iGen 06-01smc
Stereoisometry of Amino Acids

    The four bonds of the central or alpha carbon (C) of an amino acid are directed towards the four corners of a tetrahedron. With respect to the carboxyl (COO-) and amino (NH3+)groups, there are two possible arrangements of the H and Radical group.  These arrangement are literally mirror images of each other, and are called stereoisomers (AKA enantiomers). Stereoisomers are designated D (dextro-rotatory) or L (levo-rotatory) according to the direction in which the crystalline forms rotate polarized light, to the right and left, respectively. Naturally-occuring amino acids are exclusively of the L form

     The example shows an L-form amino acid. Imagine holding the model with the carboxyl at the top, the amino at the bottom, so that the C projects away from you: the R radical group will be on the left in L form.  In the mirror-image R-form, it will be on the right.

Figure © 2010 PJ Russell, iGenetics 3rd ed.; all text material © 2012 by Steven M. Carr