Cation dependency

Although tellurium resembles sulfur and selenium chemically, it is more basic, more metallic, and more strongly amphoteric. Its behavior as an anion or a cation depends on the medium, eg ... 

The order of enolate reactivity also depends on the metal cation which is present. The general order is BrMg < Li < Na < K. This order, too, is in the order of greater dissociation of the enolate-cation ion pairs and ion aggregates. Carbon-13 chemical shift data provide an indication of electron density at the nucleophilic caibon in enolates. These shifts have been found to be both cation-dependent and solvent-dependent. Apparent electron density increases in the order > Na > Li and THF/HMPA > DME > THF >ether. There is a good correlation with observed reactivity under the corresponding conditions. 

Polyelectrolytes are classified into three main groups nonionic, anionic, and cationic depending upon the nature of the residual charge on the polymer in aqueous solution as shown in Table 1. 

Some microbial exopolysaccharides contain the inorganic substituents phosphate and sulphate. Phosphate has been found in exopolysaccharide from bacteria of medical importance, including Escherichia coli. Sulphate is far less common than phosphate and has only been found in spedes of cyanobaderia. In addition to these inorganic components, which form part of the structure of some exopolysaccharides, all polyanionic polymers will bind a mixture of cations. Exopolysaccharides are, therefore, purified in the salt form. The strength of binding of the various cations depend on the exopolysaccharide some bind the divalent cations calrium, barium and strontium very strongly, whereas others prefer certain monovalent cations, eg Na ... 

Table 1. Relative energies E (kJ mol 1) of the ethyl cation dependent on calculation method used (data from 43) if not otherwise indicated)...

Chemical assemblies around hexacoordinated phosphorus atoms can be anionic, neutral or cationic depending upon the nature of the hgands. This is described in the following paragraphs. 

Experience shows that the potentials of metal electrodes in melts of their own salts (i.e., the activities of the cations) depend on the natnre of the anions. However, the variation in the valnes of activity in melts is not very pronounced. This is dne to the relatively small spread of interionic distances fonnd in different melts (their entire volume is filled up with ions of similar size) compared to the spread found in aqueous solutions. For this reason the electrostatic forces between the ions (which are very significant) do not differ greatly between different melts. 

The same expression is obtained if electrodes of the second kind are considered as electrodes of the first kind, where the activity of the metal cations depends on the solubility product of the given insoluble salt (cf. Eq. 3.1.26) ... 

A particularly interesting aspect of the reaction is its apparent counter-cation dependence. Our initial screen of bases included inorganic bases with a variety of counterions, including Li+, Na+, K+, and MgX+, but only potassium bases were effective. The precise role of the counterion is not completely understood, but it is critical for success. To date, the use of KOf-Bu or KH have consistently provided the best results, with KHMDS used as a lower-yielding alternative if absolutely necessary (in cases where the substrate was unstable to alkoxide bases). 

The biochemical properties of these structures are known. Desmosomes display protease sensitivity, divalent cation dependency and osmotic insensitivity and their membranes are mainly of the smooth type. In direct contrast to desmosomes, the tight junctions as well as gap junctions and synapses display no protease sensitivity, divalent cation dependency or osmotic sensitivity, while their membranes are complex. These facts have been used in the development of techniques to isolate purified preparations of junctional complexes. 

Within each of these adhesion molecule families, membership has been defined largely by amino-acid sequence similarity, which is reflected in common structural features. Consequently, distinct binding requirements also characterize each family. For example, cadherins interact in a Ca2+-dependent, usually homophilic manner. Binding of the members of the Ig family is Ca2+-independent and, although frequently homophilic, can be heterophilic. Integrin binding is also divalent cation-dependent (Ca2+, Mg2+) but always heterophilic. 

In the case of the butene isomers, the addition will lead to different isooctyl cations, depending on the isomer and the type of carbenium ion. The reactions involving s-butyl ions are likely to be negligible for liquid acid catalysts and of minor importance for zeolites. 

Sur, C., Betz, H., and Schloss, P. (1997) A single serine residue controls the cation dependence of substrate transport by the rat serotonin transporter. Proc. Natl. Acad. Sci. USA 94, 7639-7644. 

Table 4 Electrochemical data and group I metal cation dependence of ferrocene amide aza crown ethers and model analogues.

Table 6 Electrochemical data and ammonium cation dependence for compounds [19]—[23].

Table 14 Electrochemical cation dependence of reducible molybdenum macrocycles.

Recently another signal, the phenylalanine-tryptophan motif (or the motif containing two aromatic residues, such as (F/Y)X(F/Y), in yeast), was shown to direct some proteins including cation-dependent mannose 6-phosphate receptor (see Section IIIJ,2) from endosomes to the TGN (Schweizer et al., 1997 Burd et al., 1998). Most probably, it is used for the (retrograde) recycling processes. A putative coat complex was also discovered and named retromer (Seaman et al., 1998). 

Schweizer, A., Kornfeld, S., and Rohre, J. (1997). Proper sorting of the cation-dependent mannose 6-phosphate receptor in endosomes depends on a pair of aromatic amino acids in the cytoplasmic tail. Proc. Natl. Acad. Sci. U.S.A. 94, 14471-14476. 

SigurdsonSLandLwebuga-MukasaJS [1994] Divalent cation dependent regulation of rat alveolar epithelial cell adhesion and spreading. Exp Cell Res 213 719-727... 

One-electron oxidation of organoselenium and organotellurium compounds results in initial formation of a radical cation (equations (19) and (20)). The eventual fate of the radical cation depends on several variables, but is typically a Se(lV) or Te(lV) compound. The scope of this section will be the one-electron oxidation of selenides and tellurides that are not contained in a heteroaromatic compound, and ones in which the Se and Te are bonded to two carbons, rather than to other heteroatoms. Tellurium- and selenium-containing electron donor molecules have been reviewed. 

The significance of the supporting electrolyte cation depends crucially on whether a divided or an undivided cell is used. In a divided cell, the choice of cation is of minor importance but in an undivided cell the cathode process should not lead to formation of base and thereby to buffering of the solution. Metal cations such as Li+, Na+ or Mg + are often the choice since in aprotic solvents the metal cation may be the most easily reduced component. This has been observed as deposits of metal on the surface of the cathode arising from... 

Shfe, C.W., Wang, E., Hunter, R., Wang, S., Burgess, C., Liotta, D.C., and Merrill, A.H.Jr., 1989, Free sphingosine formation from endogenous substrates by a liver plasma membrane system with a divalent cation dependence and a neutral pH optimum. J. Biol. Chem. 264 10371-10377. 

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