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
These arrangement are literally mirror images of each other, and are
stereoisomers (AKA enantiomers) Stereoisomers are
to the direction in which the crystalline forms rotate
polarized light, to the right and left, respectively.
comprise exclusively the L forms of
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
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."