Glyceraldehyde, the simplest carbohydrate, exhibits properties
of a chiral or optical isomer compound. Review
Chiral Compounds. This molecule forms the basis for the designation
of the isomers of all of the carbohydrates.
Glyceraldehyde can exist in two isomeric forms that are mirror
images of each other which are shown in the graphic on the left.
The absolute configuration is defined by the molecule on the
far left as the D-glyceraldehyde. With the aldehyde group in
the "up" direction, the the -OH group must project
to the right side of the molecule for the D isomer.
Chemists have used this configuration of D-glyceraldehyde
to determine the optical isomer families of the rest of the carbohydrates.
All naturally occurring monosaccharides belong to the D optical
family. It is remarkable that the chemistry and enzymes of all
living things can tell the difference between the geometry of
one optical isomer over the other.