Research Basics
How to Read a Peptide Sequence: Codes, Ends and Molecular Weight
A peptide sequence is a compact description of the molecule. This guide shows how to read one, and how to check a molecular weight against it.
One-letter and three-letter codes
Each amino acid has a three-letter abbreviation and a one-letter code. Sequences are written from the N-terminus to the C-terminus.
| Amino acid | Three-letter / one-letter |
|---|---|
| Alanine | Ala / A |
| Arginine | Arg / R |
| Asparagine | Asn / N |
| Aspartic acid | Asp / D |
| Cysteine | Cys / C |
| Glutamic acid | Glu / E |
| Glutamine | Gln / Q |
| Glycine | Gly / G |
| Histidine | His / H |
| Isoleucine | Ile / I |
| Leucine | Leu / L |
| Lysine | Lys / K |
| Methionine | Met / M |
| Phenylalanine | Phe / F |
| Proline | Pro / P |
| Serine | Ser / S |
| Threonine | Thr / T |
| Tryptophan | Trp / W |
| Tyrosine | Tyr / Y |
| Valine | Val / V |
Modified ends and unusual residues
- A leading “H-” marks a free N-terminus and a trailing “-OH” a free C-terminal acid.
- “Ac-” means an acetylated N-terminus, and “-NH2” an amidated C-terminus.
- A “D-” prefix marks a mirror-image (D) amino acid.
- A “γ-” prefix, as in the γ-Glu of glutathione, marks a link through a side chain instead of the usual backbone.
Worked example: the molecular weight of Semax
A peptide’s average molecular weight is the sum of the average masses of its residues plus one molecule of water (18.015). Semax is Met-Glu-His-Phe-Pro-Gly-Pro.
| Residue | Average residue mass (Da) |
|---|---|
| Met | 131.193 |
| Glu | 129.116 |
| His | 137.141 |
| Phe | 147.177 |
| Pro | 97.117 |
| Gly | 57.052 |
| Pro | 97.117 |
| Water | 18.015 |
| Total | 813.93 |
The total matches the 813.9 g/mol on the Semax product page.
Why it matters
A mass on a certificate of analysis that does not match the sequence’s calculated mass is a reason to ask questions.
For laboratory research use only. Not for human or veterinary use. See our Research Use Only page.