Stereisomers of Alanine

Stereoisometry of Amino Acids

    The four bonds of the central (alpha) carbon (C) of an amino acid are directed towards the four corners of a tetrahedron. With respect to the carboxyl (COOH) and amino (NH2) 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 proteins comprise exclusively the L forms of amino acids.

     In this example, Alanine has a CH3 as a radical group. If you imagine holding the model with the COOH at the top and the NH2 at the bottom, the CH3 radical group in the D form will be on your right. In the mirror image L form, it will be on the left.

    For an interesting science fiction treatment of some possible consequences of stereoisometry, see the 1969 film "Journey to the Far Side of the Sun."


All text material ©2010  by Steven M. Carr