An aliphatic amino acid is an amino acid that contains only carbon and hydrogen atoms in its side chain functional group. Aliphatic amino acids repel water and are nonpolar.
Amino acids that are classified as aliphatic include glycine, alanine, valine, leucine and isoleucine. The latter three are essential amino acids for humans. Essential amino acids cannot be synthesized by the body and must be procured from the diet. Methionine, in addition to having carbon and hydrogen atoms, also has a sulfur atom in its side chain, yet it is still classified as an aliphatic amino acid. This classification is due to the fact that it shares the same nonreactive properties of all other aliphatic amino acids. Other amino acid classifications include aromatic, acidic, basic, cyclic and hydroxyl.