Carbohydrate, solvent effects

Polyphenols are ubiquitous in all plant organs where they are found as monomers or in polymerised forms (Schofield et al, 2001). In addition to the beneficial effect of poljq)henols, they also bind minerals and precipitate proteins and carbohydrates, in effect reducing the nutritive value of foods. Polyphenols have been classified for nutritional purposes into extractable and non-extractable types (Bravo, 1998). Extractable polyphenols are low-and intermediate-weight phenolics while non-extractable polyphenols have high molecular weight and are insoluble in normal solvents. 

Intermolecular solvent-solute interactions influence the charge distribution on a carbohydrate molecule. Subtle electronic changes that occur as a result of these interactions are responsible for the solvent dependence of carbon -proton coupling constants. The general aspects of solvent effects on NMR parameters have been reviewed,78-79 and consequently, only a very brief outline of the theoretical model within FPT INDO SCF MO formalism is considered here. 

In general, any satisfactory theoretical calculation of a nuclear coupling constant requires reliable calculation of the molecular wavefunction. As a consequence, a realistic approximation to the actual charge distribution in the carbohydrate molecule must presumably enter any theoretical model that attempts to provide a quantitative interpretation of solvent effects. The simplest treatments, and those that have been proposed most frequently to account for the solvent effect in the absence of specific effects, are those in which the solvent is treated as a continuum surrounding the solute molecule. Several different models where the solvent dependence of coupling interactions is related to the polarity of the medium have been proposed.78-79 The solvation theory80,81 has been successfully used within the FPT formalism to interpret the effect of solvent on Jc H and 3/CH. On the basis of this model, the Hamiltonian of a particular molecule includes the solvent-solute interaction term //so,v ... 

Solvent effects. DMF is a useful solvent for carbohydrates because mutarotation is slow in this medium. It accelerates the alkylation of potassium phthalimide, of sodio raalonates, of sodio acetoacetic esters, and of alkali salts of other active methylene compounds. In a general procedure 1 mole of the active methylene... 

K. Bock and C. Pedersen, Solvent effects on one-bond, C- H coupling constants of carbohydrates, Carbohydr. Res., 71 (1979) 319-321. 

H. Greenberg and A. S. Perlin, 1,4 Dioxane-2,6-diol from anhydroalditols Solvent effects on its formation and conformation, Carbohydr. Res., 35 (1974) 195-202. 

Only some aspects of the solvent perturbation on the conformational properties of carbohydrate polymers have been covered in this chapter. One of the maj or concerns has been to develop a description of these solvent effects starting with the complex conformational equilibria of simple sugars. In fact, only recently it has been fully appreciated the quantitative relationship between conformational population and physical properties, e.g. optical rotation. 

Formation and migration of cyclic acetals of carbohydrates has been reviewed. Molecular mechanical calculations have been used to calculate the energies of various conformations of bicyclic acetals of C4-C6 alditols with formaldehyde. The thermodynamic stabilities of the [4.4.0] products were predicted to be higher than for the [5,3.0] products in the gas phase. Discrepancies with experimentally observed data were ascribed to solvent effects. 

In addition to these advantages, there are several synthetic uses of water as a solvent. First, simple operation systems can be attained by the use of water. That is, workup procedures can be simplified because many organic compounds are lipophilic and may be separated easily from the aqueous phase. Second, control of reaction temperature is easier because the heat capacity of water is extremely high compared with those of most organic solvents. Third, the need for protective groups is reduced in water as amino acids, carbohydrates, and other water-soluble materials can be used as they are. Finally, the unique solvent effects of water are expected to influence the course of many reactions in water. 

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