Amino Acid Code Chart: One-Letter and Three-Letter Codes

The amino acid code chart

The mappings follow the IUPAC–IUB amino acid notation recommendations. This table covers the standard set; it is not a list of every amino acid or modified residue encountered in research.

How to read a short peptide sequence

For a simple linear sequence, read from the N-terminus toward the C-terminus. Replace each one-letter symbol with its matching three-letter code, keeping the order unchanged.

For example, the illustrative sequence AGKS becomes:

Ala–Gly–Lys–Ser

That is alanine, glycine, lysine, and serine, in that order. The example contains four residues. It is a notation exercise, not a product specification or a statement about biological activity.

If a document instead gives SGKA, the residue order is different. A useful manual check is to number the positions before comparing two records:

Here, positions 1 and 4 differ. Keep the original sequence beside your conversion so that a transcription check can be repeated by another reader.

D is a code; D- is a configuration prefix

In one-letter sequence notation, D means aspartic acid. It does not mean that the following residue has the D configuration.

In three-letter notation, a prefix such as D-Ala specifies D-alanine. A plain amino-acid symbol normally denotes the L configuration for a chiral amino acid unless another configuration is stated. The conventional three-letter spelling uses one capital letter and two lowercase letters, such as Ala or Gln. See the IUPAC–IUB guidance on three-letter symbols.

When reading a supplier's sequence field, check its notation key before translating lowercase letters, braces, or other special characters. Preserve the original notation in your records if the convention is unclear.

Preserve terminal groups and modification notes

A string of standard residue codes may be only one part of a peptide description. Terminal groups and modifications can require additional notation.

For example, Ac-Gly specifies N-acetylglycine. A terminal -NH₂ can indicate an amide in the relevant sequence notation; it should not be silently discarded when copying a specification. IUPAC provides separate conventions for substituted amino acids and terminal groups.

Keep residue order, terminal annotations, and modification positions in separate, clearly labeled fields when you compare documents. Do not reduce an annotated sequence to a bare letter string and then treat the two descriptions as interchangeable.

Using the chart when comparing a catalog item

Treat the chart as a reading aid. A correct code conversion does not confirm a material's identity, purity, available quantity, or current-lot documentation.

For a procurement inquiry, record the catalog identifier and copy the complete sequence or identity description from the source document. Flag any discrepancy for clarification before assuming that two listings refer to the same material.

NorthVerity Bio's product catalog and contact page provide the next steps for an item-specific inquiry. Include the catalog reference and the exact field you need clarified.