From amino acid to peptide

Reading time: ~2 min · Part of the free certification in peptide science

A peptide is a polymer of amino acids joined by peptide bonds. It shares its raw material with proteins; the difference is length and, with it, functional specificity.

Each amino acid has an alpha carbon bonded to an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen and a variable side chain (R group). The 20 proteinogenic amino acids are classified by the nature of their R group: nonpolar/hydrophobic (Ala, Val, Leu, Ile, Phe), polar uncharged (Ser, Thr, Asn, Gln), acidic (Asp, Glu), basic (Lys, Arg, His) and special cases (Gly, Pro, Cys). This side-chain chemistry governs folding and interaction with receptors.

Acid-base nature: zwitterion, pKa and pI

In solution, an amino acid exists as a zwitterion: the protonated amino group (–NH₃⁺) and the deprotonated carboxyl group (–COO⁻) coexist. The ionization state depends on pH and on the pKa values of each group. The isoelectric point (pI) is the pH at which the molecule has zero net charge; it is a critical property in peptides because it determines their solubility (minimal near the pI), their behavior in ion-exchange chromatography and their stability in formulation. A peptide rich in basic residues will have a high pI; one rich in acidic residues, a low pI.

The peptide bond

The carboxyl of one residue reacts with the amino group of the next in a condensation that releases water and forms an amide bond. That bond has partial double-bond character: it is planar and rigid, with restricted rotation, which defines the dihedral angles (phi/psi) on which the entire three-dimensional structure is built. By convention the chain is read from N-terminus to C-terminus.

Educational content for scientific purposes. It distinguishes preclinical from clinical evidence and is not medical advice, diagnosis or a recommendation for use. NeoPeptidos products are sold labelled for research use only (RUO).