What is Amino Acid? (Peptide Glossary)

Amino acids are organic molecules with a characteristic structure: an amino group, a carboxyl group, and a variable side chain that gives each amino acid its properties. Twenty standard amino acids - from simple glycine to bulky tryptophan - combine in defined orders to build every peptide and protein. The side chains determine how a peptide folds, what receptors it binds, and how stable it is.

In peptide synthesis, amino acids are linked one at a time through peptide bonds. Research peptides often include modified or non-standard forms: D-amino acids (mirror-image versions that resist enzymatic breakdown), Aib (aminoisobutyric acid, used in semaglutide and tirzepatide for stability), and N-methylated variants. These modifications are why engineered research peptides outlast their natural counterparts.

Amino acid composition also explains handling differences between peptides. Sequences rich in cysteine (like oxytocin) form disulfide bonds and are oxidation-sensitive. Hydrophobic sequences can be difficult to dissolve. Charged sequences behave differently in solution. When a supplier provides reconstitution guidance specific to a compound, this chemistry is the reason.