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 formThe 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.