Reading a peptide sequence and molecular formula
By BioGenisys Research Team · 4 min read · Updated 2026-10-27
A peptide's sequence is the order of its building blocks, and it works like an ID card. With a little notation you can read how long a peptide is, what has been changed about it, and where the chemical formula and weight on a compound note come from.
The short version
- Each amino acid has a short code: a three-letter one (Gly) and a one-letter one (G).
- A sequence is read left to right, from the start of the chain to the end.
- The chemical formula counts every atom in the molecule. The weight comes from adding them all up.
Amino acid codes
There are 20 common amino acids, and each has a three-letter code and a one-letter code. Glycine is Gly or G, proline is Pro or P, lysine is Lys or K. A sequence is written from the chain's starting end on the left (chemists call it the N-terminus) to its finishing end on the right (the C-terminus). So Gly-Pro-Lys and GPK describe the same three-amino-acid peptide.
| Amino acid | Three-letter | One-letter |
|---|---|---|
| Glycine | Gly | G |
| Alanine | Ala | A |
| Proline | Pro | P |
| Lysine | Lys | K |
| Arginine | Arg | R |
| Histidine | His | H |
| Aspartic acid | Asp | D |
| Glutamic acid | Glu | E |
Common changes
- A cap on the starting end (written Ac-): a small chemical cap added to one end, often to make the peptide sturdier.
- A changed finishing end (written -NH2): the end is turned into a slightly different chemical group, called an amide.
- A ring: the chain is closed into a loop, end to end or through its side branches.
- Unusual building blocks: mirror-image versions of normal amino acids (marked D-) or lab-made ones, often marked with a prefix or in brackets.
- Attached tails: a fatty-acid chain or another group fixed to one amino acid, noted in the name or the sequence.
From sequence to formula
A chemical formula counts the atoms of each element. C2H6O, for example, is ordinary alcohol: 2 carbon, 6 hydrogen and 1 oxygen atom. When two amino acids join, they give off one molecule of water. So a peptide's formula is all its amino acids added together, minus one water for each link. A chain of 15 amino acids has 14 links. Any changes then add or remove their own atoms.
Two kinds of weight
Molecular weight, shown in grams per mole (g/mol), is usually the average weight, based on the natural mix of slightly heavier and lighter atoms. That's the figure on our compound notes. Mass spectrometry often reports a slightly different figure that counts only the most common form of each atom. The two differ a little, and more for larger peptides, so compare like with like.
Salt left from manufacturing adds its own weight on top. See Peptide salt forms. Formulas and weights for each compound are in our compound notes.
For research use only. Not for human or veterinary use.