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
are two possible arrangements of the H and Radical
These arrangement are literally mirror images of each other, and
stereoisomers (enantiomers). Stereoisomers
to the direction in which the crystalline forms rotate
polarized light, to the right and left, respectively.
comprise exclusively the L
example, Alanine has a
CH3 as a radical group. If you
holding the model with the COOH at the
top and the NH2
at the bottom, the CH3 radical group in the D
be on your right. In
mirror image L form, it will
be on the left.
For an interesting science fiction treatments of possible consequences of stereoisometry, see the 1969 film "Journey to the Far Side of the Sun."