What Is a Peptide Sequence?

A peptide sequence is the ordered list of amino acids that make up the peptide chain. Just as letters form words, amino acids form peptides — and the specific order determines the peptide's biological properties. Changing even one amino acid can dramatically alter function.

Amino Acid Codes

Amino acids are represented using two systems. The three-letter code uses abbreviations like Gly (glycine), Ala (alanine), Leu (leucine). The one-letter code uses single letters: G (glycine), A (alanine), L (leucine). Research publications use both, but COAs and product descriptions typically use three-letter codes for clarity.

The 20 standard amino acids and their one-letter codes are: G-Glycine, A-Alanine, V-Valine, L-Leucine, I-Isoleucine, P-Proline, F-Phenylalanine, W-Tryptophan, M-Methionine, S-Serine, T-Threonine, C-Cysteine, Y-Tyrosine, H-Histidine, D-Aspartic acid, E-Glutamic acid, N-Asparagine, Q-Glutamine, K-Lysine, R-Arginine.

Reading Direction

Peptide sequences are written from left to right, starting with the N-terminus (amino end) and ending with the C-terminus (carboxyl end). This convention is universal in biochemistry. For example, GHK-Cu's sequence is written Gly-His-Lys, meaning glycine is at the N-terminus and lysine is at the C-terminus.

Common Modifications

Acetylation (Ac-): An acetyl group added to the N-terminus. Protects against exopeptidase degradation and can improve stability. Written as "Ac-" before the sequence.

Amidation (-NH2): The C-terminal carboxyl group is converted to an amide. Increases resistance to carboxypeptidases. Written as "-NH2" after the sequence.

Disulfide bonds: Covalent bonds between cysteine residues. Important for structural peptides. Noted in the sequence annotation.

Why Sequence Matters for Purchasing

When evaluating a peptide supplier, the published sequence should match the known literature for that compound. This is your first quality check — before purity data or mass spectrometry. If a supplier does not publish the sequence for their product, or if it differs from published references, this warrants further investigation.