Carbohydrate-metal salt complexes

II. Complexes of Carbohydrates with Metal Salts 1. Proof of Existence... 

Adducts prepared in aqueous media generally possess one or more molecules of water of hydration per molecule, the number being a function of cation, anion, and the combining ratio of carbohydrate to salt. Available data on complexes of simple carbohydrates indicate that three molecules of water per molecule may be the maximum for adducts of alkali metal salts as many as seven have been reported for those of the alkaline-earth metal salts. Most complexes, however, possess only one or two molecules per molecule. Generally, the higher the combining ratio, the smaller is the number of water molecules that can be accommodated by a molecule of the adduct. 

TiTany commercial uses of carbohydrates depend upon reversible inter- actions of the hydroxyl groups with ions to form complexes. The processes are intricate, and analyses of the equilibria involved have been difficult or impossible (I). For example, the nature of the complexes involving polysaccharides and polyanions, such as borate, can only be hypothesized (2). Similarly the gelling of pectates with Ca(II) (3), the reactions of heptono-y-lactones with Fe(III), or the nature of Fe(III)-dextran complexes in intramuscular injectable solutions are awaiting proper description. Although the systems involving monomeric components are more amenable to study, as for the reactions of metal salts or hydroxides with monosaccharides or their simple derivatives (4,5, 6, 7)... 

Figure 6.47 Synthesis of a carbohydrate functionalised imidazolium salt and its transition metal NHC complexes.

The unique character of liquid crystals allows the sensitive change of molecular orientation with both temperature and chemical environment. Steroidal crown compounds entrapped in a mixture of cholesteryl nonaoate and cholesterol thus form a liquid crystalline phase whose helical pitch can be modified by different complexed metal salts (Fig. 3.5.4). Potassium R-mandelate, for example, induces a green assembly color, potassium S-mandelate a blue one. For the detection of carbohydrates by steroidal borates, see Figure 4.5.1 (Shinkai et al., 1991). 

Sihca impregnated with saturated and unsaturated hydrocarbons (squalene, paraffin oil) silicone and plant oils complexing agents (silver ions, boric acid, and borates carbohydrates unsaturated and aromatic compounds) chelating compounds [ethylene diamine tetra-acetic add (EDTA), digitonin] transition metal salt synthetic peptides 18-crown-6 and ammonium sulfate silanized sdica gel impregnated with anionic and cationic surfactants Cross-hnked, polymeric dextran gels (Sephadex)... 

The effectiveness with which the solvent promotes migration decreases in the order methanol > ethanol water. The extremely low stability of complexes in water can be explained by the relatively great tendency of metal ions to associate with water molecules.70 The difference between the rate in ethanol and that in methanol can be attributed, at least partly, to the fact that salts are more highly dissociated into free ions in methanol71 a higher concentration of free cations would permit a higher concentration of positively charged carbohydrate species. 

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.)... 

The food fiavorist also must be aware of metal complexes, which carbohydrates are capable of forming, Iron salts not only form complexes with dietary fiber, but with nearly all of the known natural sugars. Fructose, maltitol. sorbitol, and xylitol can easily form complexes with the ferric... 

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