Isoleucine

Isoleucine is one of the 20 most common natural amino acids, and coded for in DNA. Its chemical composition is identical to that of leucine, but the arrangement of its atoms is slightly different resulting in different properties. Nutritionally, in humans, isoleucine is an essential amino acid.

Isoleucine has two chiral centers, therefore there are four possible stereoisomers of isoleucine. So there are two possible diastereomers of L-isoleucine. However, isoleucine present in nature exists in one enantiomeric form, (2S,3S)-2-amino-3-methylpentanoic acid.