Optically active compounds exist in two isomeric forms. The
isomer that rotates the plane of polarized light to the left
(counterclockwise) is called levorotatory (l). The other isomer
that rotates the light to the right (clockwise) is called dextrorotatory
The optical isomers are mirror images of each other. The isomers
result from the tetrahedral geometry around the chiral carbon
center. To draw mirror images, write the structure of the first
isomer, draw a plane (dotted line) to represent the mirror, and
finally draw the mirror image behind the plane.
See the graphic on the left.
Optical isomers are also designated with the letters (D) and
(L) to indicate the absolute configuration. In an absolute configuration
the position of the groups is compared to a commonly agreed upon
standard molecule. In amino acids, this molecule is serine and
for carbohydrates, it is glyceraldehyde. The capital (D) and
(L) designations do not necessarily agree with the (d) or (l)
designations which are based upon the actual rotation of polarized
L-Alanine is a representative amino acid that is used in human
and animal metabolism. The D isomer is not used. In the metabolism
of carbohydrates only the D isomer is utilized and not the L
Example: Chiral Drugs - Ibuprofen (Advil)
As a further example, the drug Ibuprofen, is sold as a mixture
of isomers, although the (S) isomer is most effective and the
(R) isomer is also utilized after conversion to the (S) isomer
in the body.
See ibuprofen graphic
in new window
Quiz: Which carbon in ibuprofen is chiral? Check answer
using pull down menu.