Levorotation and dextrorotation

Dextrorotation and levorotation (also spelled as laevorotation) are terms describing circular direction of rotating plane-polarized light. If the light rotates clockwise as it approaches an observer, this is known as dextrorotation (rotation to the right); if it rotates counterclockwise, then it exhibits levorotation (rotation to the left).

A compound with dextrorotation is called dextrorotatory or dextrorotary, while a compound with levorotation is called levorotatory or levorotary. Compounds with these properties are said to have optical activity and consist of chiral molecules. If a chiral molecule is dextrorotary, its enantiomer (geometric mirror image) will be levorotary, and vice versa. The enantiomers will rotate plane polarized light the same number of degrees, but in opposite directions.

Apart from direct measurement of the optical rotation of an actual sample, it is only possible to determine whether a given chiral molecule will be levorotatory or dextrorotatory directly from its absolute configuration, via detailed computer modeling. That is to say, both "R" and "S" stereocenters have the ability to be dextrorotatory or levorotatory.

A dextrorotary compound is often prefixed "(+)-" or "d-". Likewise, a levorotary compound is often prefixed "(−)-" or "l-". These "d-" and "l-" prefixes are distinct from the uppercase (though SMALL CAPS) "D-" and "L-" prefixes, which are based on the actual configuration of each enantiomer, with the version synthesized from naturally occurring (+)-glyceraldehyde being considered the D-form. For example, nine of the nineteen L-amino acids commonly found in proteins are dextrorotatory (at a wavelength of 589 nm), and D-fructose is sometimes called "levulose" because it is levorotatory.

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