Specific rotation

The rotation value or specific angle of rotation is a physical measure in analytical chemistry, which indicates the optical activity of a chemical substance or its solution in degrees. The rotation value depends on the measurement conditions ( inter alia measuring temperature and wavelength of the light used ), so include this information for each rotation value.

The optical rotation is measured with a polarimeter. Most pure substances have a rotation value of 0 °, and are therefore optically inactive. Many natural products ( alkaloids, amino acids, terpenes, sugars, etc.) are in contrast, however, chiral and occur in nature almost always in front of enantiomerically pure substances which have a rotation value.

Enantiomers of a substance have the same rotation value, but with different signs:

• " " Or for right-handed materials ( clockwise rotation when the light comes on to the viewer )
• "-" Or for left-handed materials ( counterclockwise rotation ).

Racemates ( 1:1 mixtures of enantiomers on) have a rotation value of 0 °, since the rotations of the enantiomers cancel each other out.

There is no direct link between D- and L- configuration ( Fischer projections ) and the rotation value. For example, the D- enantiomer of the left - and the L-enantiomer is dextrorotatory in tartaric acid. Also, a single enantiomer which, depending on pH, a different rotation direction: so has the amino acid L- leucine at room temperature in 6 M hydrochloric acid, a specific angle of rotation of 15.1 ° and in neutral water from a -10.8 °; in 3- molar sodium hydroxide solution, however, he is again 7.6 °.

Application in pharmacy and chemistry

The rotation value is a measure by which you can identify and control their purity chiral drugs.

For enantioselective syntheses have been previously determined by the amount of rotation, the optical purity op ( english optical purity ) of the substance produced: if the measured optical rotation with known substances - eg natural substances - was identical with the highest literature optical rotation, classified to the optical purity 100%. Today, such analytical studies usually by chromatography ( thin layer chromatography, the amino acid L -proline and the coated thin layer chromatographic direct separation of enantiomers on the principle of ligand exchange chiral gas chromatography, high performance liquid chromatography allow. ) Are carried out using a chiral stationary phase.

In Food Chemistry, the concentration of sugar solutions determined ( saccharimetry ) by measuring the amount of rotation.