Molecular recognition and metal ion template synthesis

Methods for the design and synthesis of ligands intended to be specific for a metal ion have been a recent chemical development. This article describes how this process can be inverted so that the specifics of the coordination environment around the metal ion can be used as a template in large-scale ligand synthesis. The synthesis of macrobicyclic ligands for ferric ion has been accomplished by using active esters of catechol ligands in which catecholate coordination to iron is a prelude to the organic chemical reactions that link the coordination subunits together into one ligand system surrounding a central metal ion coordination site. The lanthanide(III) ions, which are among the most labile metal ions known, have coordination numbers of 8 or higher, and thus their encapsulation into a macrobicyclic structure is a challenging problem. Lanthanide amine complexes have been used as metal templates in the synthesis of such macrobicyclic lanthanide complexes. There is evidence that such a complex is inert to exchange in aqueous solution.     

Gas-phase ion chromatography: transition metal state selection and carbon cluster formation

Gas-phase ion chromatography can separate ions that have the same mass but differ in isomeric structure or electronic configuration. The main features of this technique are briefly outlined, and applications to a series of problems in transition metal chemistry and carbon cluster chemistry are described. Examples in transition metal chemistry include state-selective reactivity, excited state deactivation, and state-selective ligand binding energies. For clusters, ion chromatography was used to determine the structure of pure carbon cluster ions as a function of size from C(4) to C(84). The results indicate that carbon grows first in linear chains, transforms to monocyclic planar rings at about C(10), and forms new families of planar bi-, tri-, and tetracyclic rings at C(20), C(30), and C(40), respectively. Fullerenes, which mysteriously appear at C(30) and dominate by C(50), are generated by heating the planar ring systems above an isomerization barrier rather than by growth of graphite precursors.     

Large clusters of metal ions: the transition from molecular to bulk magnets

Clusters of metal ions are a class of compounds actively investigated for their magnetic properties, which should gradually change from those of simple paramagnets to those of bulk magnets. However, their interest lies in a number of different disciplines: chemistry, which seeks new synthetic strategies to make larger and larger clusters in a controlled manner; physics, which can test the validity of quantum mechanical approaches at the nanometer scale; and biology, which can use them as models of biomineralization of magnetic particles.     

Research advance on the physiologically molecular mechanisms for plant heavy metal tolerance

植物可以通过一系列动态的分子生理机制来减少重金属的胁迫 这些机制包括重金属离子与细胞壁的结合 ,限制重金属离子透过细胞膜 ,重金属离子或化合物的排出 ,重金属在植物体内的转移 ,重金属的络合与区隔化以及植物对胁迫的基本的反应机制如胁迫蛋白含量的变化等 本文就植物耐重金属的分子生理机制进行综述     

植物耐重金属分子生理机制研究进展

植物可以通过一系列动态的分子生理机制来减少重金属的胁迫.这些机制包括重金属离子与细胞壁的结合,限制重金属离子透过细胞膜,重金属离子或化合物的排出,重金属在植物体内的转移,重金属的络合与区隔化以及植物对胁迫的基本的反应机制如胁迫蛋白含量的变化等.本文就植物耐重金属的分子生理机制进行综述.     

THE METAL CARBONYLS

When the metal carbonyls were first discovered, their properties were startling because they seemed to violate nearly all the previously recognized generalizations of chemistry. Even to-day the existence of the carbonyls is not particularly emphasized in elementary courses of chemistry because it is rather hard to reconcile them with the first presentations of the generalizations of chemistry. Nevertheless, as the student progresses deeper into the knowledge of chemistry it becomes desirable to include the knowledge of the carbonyls both because they become more comprehensible when viewed in the light of Werner's system of coordination and because they themselves contribute to the comprehension of the Werner theory. As long ago as 1931, Reiff in his discussion of cobalt nitrosyl carbonyl recognized the correlation between the effective atomic number and the volatility of carbonyls. A more recent study of charged Werner coordination complexes, that is, of complex ions, has shown a similar role of the effective atomic number. We are standing on fairly firm ground when we point out the correlation between E.A.N. and the volatility of the carbonyl complexes and the existence of complex ions. Be it noted that we have made no postulates as to the arrangement of the electrons in quantum levels. In the inert gases the outer principal quantum group is supposed always to contain eight electrons. In the carbonyls and other Werner complexes there is no compelling reason to suppose that the electrons in the coordinating layer, be this layer of eight, ten, twelve or sixteen electrons, are not all at the same energy level. Although we have confined our discussion almost exclusively to the property of volatility, the carbonyls are very interesting from the standpoint of several other properties, for example, magnetic susceptibility and dielectric constant. Enthusiasts in the interpretation of such properties try to draw conclusions as to the condition of the electrons, sometimes they become so dogmatic as to seem really.to believe in the actual existence of the condition they postulate. As Professor Smith said, "Theories come and theories go, but facts live on forever." The facts of chemistry are so multitudinous that we would be utterly helpless to use them had we not means of correlating them. Any postulates which reach beneath the surface of the directly observable to give a mechanism to correlate the facts are helpful. But a scientist without a sense of humor is pretty hopeless. Who in his right mind can regard as other than absurd the idea that an electron pair can simultaneously occupy positions in two atom shells to make up the supposedly necessary number in each atom? In fact, is not the electron itself a pretty ridiculous figment of the imagination? To be sure, we recognize the electron as a discrete entity with certain very definite properties, but in the light of comparison with any mechanism within our comprehension is not the electron perfectly impossible? By all means let us use a postulate which allows us to make a useful classification of facts, but never let us lose the sense to see how utterly ridiculous the postulate will look to one who has not like ourselves grown attached to it. We do not expect ever to discover the ultimate reason for things, but we do expect ever to make progress in correlating and classifying the facts which we have already discovered and shall continue to discover. Dogmatic belief in ridiculous postulates retards this progress.     

Visualizing the higher order folding of a catalytic RNA molecule

The higher order folding process of the catalytic RNA derived from the self-splicing intron of Tetrahymena thermophila was monitored with the use of Fe(II)-EDTA-induced free radical chemistry. The overall tertiary structure of the RNA molecule forms cooperatively with the uptake of at least three magnesium ions. Local folding transitions display different metal ion dependencies, suggesting that the RNA tertiary structure assembles through a specific folding intermediate before the catalytic core is formed. Enzymatic activity, assayed with an RNA substrate that is complementary to the catalytic RNA active site, coincides with the cooperative structural transition. The higher order RNA foldings produced by Mg(II), Ca(II), and Sr(II) are similar; however, only the Mg(II)-stabilized RNA is catalytically active. Thus, these results directly demonstrate that divalent metal ions participate in general folding of the ribozyme tertiary structure, and further indicate a more specific involvement of Mg(II) in catalysis.     

混合络合物的形成及生物学意义

自然界特别是在生物体系中广泛地存在着混合型络合物。它的形成可以是一步同时生成或逐步生成,也可以是由单一型络合物混合生成。混合络合物的稳定性是由如下因素所决定的,即:统计效应,静电效应,共价键性,立体效应和配体的合作效应。在生物体内,混合络合物的形成涉及到:生物分子的稳定化和三维构象的稳定化,金属离子和配体分子的分布和运转,某些金属酶的催化作用(如脱羧酶、水解酶,羰基水化酶,碳酸酐酶等)及生物固氮等等问题。     

尿素配合物微量元素肥料的合成

用铁、锰、铜、锌的硝酸盐分别与尿素反应 ,合成了四种固体配合物。它们均易溶于水。根据物质结构理论 ,这些金属离子与尿素中的氮原子发生了配位 ,使金属更易被植物吸收 ,使氮缓释。因而合成的此类配合物不仅是优良的微量元素肥料 ,而且也是一种长效的氮肥。     

Laser-induced fluorescence and bonding of metal dimers

This article describes a technique for the spectroscopic of metal clusters and intermetallic compounds. First, metallic samples are vaporized by a pulsed YAG (yttrium-aluminum garnet) laser, and then the gaseous products are excited with a pulsed-dye laser until they fluoresce. A time-resolved, fluorescence spectrum is then measured by the product. The application of this technique to the study of metal dimers is reviewed, with emphasis on recent results from Be(2) and Cr(2). Studies of such species often yield insights into the chemistry of metals and metal-metal bonding.     

Supramolecular chemistry: receptors, catalysts, and carriers

Supramolecular chemistry is the study of the structures and functions of the supermolecules that result from binding substrates to molecular receptors. Macropolycyclic receptors and coreceptors have been designed that form cryptate inclusion complexes and display molecular recognition towards spherical, tetrahedral, and linear substrates of various kinds (metal cations, inorganic anions, and organic or biological cations or anions). Anion binding has led to the development of anion coordination chemistry. Metalloreceptors simultaneously bind organic molecules and metal ions; speleands combine polar and nonpolar binding subunits. Receptors bearing reactive functional groups may act as molecular reagents or catalysts, performing a chemical transformation on the bound substrates (by such reactions as hydrogen transfer, ester cleavage, and protoadenosinetriphosphatase and protokinase activities). Receptors fitted with lipophilic groups can operate as molecular carriers, translocating bound species through a membrane; this transport can be coupled to chemical potentials (proton and redox gradients).     

天然有机质和金属离子在矿物表面的共吸附

天然有机质(NOM)在土壤、沉积物和水体等环境中无处不在,其中富里酸和胡敏酸是主要形态。富里酸及胡敏酸活性高,易与天然矿物颗粒和金属离子发生相互作用,影响矿物的表面化学特性以及金属离子的形态与迁移性,进而在控制环境中金属离子的生物有效性和毒性等方面起重要作用。本文主要综述了富里酸和胡敏酸等NOM和金属离子在矿物表面共吸附特性与主要影响因素,归纳了表面络合模型和现代光谱技术在上述三元体系研究中的应用及其反应机制研究进展。NOM在较大程度上改变了金属离子在矿物表面的吸附特性和反应机制,并受体系pH、金属离子类型和浓度、NOM浓度、NOM和金属离子的添加顺序、矿物类型等因素的影响。低pH时,NOM通常促进矿物对金属离子的吸附。NOM和金属离子在矿物表面的共吸附机制包括:NOM和金属离子竞争吸附表面活性吸附位点;在溶液中形成NOM-金属离子络合物;形成金属离子桥接矿物表面位点与NOM的A型三元络合物(矿物-金属离子-NOM)或NOM联接矿物表面与金属离子的B型三元表面络合物(矿物-NOM-金属离子);静电作用改变表面电荷特征。最后展望了天然有机质等配体与金属离子在矿物表面共吸附有关的研究热点和方向。     

Ultrafast dynamics at semiconductor and metal surfaces

A variety of important dynamical phenomena at metal and semiconductor surfaces are now being investigated with the use of new ultrafast measurement techniques involving lasers and nonlinear optics. Understanding of the rates and mechanisms for relaxation of optical excitations of the surface itself as well as those of adsorbates on the surface is providing new insight into surface chemistry, surface phase transitions, and surface recombination of charge carriers in semiconductors.     

A main group metal sandwich: five lithium cations jammed between two corannulene tetraanion decks

Lithium-coordinated polyaromatic anions such as tetrareduced corannulene, C(20)H(10)(4-) (1(4-)), are useful substrates to model and ultimately improve the graphitic electrodes in lithium-ion (Li(+)) batteries. Previous studies suggested that 1(4-) forms dimers encasing four Li(+) ions in solution. Here, we report a single-crystal x-ray diffraction analysis confirming the formation of a sandwich-type supramolecular aggregate with a high degree of alkali metal intercalation. In contrast to the prior model, our data reveal that five Li(+) ions are sandwiched between the two tetrareduced corannulene decks, and (7)Li nuclear magnetic resonance spectroscopy delineates a conserved structure in tetrahydrofuran solution. Remarkably, the sandwich is robust in both solution and solid states even in the presence of crown ethers that compete for Li(+) coordination. These results should help elucidate Li(+) intercalation motifs between curved carbon surfaces more broadly.     

Molecular borromean rings

The realization of the Borromean link in a wholly synthetic molecular form is reported. The self-assembly of this link, which is topologically achiral, from 18 components by the template-directed formation of 12 imine and 30 dative bonds, associated with the coordination of three interlocked macrocycles, each tetranucleating and decadentate overall, to a total of six zinc(II) ions, is near quantitative. Three macrocycles present diagonally in pairs, six exo-bidentate bipyridyl and six endo-diiminopyridyl ligands to the six zinc(II) ions. The use, in concert, of coordination, supramolecular, and dynamic covalent chemistry allowed the highly efficient construction, by multiple cooperative self-assembly processes, of a nanoscale dodecacation with an approximate diameter of 2.5 nanometers and an inner chamber of volume 250 A(3), lined with 12 oxygen atoms.     

Binding of alkali metal ions by cyclic polyethers: significance in ion transport processes

Values for the formation constant (log K), the change in enthalpy (triangle upH degrees ), and the change in entropy (triangle upS degrees ) have been determined for the interaction of lithium, sodium, potassium, rubidium, and cesium ions with the two isomers of the cyclic polyether, 2,5,8,15,18,21-hexaoxatricyclo[20.4.0.0(9,14)] hexacosane. The stability order of these metal ions with either isomer is identical to the permeability order for these same metal ions with the structurally related antibiotics, valinomycin and monactin.     

CHEMISTRY: Harnessing the Power of Diatomics

When a diatomic molecule is broken up into its constituent atoms, these very energetic atoms provide a large driving force for further reaction. Diatomic molecules often do not undergo productive chemistry, however, because the energy needed to break the diatomic bond is also high. In his Perspective, Thorp discusses the work by MacBeth et al., who have synthesized a nonheme iron complex that reacts with O2 to produce two equivalents of a metal-oxo complex. The complex elegantly mimic the ability of some enzymes to influence metal ion coordination spheres.     

Linear metal nanostructures and size effects of supported metal catalysts

Nickel metal catalysts composed of nanometer by micrometer strips have been produced with solid-state microfabrication techniques. The strips are actually the edges of nickelcatalyst thin films, which are sandwiched between separating support layers, which are also nanometers thick. These linear nanostructures constitute well-defined and well-controlled catalytic entities that reproduce the size of traditional supported metal clusters in one dimension, thus separating size from total number of atoms in the catalyst. Examination of their catalytic activity showed that they duplicate the behavior of conventional supported clusters. A specific rate maximum was observed for the hydrogenolysis of ethane at a nanoscale dimension similar to the cluster size at which the rate is maximum in the case of the supported cluster studies, whereas the hydrogenation of ethylene shows no such size dependency. The results suggest that the surface-to-volume ratio or the number of atoms in the catalytic entity cannot be the source of these size effects and that either support effects or nonequilibrium surface structures are the determining factors.     

金属离子对蜜蜂球囊菌几丁质酶活力的影响

探讨了不同金属离子对蜜蜂球囊菌几丁质酶活力的影响.结果表明:在0-100 mmol.L-1范围内,一价金属离子Na+、K+随离子浓度的增大,对几丁质酶活力的影响表现为先促进后抑制;在1-10 mmol.L-1范围内,二价金属离子Mg2+、Mn2+起抑制作用,Zn2+基本没有影响,Ca2+、Fe2+起促进作用;在1-10 mmol.L-1范围内,三价金属离子A l3+起促进作用,Fe3+基本没有影响.     

Boon and bane of metal ions in medicine

In biological systems metal ions promote responses that range from deficiency to toxicity. Some, such as iron and zinc, have a known optimal intake range for normal, healthy individuals. Metal ions contained within well-designed molecules already constitute a great boon for the medicinal pharmacopoeia. However, whether essential or not, the threshold for toxicity can be very low. One of the challenges of designing metal-based drugs is to balance the potential toxicity of an active formulation with the substantial positive impact of these increasingly common therapeutic and diagnostic aids.