Naming conventionsAn optical isomer can be named by the spatial configuration of its atoms. The D/L system
does this by relating the molecule to glyceraldehyde. Glyceraldehyde is chiral itself, and
its two isomers are labeled D and L. Certain chemical manipulations can be performed
on glyceraldehyde without affecting its configuration, and its historical use for this pur-
pose (possibly combined with its convenience as one of the smallest commonly-used chiral
molecules) has resulted in its use for nomenclature. In this system, compounds are named
by analogy to glyceraldehyde, which generally produces unambiguous designations, but is
easiest to see in the small biomolecules similar to glyceraldehyde.
Figure 90 Optical isomers
One example is the amino acid alanine: alanine has two optical isomers, and they are
labeled according to which isomer of glyceraldehyde they come from. Glycine, the amino
acid derived from glyceraldehyde, incidentally, does not retain its optical activity, since its
central carbon is not chiral. Alanine, however, is essentially methylated glycine and shows
optical activity.
The D/L labeling is unrelated to (+)/(-); it does not indicate which enantiomer is dextro-
rotatory and which is levorotatory. Rather, it says that the compound’s stereochemistry is
related to that of the dextrorotatory or levorotatory enantiomer of glyceraldehyde. Nine of
the nineteen L-amino acids commonly found in proteins are dextrorotatory (at a wavelength
of 589 nm), and D-fructose is also referred to as levulose because it is levorotatory.
The dextrorotatory isomer of glyceraldehyde is in fact the D isomer, but this was a lucky
guess. At the time this system was established, there was no way to tell which configuration
was dextrorotatory. (If the guess had turned out wrong, the labeling situation would now
be even more confusing.)
A rule of thumb for determining the D/L isomeric form of an amino acid is the ”CORN”
rule. The groups:
COOH, R, NH2 and H (where R is an unnamed carbon chain)are arranged around the chiral center carbon atom. If these groups are arranged clockwise
around the carbon atom, then it is the L-form. If counter-clockwise, it is the D-form.This
rule only holds when the hydrogen atom is pointing out of the page.^3
3 http://www.chemguide.co.uk/organicprops/aminoacids/background.html