An amino acid is the monomer unit of every peptide: a central alpha-carbon carrying an amino group, a carboxyl group, a hydrogen and a variable side chain (the R group) that gives the residue its chemistry. Twenty proteinogenic L-amino acids build native sequences. Research peptides routinely add non-standard residues as well — D-amino acids, Aib (2-aminoisobutyric acid) and other unnatural analogs — inserted specifically to resist enzymatic cleavage.
Side chains decide handling
Side-chain chemistry predicts most practical laboratory behaviour. Methionine and cysteine oxidise; asparagine and glutamine deamidate; paired cysteines form disulfide bridges; and the balance of acidic residues (Asp, Glu) against basic ones (Lys, Arg, His) sets the isoelectric point, which in turn governs solubility at a given pH. Aib sits at position 2 of semaglutide and at positions 2 and 13 of tirzepatide precisely because it blocks DPP-4 cleavage — a structural design choice, not a formulation additive.
Why it matters
Sequences are written N-terminus to C-terminus in one-letter or three-letter code, and a certificate's mass result is only interpretable against the theoretical mass computed from that residue list. Substituting a single residue shifts molecular weight by a few daltons — enough for MS to catch.
Related terms
peptide · peptide bond · oxidation (methionine). See how to read a peptide sequence and Tirzepatide.