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

Every peptide guide on this site writes sequences as strings of single letters: GHK, AEDG, YNWNSFGLRF. Those letters are shorthand for the 20 amino acids that life uses to build peptides and proteins. This reference lists all 20 with their one-letter and three-letter codes, explains what makes each one chemically distinct, and shows how to read a peptide sequence the way researchers do.

The short answer

• There are 20 standard amino acids in the genetic code, each with a three-letter abbreviation, such as Ala, and a one-letter code, such as A.
• The one-letter code was popularized by Margaret Dayhoff in the 1960s to make long sequences compact enough for early computer databases, and was adopted as an international standard in 1968.
• Where possible the letter is the first letter of the name (A for alanine). Where names clash, other letters were assigned, such as K for lysine, W for tryptophan and Y for tyrosine.
• Sequences are always written from the N-terminus on the left to the C-terminus on the right.
• Nine of the 20 are essential in human nutrition because the body cannot make them.

The amino acid code chart

The nine essential amino acids for humans are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. That is a nutrition fact rather than a laboratory one, but it is often the first thing people ask.

Why some letters seem odd

Ten amino acids start with the same letters as others, so the code had to improvise. The memory aids below are widely taught, though they are teaching tricks rather than official reasons:

• F for phenylalanine: “fenylalanine”.
• R for arginine: it sounds like “aRginine”.
• Y for tyrosine: “tYrosine”.
• W for tryptophan: its double ring, or “tWo rings”.
• K for lysine: L was taken by leucine, and K sits next to L in the alphabet.
• Q for glutamine: “Q-tamine”.
• D and E for aspartic and glutamic acid: “asparDic” and “glutEmic”.
• N for asparagine: “asparagiNe”.

Extra and ambiguity codes

The J code exists for a reason researchers meet on certificates of analysis: leucine and isoleucine have exactly the same mass, so mass spectrometry alone cannot distinguish them.

How to read a peptide sequence

A sequence reads left to right from the N-terminus, the end with a free amino group, to the C-terminus, the end with a free carboxyl group. On top of the letters, chemists use a few standard marks:

• Ac- at the start means the N-terminus is capped with an acetyl group.
• -NH₂ at the end means the C-terminus is an amide rather than an acid, common in natural peptide hormones.
• D- before a residue, or a lowercase letter, means the mirror-image D form. See our guide to D-amino acids.
• pGlu or pE means pyroglutamate, a cyclized N-terminal glutamine or glutamic acid.
• Bracketed or multi-letter names, such as Aib or 2Nal, mark non-standard building blocks.

Why the chemistry matters to researchers

• Charge drives behaviour. Lysine, arginine and histidine carry positive charge and attract counter-ions, which affects solubility and how much of the powder is actually peptide. See our net peptide content guide.
• Some residues are fragile. Methionine, tryptophan and cysteine oxidize; asparagine and aspartic acid rearrange. See how peptides degrade.
• Mass checks have blind spots. Leucine and isoleucine, and D- and L-forms, share masses, so identity testing relies on more than mass alone. See our HPLC testing guide.
• Molecular weight follows from the sequence. That is the number to check on a COA and to use in our peptide calculator.

Frequently asked questions