Chelating cation interaction

These ligands are frequently used as metal ion traps to bind undesired metal ions, e. g. in formulations of X-ray contrast agents. Chelated cations may associate electrostatically with anionic compounds and with solvent molecules by weak interactions. This results in the formation of an outer coordination sphere. Instead of the term complex the term chelate is very frequently used meaning that the metal ion is covered by the ligand like a claw ( chela word from the Greek for claw). 

In the lithiation of fluoroanisoles (15) and (16) and their derivatives, butyllithium exclusively deprotonates the less acidic protons from the position ortho to the alkoxyl group. On the other hand, deprotonation takes place at the more acidic site, i.e. the ortho-position next to the fluorine substituent on reaction of the substrates with super bases, such as BuLi— t-BuOK or BuLi—N,N,N/,N//,N//-pentamethyldiethylenetriamine, in which lithium cation is stabilized by chelation in the combined base-system (Scheme 3.5) [ 14]. The lithium cation interacts preferentially with the more Lewis basic alkoxyl oxygen to locate butyllithium close to the position ortho to the alkoxyl group, enhancing kinetic deprotonation (see 17 in Scheme 3.5). 

Evidence is presented which shows that a chelated cation is a distinct, long-lived chemical species and that different chelated cations may coexist in solution as discrete observable species. Investigation of the anion-cation interaction shows that chelated salts in benzene exist as tight ion pairs down to the limit of spectrometer sensitivity. The effect of chelating agent on ion pair separation is considered. Finally we describe a series of experiments conducted in mixed solvents, the results of which reveal a stereospecific association of aromatic solvent molecules with a chelated lithium salt. 

Further evidence of the strength of the chelating agent-cation interaction comes from the H NMR spectra of chelated LiBr systems which have an excess of chelating agent. We have already noted the large up-field shift of the N-CH2- protons of iso-HMTT LiBr in benzene relative to free iso-HMTT (Table II). In several experiments distinct peaks for free and complexed iso-HMTT could be observed. In order to study this system without the interference of the N-CH3 protons, selectively deu-terated iso-HMTT-di8 was used (> 99% N-CD3). Figure 7 shows the... 

We have looked at the chelating agent-cation interaction from the point of view of the chelating agent NMR), the cation (7Li NMR), and the anion (ESR). In each case we have seen clear-cut evidence that a chelated Li+ or Na+ cation is a well defined chemical species. 

We note a very interesting manifestation of the strong chelating agent-cation interaction, i.e. the ESR linewidths observed for Chel Na+C10H8" in the presence of excess Ci0H8. We consider two processes which can broaden the ESR lines, anion-neutral molecule electron transfer... 

We conclude that chelated salts are tight ion pairs in aromatic hydrocarbon solution with an anion-cation interaction which is a sensitive function of the chelating agent-cation interaction. 

Another possible explanation of the results invokes steric effects in the intermediate allyl anion (Figure 3). The steric interaction between the bulky chelated cation and the benzyl group probably pushes the... 

Divalent cations, univalent cations, or both are essential cofactors for a large number of enzymes. Kinases as a class, for example, share the requirement for such cations, while in other instances, other metal ions are inhibitors of metal-activated enzymes, e.g., activation by Mg " and inhibition by Ca for many kinases and synthetases. Mildvan (1970) has reviewed the models that have been proposed to account for activation (or inhibition) of enzymes by metal ions. The "substrate bridge" and "metal bridge" models conceive of the metal ions either combining with the substrate to form a chelate or interacting with the enzyme to complete the required binding site. These complexes usually involve the active site, but Schramm (1974) has demonstrated activation of AMP nucleosidase by MgATP at a modifier site instead. 

Molecular Interactions. Various polysaccharides readily associate with other substances, including bile acids and cholesterol, proteins, small organic molecules, inorganic salts, and ions. Anionic polysaccharides form salts and chelate complexes with cations some neutral polysaccharides form complexes with inorganic salts and some interactions are stmcture specific. Starch amylose and the linear branches of amylopectin form inclusion complexes with several classes of polar molecules, including fatty acids, glycerides, alcohols, esters, ketones, and iodine/iodide. The absorbed molecule occupies the cavity of the amylose helix, which has the capacity to expand somewhat to accommodate larger molecules. The starch—Hpid complex is important in food systems. Whether similar inclusion complexes can form with any of the dietary fiber components is not known. 

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