Lactose optical rotation

Natural Occurrence of ( — )-proto-Quercitol. Although the dextrorotatory form (12) of proto-quercitol was discovered in acorns more than a century ago by Braconnot (5), who at first thought that it was lactose, the levorotatory form (13) remained unknown until 1961. In that year, Plouvier isolated it from leaves of the tree Eucalyptus populnea the yield was 0.55% (36). The optical rotation of the new compound was equal and opposite to that of the dextro enantiomer, and it was identical to the latter in its crystal form, melting point, solubilities, molecular formula and infrared spectrum. 

The specific rotation, [a]o°, of lactose in solution at equilibrium is +55.4° expressed on an anhydrous basis ( + 52.6° on a monohydrate basis). The specific rotation is defined as the optical rotation of a solution containing 1 g ml"1 in a 1 dm polarimeter tube it is affected by temperature (20°C is usually used indicated by superscript) and wavelength (usually the sodium D line (589.3 nm) is used indicated by subscript). 

Acid Hydrolysis. Lactose is resistant to acid hydrolysis compared to other disaccharides such as sucrose. In fact, organic acids, such as citric acid, that easily hydrolyze sucrose are unable to hydrolyze lactose under the same conditions. This is useful in analyzing a mixture of these two sugars, because the quantity of sucrose can be measured by the extent of these changes in the optical rotation of reducing power as a result of mild acid hydrolysis. The speed of hydrolysis of lactose varies with time, temperature, and concentration of the reactant, as shown in Table 6.8. 

In 1855, Berthelot noticed that freshly prepared lactose solutions show a progressive fall in optical rotation with time. This phenomenon, which was soon confirmed," had previously been reported for D-glucose, and was later given the name mutaroiation Several crystalline forms of lac-... 

Melting Point and Optical Rotation of the Lactose Anomers... 

Changes in optical rotation have been popular with those investigating glycoside hydrolysis since Wilhemy in 1850, using the newly invented polarimeter, demonstrated that hydrolysis of glycosides follows the first-order rate equation, k = 1/t log a/(a — x). Rate constants were then ob-tmned for the acid-catalyzed hydrolysis of salicin, - starch, maltose, lactose, methyl a- and /9-n-glucopyranoside, methyl a- and /S-n-galacto-pyranoside, - sucrose, and raffinose. ... 

Galactose was discovered by Pasteur, who called it lactose , by the hydrolysis of milk sugar (lactose). The present name is due to Berthelot. E. O. Erdmann in a Berlin dissertation reported that the optical rotation of the product of hydrolysis of milk sugar does not correspond with that of glucose. G. Bouchardat discovered dulcitol by the reduction of galactose. 

The potential of a recently developed model for saccharide optical rotation calculations has been tested by application to lactose. The new method readily accounted for the large difference between the optical rotations of lactose and cellobiose which have the same structures in the linkage region. ... 

A newly developed calculation model for optical rotation has been applied to the determination of the conformation in solution of jS-lactose and trehalose. ... 

The optical rotations of a-lactose, jS-lactose, and equilibrated solutions thereof have been measured at 546 and 589 nm between 10 and 35 "C. Although the measured optical rotation of an equilibrated solution of lactose showed good agreement with literature values, revised values were reported for the pure a- and j8-forms. 

Several methods are available to measure lactose content. As lactose is optically active and rotates the plane of polarized light in proportion to its concentration, polarimetry may be used to quantify it. The proteins are first removed by precipitating them with phosphotungstic acid. This method is much less used now. 

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