess ligand phosphine. By-products are branched aldehyde (2), 2-hydroxymethylpropionaldehyde [38433-80-6] propionaldehyde produced by isomerization of aHyl alcohol and / -propanol produced by hydrogenation of aHyl alcohol The types of by-products and the yield are affected by the reaction temperature, the molar ratio between phosphine and rhodium, the kind of phosphine, and the molar ratio between H2 and CO. The yield of linear aldehyde (1), 4-hydroxybutyraldehyde [25714-71-0] depends on the kind of ligand. A yield of 60—70% is obtained even with excess triphenylphosphine, but a yield of more than 80% is obtained with  [c.73]

The f3-J. drenergic Teceptor An excellent example of the appHcation of protein engineering techniques to an integral membrane receptor protein is found in the P-adrenergic receptor (PAR). The P-adrenergic receptor belongs to the family of G-protein coupled receptors (GPCRs). Upon agonist binding, the receptor activates a G-protein by catalyzing the exchange of guanosine diphosphate (GDP) for guanosine triphosphate (GTP) to give the active GTP-bound form, which then initiates various intracellular events. Initial ligand specificity is deterrnined by the receptor. Different types of GPCRs couple to specific G-proteins, which in turn stimulate distinct intracellular effector systems. In the case of the P-adrenergic receptor, binding of a catecholarnine agonist, such as the biogenic amines epinephrine and norepinephrine (qv), activates the G-protein G, which then stimulates the enzyme adenylate cyclase, resulting in production of cycHc adenosine monophosphate (cAMP), the increased concentration of which affects other cellular events, etc.  [c.205]

Other types of bonding include donation by Ligand TT-orbitals, as in the classical Zeiss s salt ion [Pt( 7 -CH2=CH2)Cl3] [12275-00-2] and sandwich compounds such as ferrocene. Another type is the delta (5) bond, as in the Re2Clg ion, which consists of two ReCl squares with the Re—Re bonding and echpsed chlorides. The Re—Re 5 bond makes the system quadmply bonded and holds the chlorides in sterically crowded conditions. Numerous other coordination compounds contain two or more metal atoms having metal—metal bonds (11).  [c.168]

Metal catalysis seems not to have been employed extensively in the preparation of four-membered heterocycles by [2 -1- 2] cycloaddition. As has been pointed out for all-carbon systems, processes which are orbital symmetry forbidden can be changed to allowed ones by making certain types of metal complexes of the systems for which the desired process is forbidden. Nonreacting ligands in such a complex may split orbitals involved in the desired reaction, changing the symmetry of the process to an allowed one. Interaction with the ligand fields of nonreacting ligands also may create activation barriers for (perhaps undesired) conversions and dictate a different reaction pattern (or render the system inert) (71JA1123). Exploitation of these possibilities may prove very fruitful. One example is the addition of diethyl azodicarboxylate to the tricarbonyliron complex of cyclobutadiene, which yields l,2-bis(ethoxycarbonyl)-l,2-diazabicyclo[2.2.0]hex-4-ene (Scheme 15) (78TL2469).  [c.39]

After collection, liquid particles coalesce and must be drained from the unit, preferably without reentrainment. Calvert (R-12) has studied the mechanism of reentrainment in a number of liquid-particle collectors. Four types of reentrainment were typically observed (1) transition from separated flow of gas and hquid to a two-phase region of separated-entrained flow, (2) rupture of bubbles, (3) liqmd creep on the separator surface, and (4) shattering of hquid droplets and splashing. Generally, reentrainment increased with increasing gas velocity. Unfortunately, in devices coUecting primarily by centrifugal and inertial impaction, primaiy collection efficiency increases with gas velocity thus overall efficiency may go through a maximum as reen-trainment overtakes the increment increase in efficiency. Prediction of collection efficiency must consider both primaiy collection and reentrainment.  [c.1428]

Tamper-eviaent seals and closures are commonly added to all packages. For certain products, child resistant closures are also added. There are several types of tamper-evident closures of which three are most common to liqmd chemical products. First is a metal enclosure which covers the drum bung and which is crimped to it. The pulling of a tab breaks the tampered-evident enclosure which cannot be used again. It is customary to have the tamper-evident seal on both drum bungs. This holds also for pails. For small glass or plastic packages a seal is fitted into the cap which is then heat sealed to the bottle by means of an induction sealer. The construction of this seal consists of an outer layer of a thermoplastic material such as polyethylene, followed by aluminum foil, followed by a bleached kraft paper. The induction sealer induces eddy currents in the aluminum foil which raises its temperature to above the melting point of the polyethylene. The polyethylene melt then fuses the seal to the bottle. Another type also used for plastic and glass bottles is an external sleeve of shrinkable PVC (polyvinylchloride). This is usually apphed by machine as the bottles move down the packaging hue after being capped. They pass through a heated tunnel whidi raises the temperature of the se to where it shrinks tightly around the closure, thereby providing tamper  [c.1953]

Names for radicals are of more general importance than names for anions and cations, because the same names are generally used for naming substituents as well as free radicals, and such names therefore have a longer history. The suffix -yl is the sign of a radical in heterocyclic compounds just as in other types of compounds. In the most formal procedure, it is added to the end of the name for the parent compound, eliding a terminal e if there is one, and including a locant if it is required. For many of the most commonly encountered heterocycles, however, an entire terminal syllable is customarily elided, and in a few cases the parent name is altered more substantially in order to avoid ambiguity, e.g. (156), (157) and (158).  [c.39]

The protein kinase domains of groups 1, 2, and 4 all have homologous amino acid sequences and belong to one of the largest superfamilies known the cyclin-dependent protein kinase CDK2 described in Chapter 6 (see Figure 6.16a) is one such member. In the complete yeast genome sequence more than 100 genes for members of this superfamily have been identified by sequence similarities, and it has been estimated that there are about 1000 members in humans. The first two types of receptors are by far the most numerous and they are thought to work in a similar way ligand binding induces the receptors to oligomerize, which activates the tyrosine kinase activity of either the receptor itself or its associated nonreceptor tyrosine kinase. When activated, receptor tyrosine kinases usually cross-phosphorylate themselves on multiple tyrosine residues, with the phosphotyrosine residues then serving as docking sites for intracellular proteins. This results in the formation of an ensemble of proteins immobilized at the cell membrane, bringing together a diverse range of proteins such as kinases, phosphatases, phospholipases and G proteins such as Ras. In this way, a multisignaling complex is activated from which the signal spreads from the membrane to the cell interior.  [c.271]

The binding of hormone to the receptor triggers a number of changes in the receptor that allow the hormone-receptor complex to bind to DNA and stimulate transcription. The occurrence of this ligand-induced transcriptional activity is termed receptor activation. The binding of progestin to the receptor induces the phosphorylation of serine residues in the receptor (193,194), the release of heat shock proteins normally associated with the hormone-free receptor (195), and the dimerization of activated receptors (196). Because there are A and B isoforms of the progestin receptor, three species of dimers may exist A A, A B, and B B. AH three dimers can bind to the same specific DNA sequences, known as progesterone response elements (196). However, there is evidence that the three dimeric forms may differentiaHy regulate target genes (197,198). To study the A and B isoforms in isolation, molecular biological techniques have been used to produce ceUs which express either the A isoform, the B isoform, or some known ratio of the two. In some ceU types, the B isoform but not the A isoform activates a specific gene promoter (198). When the A isoform is coexpressed in the same ceUs as the B isoform, the transcriptional activity of the B isoform is repressed in dkect proportion to the amount of A isoform present. Apparendy, the A isoform has dual activities, acting both to promote and repress the expression of certain genes. The A and B isoforms show additional differences in thek response to progestin antagonists. The A isoform is not transcriptionaHy activated by progestin antagonists, but only by progestin agonists. In contrast, the B isoform, in the absence of the A isoform, is activated to induce gene transcription by both progestin agonists and antagonists (199,200). The binding of progestin antagonists induces changes in receptor stmcture and phosphorylation different from those due to progestin agonist binding (201—203). A monoclonal antibody dkected to the carboxyl terminus of the human progestin receptor can differentiate between agonist- and antagonist-receptor complexes (204). It is these ligand-induced changes in receptor stmcture which are beHeved to be the basis for the biological effects of progestin agonists and antagonists.  [c.220]

See pages that mention the term Types of Ligand : [c.906]    [c.907]    [c.596]    [c.511]    [c.443]    [c.185]    [c.150]    [c.436]    [c.304]    [c.87]    [c.115]    [c.250]    [c.312]    [c.930]    [c.166]    [c.210]    [c.217]    [c.300]    [c.239]    [c.706]    [c.168]   
See chapters in:

Chemistry of the elements  -> Types of Ligand