Amino Acid

Amino acids are the building blocks from which all peptides are assembled. Each has an alpha-carbon at its centre bearing an amino group, a carboxyl group, a hydrogen atom and a side chain (the R group) whose structure defines the residue’s chemical personality. Native sequences draw on twenty proteinogenic L-amino acids. Research peptides frequently include non-standard building blocks too — D-amino acids, Aib (2-aminoisobutyric acid) and other unnatural analogs — placed there on purpose to withstand enzymatic cleavage.

How side chains shape lab handling

Most of what a peptide does on the bench can be predicted from its side chains. Methionine and cysteine are prone to oxidation; asparagine and glutamine undergo deamidation; cysteine pairs link up as disulfide bridges; and the ratio of acidic residues (Asp, Glu) to basic residues (Lys, Arg, His) fixes the isoelectric point, which then controls solubility at any given pH. Semaglutide carries Aib at position 2, and tirzepatide at positions 2 and 13, exactly because this residue prevents DPP-4 cleavage — a deliberate structural choice rather than a formulation ingredient.

Relevance for identity testing

A sequence is written from the N-terminus to the C-terminus using one-letter or three-letter codes, and the mass value on a certificate only makes sense when compared with the theoretical mass calculated from that list of residues. Exchanging just one residue alters the molecular weight by a few daltons — a difference MS readily detects.

Related terms

peptide · peptide bond · oxidation (methionine). See how to read a peptide sequence and Tirzepatide.