Heavy-metal complexes

MIR), requires the introduction of new x-ray scatterers into the unit cell of the crystal. These additions should be heavy atoms (so that they make a significant contribution to the diffraction pattern) there should not be too many of them (so that their positions can be located) and they should not change the structure of the molecule or of the crystal cell—in other words, the crystals should be isomorphous. In practice, isomorphous replacement is usually done by diffusing different heavy-metal complexes into the channels of preformed protein crystals. With luck the protein molecules expose side chains in these solvent channels, such as SH groups, that are able to bind heavy metals. It is also possible to replace endogenous light metals in metal-loproteins with heavier ones, e.g., zinc by mercury or calcium by samarium. 

How is that knowledge used to find the phase of the contribution from the protein in the absence of the heavy-metal atoms We know the phase and amplitude of the heavy metals and the amplitude of the protein alone. In addition, we know the amplitude of protein plus heavy metals (i.e., protein heavy-metal complex) thus we know one phase and three amplitudes. From this we can calculate whether the interference of the x-rays scattered by the heavy metals and protein is constructive or destructive (Figure 18.10). The extent of positive or negative interference plus knowledge of the phase of the heavy metal together give an estimate of the phase of the protein. 

Grill, E., Winnacker, E.-L. Zenk, M.H. (1985). Phytochelatins The principal heavy-metal complexing peptides of higher plants. Science, 230, 674-6. 

Several heavy metal complexes of the hydrazide (ML2.2H20) exploded around... 

In addition, dissolved organic carbon (DOC) is also an important soil solution solute affecting speciation and bioavailability of many trace elements in soil solution. Many trace elements and heavy metals complex with dissolved organic carbon. This is especially important in arid and semi-arid environments since high soil pH increases the solubility of organic molecules and accordingly increases concentrations of dissolved organic carbon in soil solution. 

Wang, Z.-J., and W. Stumm (1987), "Heavy Metal Complexation by Surfaces and Humic Acids A Brief Discourse on Assessment by Acidimetric Titration", Netherlands J. of Agricult. Sci. 35, 231 -240. 

Due to the various health risks of different element species, there are a multitude of applications for natural water samples in this field (e.g., Cr and Sb speciation or Br and I determination).19 The investigation of heavy metal complexes with humic substances by isotope dilution SEC-ICP-MS has been described, for example, by McSheehy and Mester.20 Copper, zinc and molybdenum were found to form complexes with similar size fractions of humic substances in seepage water samples from soils. Sturgeon s group proposed the use of solid phase microextraction (SPME)... 

The preparation and properties of the dithiocarboxylic acids and their metal complexes have been reviewed several times.38"11 The formation of C—C bonds in the direct reaction of CS2 requires sufficiently nucleophilic carbon bases, directly or potentially accessible in the form of ambifunction-al phenoxides, organometalfic compounds, CH acidic compounds, enamines or ketimines. Carba-nions react with CS2 to give dithiocarboxylates. The preparation and purification of the adds is performed via their salts. Metal complexes are in general readily available. The bonding in these complexes is mostly of the type (27) but a bonding mode (28) is also found. Action of elemental sulfur upon heavy metal complexes of (29) aromatic dithiocarboxylic acids yields the perthio complexes (29) of these compounds. 

Characteristic of the heavy metal complexes is their carbon disulfide elimination, yielding mixed thiol-thioxanthato complexes with trimeric structures and thiol bridges.139 The facile nucleophilic substitution of RS by R2N is interesting.113114... 

Injection of calcium disodium EDTA does not affect blood calcium levels but heavy metal ions in the body have a higher affinity for EDTA than does calcium, and hence these metals readily exchange in vivo to form soluble EDT A-heavy-metal complexes that are excreted in the urine. Because of its poor absorption from the alimentary tract, calcium disodium EDTA is usually injected intramuscularly or intravenously. These considerations led to its being used, initially in 195270), to treat lead encephalopathy. 

Another vital type of ligand is a heavy-metal atom or ion. Crystals of protein/ heavy-metal complexes, often called heavy-atom derivatives, are usually needed in order to solve the phase problem mentioned in Chapter 2 (Section VI.F). I will show in Chapter 6 that, for the purpose of obtaining phases, it is crucial that heavy-atom derivatives possess the same unit-cell dimensions and symmetry, and the same protein conformation, as crystals of the pure protein, which in discussions of derivatives are called native crystals. So in most structure projects, the crystallographer must produce both native and derivative crystals under the same or very similar circumstances. 

When particle size is less than 100 pm, it can be dialyzed. Particles in this class include heavy metals complexed with organics and chelates, inorganic ion pairs, and free metal ions. 

The decrease in 37t-7c luminescence lifetimes by several orders of magnitude in heavy metal complexes as compared to the lifetimes of the free ligand is usually attributed to mixing between the 3LC and MLCT excited states [64]. This is borne out by the work of Frei et al. [38] which shows that the 3LC transitions of... 

Nurnberg, H.W., 1984. Potentialities of voltammetry in the study of physicochemical aspects of heavy metal complexation in natural waters. In C.3.M. Kramer and 3.C. Duinker (eds), Complexation of Trace Metals in Natural Waters. Nijhoff/3unk Publ., the Hague, pp. 95-115. 

X-ray crystallography of PQQ [28], (PQQ)2(K) [28], and (PQQ)(Na)2 [29] has been reported. Coordination of the potassium ion involves the 7-carboxyl oxygens as well as the N(6) and 0(5) atoms. In the sodium complex, additional interactions with the 0(4) atom and the 2-carboxyl group are observed. As yet, crystalline preparations of PQQ-heavy metal complexes have not been obtained [28], X-ray crystallographic data of the PQQ acetone adduct [30] and the 2,4-dinitro-phenylhydrazone of PQQ triester [31 ] have also been presented. 

Vogl, J. and Heumann, K.G. (1998) Development of an ICP-IDMS method for dissolved organic carbon determinations and its application to chromatographic fractions of heavy metal complexes with humic substances. Anal. Chem., 70, 2038-2043. 

Inulin can be modified to compounds that display good heavy metal complexing properties similar to ethylene diamine tetra-acetic acid (EDTA) but with better biodegradation properties (Bogaert et al., 1998). Inulin is first oxidized using sodium periodate to the dialdehyde, and then reduced to a polyol using Pt/C and hydrogen. The polyol can then be modified with carbon disulfide to form xanthate or with S03-pyridine to obtain an inulin sulfate. Alternatively, the dialdehyde can be animated with diaminoethane and sodium cyanoborohydride and the product reacted with monochloroacetic acid sodium salt to form carboxymethylamino inulin. Each of these compounds can be used to precipitate heavy metals. 

The coordination numbers heavy metal complexes are often greater than those of the 3d metals... 

Even if anionic chaotropes are the most popular neoteric IPRs, polarizable cations such as sulfonium and phosphonium reagents showed single selectivity toward polarizable anions their behavior was rationalized on the basis of their chaotropicity. Probe anion retention generally increases in the order of tributylsulfonium < tetrabutylammonium < tetrabutylphosphonium. Interestingly, retention was found to be influenced by the kosmotropic/chaotropic character of both the IPR and the probe anion [93] and this confirms the peculiarities of hydrophobic ion-pairing. Quaternary phosphonium salts provided increased selectivity compared to ammonium in the IPC of heavy metal complexes of unithiol [112]. 

Diketonates are used as model systems for syntheses and calculations because of their similarity to the active sites of metaUoproteins, where metal ions are coordinated to nitrogen, oxygen and sulfur atoms of amino acid residues . Heavy metal complexes are efficient phosphors in organic light-emitting diodes (OLEDs) and have attracted much... 

The photobehavior of molecules having excited states strongly perturbed by spin-orbit coupling may show large variations at low temperatures due to changes in the Boltzman populations of the various component levels in the excited states of such species. The temperature dependence of the luminescent properties of several heavy metal complexes have been interpreted in this manner (Section III-C). 

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