Enantiomers of amino acids and nucleotides. (a) Each amino acid, except glycine, has two possible stereoisomers. These nonsuperposable mirror images (enantiomers) are known as L and D forms. Only the L forms of amino acids are found in natural proteins. (b) All of the carbon atoms in ribose except C-5′ and all in deoxyribose except C-2′ and C-5′ are chiral centers. Nature uses only the D-enantiomeric form of the sugars—D-deoxyribose or D-ribose—as the building blocks for DNA and RNA.