Optical rotation: molecular specific

Boiling-point at 9 mm. Specific gravity at 20° Optical rotation. Refractive index. Molecular refraction. ... 

Boiling-point Specific gravity. Refractive index Molecular refraction Optical rotation... 

Specific gravity Boiling-point. Optical rotation Refractive index Molecular refraction... 

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 ). 

Activity in the field was, however, expanding. For example, in 1927 Drew and Haworth (65) obtaind a crystalline polymeric powder by the action of hydrogen chloride on the lactone of 2,3,4-trimethyl-l-arabonic acid. Citing the increase in melting point and molecular weight, and loss of specific optical rotation, they ascribed a cyclic, high polymer structure to this polyester. 

It is important to note that the measured optical purity (P) is not necessarily linearly related to the true enantiomeric purity (ee) when nonideal conditions prevail, e.g., when the enantiomers interact between themselves. Thus, the optical purity may markedly deviate from the true enanliomeric purity if the enantiomers undergo molecular self-association51. The oligomers formed in solution display their individual optical rotation and, depending on their concentration, contribute in a specific way to the overall optical rotation. The nonequivalence between the optical and enantiomeric purities has been experimentally demonstrated for 2-ethyl-2-methylbutanedioic acid in dichloromethane (Figure 4)51. 

An increase in the specific rotation of low-molecular weight polypeptides may be caused also by their intermolecular association (75). Such an effect should be concentration dependent, and indeed in dioxane the specific rotations of the penta- and hexamer were found to increase with their concentration, and ultracentrif-ugation studies (77) proved indeed that these oligomers are associated. However, the specific optical rotations of the dimer, trimer and tetra-mer, as well as of the hepta- and nonamer were concentration independent, and hence the abnormally high specific rotation of the hepta- and nonamers indicates that they are probably helical. 

Specific rotations in cuprammonium are based upon the weight of the glycol-containing molecule, not upon the weight of the copper glycol complex. This provision is necessary since the structures and molecular weights of the complexes are unknown. The optical rotations are the result of a reversible reaction, hence they are particularly dependent upon the concentrations of the reactants. The composition of the... 

When used as a means of characterization of a chiral compound, optical rotation is often measured at a single wavelength only (commonly the Na D line at A = 589 nm). Specific rotation [a] gives the rotation (in deg dm ) that is measured for a solution of a concentration of I g/100 cm The molar rotation [a]u is then obtained by multiplying with the relative molecular mass M and dividing by 100 it is commonly expressed in units of deg cm L mol . Then,... 

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