Monosaccharides optical rotation

A classical, and still most useful, physicochemical parameter for characterizing glycosaminoglycans is their specific optical rotation. For pure heparins from common sources, [a]80 = + 45 to + 53 ° (see Table II). The actual value depends on the relative proportions of monosaccharide constituents (that is, the L-idosyluronic acid residues make a less positive contribution than the n-glucosyluronic residues), and on the degree of sulfation of the preparation (that is, although sulfate groups may have little direct influence on the molecular rotation, they lower the value by acting as diluents ). 

Preliminary examinations of dextran structures were conducted by optical rotation, infrared spectroscopy and periodate-oxidation reactions. More detailed results can be achieved by methylation analysis [19]. The hydroxyl groups are methylated with methyl iodide after activation with sodium methylsulfinyl carbanion (Fig. 2). The methyl dextran is hydrolysed to the corresponding different methylated monosaccharides, which are furthermore reduced and peracetylated. The resulting alditol acetates of methylated sugars are separated by gas chromatography and identified by their retention times. In particular, a combined capillary gas-liquid chromatography/mass... 

Primary paramagnetic products of the radiolysis of yd-L-arabinose between 77 and 430K were Identified by e.s.r. as stabilized electron and alkoxy radicals with electrons localized at 0-2. Free radicals, formed by thermal decomposition of primary products, were identified and their mechanism of formation and subsequent transformation proposed. U.v., l.r., optical rotation, and magnetic susceptibility data on Jl irradiated fructose have been reported.E.s.r. has been used to study production of free radicals during the aut-oxidatlon of simple monosaccharides via spin-trapping with StS -dl-methyl-1-pyrrollne N-oxlde. The monosaccharides produced hydroxy and hydroxymethyl radical-derived spin adducts. At high pH, both hydroxy... 

How do we know that monosaccharides exist mainly as cyclic hemiacetals There is direct physical evidence. For example, if D-glucose is crystallized from methanol, the pure a form is obtained. On the other hand, crystallization from acetic acid gives the f3 form. The a and )3 forms of D-glucose are diastereomers. Being diastereomers, they have different physical properties, as shown under their structures in eq. 16.3 note that they have different melting points and different specific optical rotations. 

The anomeric pairs of different sugars have been obtained as stable crystalline compounds but in aqueous solution the optical rotation of the a-D-form (XCIV) decreases and that of the /S-D-form (XCVI) increases until they reach the same equilibrium value. In effect one isomeric form is being transformed into the other, most probably through the intermediate production of the aldehydic form (XCV) of which a very small proportion is considered to be present in solution . Cantor and Peniston estimated the amount of the reducible aldehydo form in solution of a number of the common monosaccharides by reduction at the dropping mercury cathode References p. 63... 

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