Chemists group the 20 amino acids by the character of their side chains. Grouping helps predict where a residue will sit in a folded chain and how it will interact.
Nonpolar side chains are built mostly from carbon and hydrogen. Examples include alanine, valine, leucine, isoleucine, methionine, phenylalanine and tryptophan. They are hydrophobic and tend to cluster away from water.
Polar uncharged side chains carry groups such as hydroxyl or amide groups that can form hydrogen bonds. Serine, threonine, asparagine and glutamine belong here, and tyrosine is often grouped with them. Acidic side chains, found in aspartate and glutamate, usually carry a negative charge at neutral pH. Basic side chains, found in lysine and arginine, usually carry a positive charge. Histidine is basic too, but it can be charged or neutral near neutral pH.
Three amino acids deserve special mention. Glycine has only a hydrogen as its side chain, so it is small and flexible. Proline's side chain loops back and bonds to its own amino nitrogen, forming a ring that makes the chain stiff and often causes bends. Cysteine carries a sulfhydryl group. Two cysteines can react to form a disulfide bond, a strong link that can tie different parts of a chain together.
Textbooks differ slightly on where a few borderline residues belong. The families are a guide, not strict boxes.
References
- Berg JM, Tymoczko JL, Stryer L. Biochemistry, 5th ed., Section 3.1: Proteins Are Built from a Repertoire of 20 Amino Acids (2002)
- IUPAC-IUB Joint Commission on Biochemical Nomenclature, Nomenclature and Symbolism for Amino Acids and Peptides (1983)
Educational content about laboratory research. Not medical advice. All Orion compounds are for laboratory research use only.
