Describe the term $D$ and $L$ configuration used for the amino acids with examples.

(N/A) The $D$ and $L$ configuration of amino acids is based on their structural relationship to the reference compound glyceraldehyde.
In the Fischer projection of an amino acid,if the $-NH_2$ group is on the right side,it is assigned the $D$-configuration.
If the $-NH_2$ group is on the left side,it is assigned the $L$-configuration.
Naturally occurring amino acids found in proteins almost exclusively possess the $L$-configuration.
Except for glycine,all amino acids are optically active due to the presence of a chiral carbon atom.