Sugar with cations, complexes

In the previous chapters the reactivity of metal ions with the monomer units of nucleic acids has been discussed. This section will deal with the binding of transition metals to the polynucleotides. There are also three types of complexes to be expected the metal-ring, the intermediate and the metal chain complex. The effect of the ribose or deoxyribose residue on the stability constants can be neglected since the reactivity of these sugars with cations is extremely low. However, as it will be seen later, the hydrolysis of polyribonucleotides is markedly facilitated by interaction of metal ions with the 2 —OH groups of the ribose. 

The prototype industrial process based on this concept is the Ruhrchemie-Rhone Poulenc process for the hydroformylation of propylene to butanal94,219,220 (see Section 7.3.1). Because of the use of appropriately modified water-soluble ligands, the catalyst resides and operates in the aqueous phase. The particular features of this process are the positive energy balance and easy catalyst recovery, namely, the simply circulation of the aqueous catalyst solution. New types of water-soluble Ir and Rh complexes with tris(hydroxymethyl)phosphine222 were described, and the biphasic hydroformylation of 1-hexene was accomplished in ionic liquids.223 A cationic sugar-substituted Rh complex displays high regioselectivity to branched aldehydes.224... 

The literature on metal complexes of carbohydrates through 1965 has been fully reviewed by Rendleman (I), and we shall therefore only discuss recent work. We shall not discuss the complexes formed with strong bases, such as calcium and barium oxide these are salts in which the sugar acts as a weak acid, losing one or several protons. Nor shall we discuss the complexes formed with anions of oxyacids—e.g., borate, stannate, periodate, etc. ions all these are compounds formed by covalent bonds in alkaline solution. We are concerned only with complexes formed with cations in neutral aqueous solution there is no evidence for the formation of complexes between sugars and simple anions in neutral aqueous solution. (For an example of complex formation between a sugar derivative and chloride ion in chloroform solution, see Reference 3.)... 

Typical applications of ion exchange in sugar analysis include (a) complexation of borates, which accentuates ionic interactions with the exchanger (27), (b) use of hydroorganic eluents, especially acetonitrile/water, with rigid, fine-particulate anion columns (28), (c) use of basic eluents, since most carbohydrates are weak acids with pKfl of 12-13 (29), (d) complexation with cations, Pb2+, Ca2+ and Ag+ being the most frequently employed, (e) the use of cation exchangers in a heavy metal form, e.g., Aminex HPX-85. 

It was in 1971, just 100 years later, that there appeared the first paper to discuss the relationship between structure and complex-formation of neutral carbohydrates with cations. Most of the developments during the intervening 100 years were covered in an article by Rendleman in this Series. The present discussion is an updating of Rendleman s chapter, and it covers the renaissance of interest in metal - sugar complexes. The information given by Rendleman will not be repeated here unless it is required for the understanding of subsequent developments. 

Table IV also includes some values determined in methanol as the solvent these are very much higher (and, hence, also more accurate) than those in water, because the polyol competes with methanol, rather than with water, for outer-sphere positions on the cation. These figures explain why carbohydrates are soluble in methanol or ethanol containing high concentrations of calcium chloride, or even potassium acetate, and in such systems as lithium chloride in 2-methoxyethanol. ° Sugar derivatives that are soluble in non-hydroxylic solvents form complexes with cations in those solvents even more readily for example, methyl 2,3-0-isopropylidene-4-0-methyl-) -L-rhamnopyranoside (24) (but not its a anomer) will form a complex with sodium iodide in acetone, the Na" " ion coordinating to 0-1,0-2, and 0-3. In aqueous solution, the concentration of this complex would be negligible.

These columns are also used for the separation of carbohydrates that do not form complexes with cations in this case, the column functions by adsorption and partition. Besides Ca, columns in the Pb and the Ag+ form are also used, but these cations may react with reducing sugars. Thus, it has been reported that, at 60°, ketoses are isomerized on a column. Columns in the La form have not been extensively used, although they are the choice for separating alditols from each other and from other carbohydrates. ... 

Smits to the Pb complexation with the substrate (16). In fact, many complexes of sugars and sugar derivatives with inorganic salts and bases were isolated in solid and often in crystalline forms. There was considerable evidence, based on various physical measurements, that sugar-cation complexes exist in solution... 

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