Every standard amino acid has a three-letter abbreviation and a one-letter symbol, and both are set by IUPAC and IUBMB. Alanine is Ala or A, glycine is Gly or G, tryptophan is Trp or W. The full table is below. After it we explain how the codes are built, how to read a peptide sequence written with them, and what the less common symbols (B, Z, X, U, O) and modified-residue notations mean.

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Key Takeaways

There are 20 standard amino acids. Each has a three-letter code (one capital, two lowercase, such as Gly) and a one-letter code (such as G).
Sequences are written from the N-terminus on the left to the C-terminus on the right.
Hyphens matter: Gly is a free amino acid, while Gly- , -Gly- and -Gly are terminal and internal residues.
B, Z and X are ambiguity symbols, not extra amino acids. U (selenocysteine) and O (pyrrolysine) are rarer relatives.
Modified peptides use square brackets and position numbers, for example [D-Tyr 4], so the substitution is explicit.

The 20 standard amino acids and their codes
How the codes are built
Reading a peptide sequence
Free amino acids versus residues
Ambiguity codes and rarer residues
Modified residues, termini and named peptides
Common mistakes with amino acid abbreviations
Why the codes matter when you buy or characterise peptides
Our perspective
Find research peptides with clear specifications
FAQ
Sources

The 20 standard amino acids and their codes

The table follows the IUPAC-IUBMB list of standard amino acid symbols, which dates from the 1984 recommendations on three-letter and one-letter symbols. It is sorted alphabetically by name, which is how most reference lists and data sheets present it.

Amino acid	Three-letter code	One-letter code
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

Two quick answers to common questions. The letter E stands for glutamic acid (glutamate), and D stands for aspartic acid (aspartate). Asparagine is N and glutamine is Q, so the amide forms never share a letter with their acid counterparts.

How the codes are built

The three-letter codes are easy to remember because they usually take the first three letters of the amino acid name: Ala, Gly, Leu, Pro. The one-letter codes generally use the initial letter when it is unique, which is why A, G, L and P are what you would expect. Where several amino acids share an initial, the assignments become mnemonic or disambiguating. Phe/F, Arg/R and Trp/W are the classic examples, and they are the ones people most often get wrong when working from memory.

A practical way to learn them is in groups:

Initial letter is the code: A, C, G, H, I, L, M, P, S, T, V.
Same initial, different letter used: R (arginine), K (lysine), F (phenylalanine), W (tryptophan), Y (tyrosine).
Acid and amide pairs: D and E for the acids, N and Q for the amides.

The original IUPAC one-letter notation paper from 1971 also explains why some letters were left unassigned: U, O and J were avoided because of visual, printing or linguistic confusion. That is why J is not used for any amino acid in the standard scheme.

Capitalisation

IUPAC recommends the three-letter style of one capital followed by two lowercase letters. Gly is correct, while GLY and gly are variants that you will see in software output but should not use in a written methods section or a product specification. One-letter codes are written as capitals.

Reading a peptide sequence

A peptide is a chain of amino acids joined by peptide bonds, and the abbreviations let you write that chain compactly. The convention is fixed: residues are written from the free-amino end (the N-terminus) on the left to the free-carboxyl end (the C-terminus) on the right. Where an end is not known to be the end of the complete molecule, IUPAC uses a slash at that end.

Take a hypothetical tripeptide, written for illustration only:

Three-letter form: Gly-Ala-Lys
One-letter form: GAK

Both describe the same molecule: glycine at the N-terminus, alanine in the middle and lysine at the C-terminus. Reversing the order, Lys-Ala-Gly (KAG), is a different peptide with different properties, even though it contains the same three amino acids. This is why direction is not optional information.

The three-letter form is easier to read for short peptides and keeps the hyphens that show the bonds. The one-letter form is the standard for long sequences, databases and software, where a 30-residue chain would be unwieldy in three-letter form.

Why sequence length and composition matter in practice

A sequence tells you what the peptide should be, but a certificate of analysis tells you whether the material in the vial matches it. Our guide on peptide identity versus purity explains the difference: identity confirms that the molecule is the intended sequence, while purity measures how much of the sample it makes up. If you want to see how a sequence becomes a physical product, how peptides are synthesised from amino acids to lyophilised powder walks through it step by step.

Free amino acids versus residues

The same abbreviation can mean slightly different things depending on context. A free amino acid is a complete molecule with its own amino and carboxyl groups. When amino acids join into a peptide, each loses the elements of water, and what is left in the chain is called an amino acid residue.

Amino acids joining into a peptide chain

IUPAC also defines the ends of the chain. The N-terminal residue is the one with a free or not-acylated amino group, and the C-terminal residue is the one with a free or non-acylating carboxyl group, as set out in the same recommendations.

The IUPAC peptide symbolism rules use hyphens to show which of these you mean:

Written as	Meaning
Gly	Free glycine
Gly-	Glycine as the left-hand (N-terminal) residue
-Gly-	Glycine as an internal residue
-Gly	Glycine as the right-hand (C-terminal) residue

In everyday use the hyphens at the very ends are often dropped, so Gly-Ala-Lys is understood without writing Gly-Ala-Lys as "Gly-Ala-Lys" with end markers. The point is that the hyphen is a bond, and it makes direction and attachment explicit.

Ambiguity codes and rarer residues

Sometimes a sequence is only partly determined, for instance when a classical analysis cannot tell an acid from its amide. IUPAC provides symbols for those cases, listed in the standard symbol table:

One-letter	Three-letter	Meaning
B	Asx	Aspartic acid or asparagine
Z	Glx	Glutamic acid or glutamine
X	Xaa	Unknown or other amino acid
U	Sec	Selenocysteine

The 1971 notation paper explains that B, Z and X exist to prevent ambiguity in partly determined sequences. X in particular can have other uses in some contexts, so IUPAC's own addendum advises defining the convention whenever ambiguity could matter. If you use X in a methods section, say what it stands for.

Less common relatives include pyrrolysine (Pyl, O) and homocysteine (Hcy), which appear in a university reference list of amino acids alongside selenocysteine. You will rarely meet them in routine peptide work, but you should recognise them if they appear in a database entry.

Modified residues, termini and named peptides

Most research peptides are not just a bare string of standard residues. Terminal groups and substitutions change the chemistry, and nomenclature has to show them.

Configuration and termini

By default, the codes refer to the L-form. To show the mirror-image form, IUPAC puts a D before the symbol, and DL for a mixture, as in D-Ala. For the ends of the chain, the free N-terminus can be written as H and the free C-terminus as OH, and these can be modified to show the ionisation state where that matters. A full unprotected tripeptide can therefore be written H-Gly-Ala-Lys-OH.

That convention is useful because a terminal modification, such as an acetylated N-terminus or an amidated C-terminus, replaces the H or OH. Two peptides with the same residues but different end groups are different compounds, with different masses and often different solubility.

Substitutions in named peptides

For peptides that have a trivial name, IUPAC describes changes with square brackets and a position number. Examples from the named-peptide recommendations include [Cit 8]vasopressin, where citrulline replaces the residue at position 8, and [D-Tyr 4] for a D-tyrosine at position 4. The same document separates other kinds of modification, each with its own abbreviated construction:

N-terminal extension
C-terminal extension
insertion
residue removal
side-chain substitution
partial sequence
sequence reversal
enantiomeric forms

In short, the bracket tells you what changed and the number tells you where. When a supplier lists a peptide with a bracketed substitution or a D-residue, that is a deliberate structural difference from the parent sequence, not a typographic quirk.

The whole system, from three-letter symbolism to one-letter notation to modified peptides, sits in one framework: the IUPAC-IUBMB Amino Acids and Peptides home page is the place to check a symbol when two sources disagree.

Common mistakes with amino acid abbreviations

The errors below turn up often in lab notes, spreadsheets and product listings.

Mixing the two systems in one sequence. Write either Gly-Ala-Lys or GAK, never GAly-K.
Confusing E and D, or Q and N. The acids (D, E) and amides (N, Q) differ in charge at neutral pH, so a swap changes behaviour.
Assuming the initial is always the code. It is not for arginine, lysine, phenylalanine, tryptophan or tyrosine.
Dropping direction. A sequence read right to left describes a different peptide.
Using X without defining it. Say whether it means an unknown residue or a non-standard one.
Writing GLY or gly in formal documents. Use Gly.

Why the codes matter when you buy or characterise peptides

The abbreviations are the shared language between a product label, a specification sheet and an analytical report. A sequence string is the first thing to compare against a certificate of analysis, followed by the molecular mass and the batch identifier. Our article on understanding peptide product specifications shows how those fields fit together, and how to verify a COA using the batch number covers the traceability side.

If you work on the analytical end, amino acid analysis confirms composition by measuring the amino acids released after hydrolysis. We cover that in validating amino acids in your lab with AOAC SMPRs and COA checks. For handling after delivery, see how to assess peptide solubility before research, since the residues in a sequence largely determine what solvent is sensible, and our notes on how temperature affects peptide stability.

Our perspective

We see the same pattern in how researchers evaluate suppliers: people who can read a sequence string and a COA together spot problems early. A label that gives a name but no sequence, or a COA whose batch identifier does not match the vial, is a reason to stop and ask questions before anything goes into an experiment.

For that reason we treat nomenclature as part of quality, not as trivia. Clear labelling, batch identification and a certificate of analysis where applicable are only useful if the reader can interpret them, which is why our educational content explains the notation, the testing and the specifications in plain terms. Our advice is simple: write the sequence in one consistent system, state the termini and any modifications, define X if you use it, and check the sequence against the documentation for the exact batch you hold.

All of our products are supplied strictly for laboratory research and analytical purposes, and are not intended for human or veterinary use.

Find research peptides with clear specifications

If you are comparing compounds for laboratory work, our research peptides catalogue is organised by research area, including tissue, skin, metabolic, longevity, nootropic, hormones and signalling, immune research and laboratory supplies. Products are supplied as research-grade materials with labelling, batch identification and COA documentation where applicable.

FAQ

What is the difference between three-letter and one-letter amino acid codes?

The three-letter codes, such as Ala or Trp, are easier to read and show the bonds between residues when hyphenated. The one-letter codes, such as A or W, are more compact and are standard for long sequences and software. Both are defined in the same IUPAC-IUBMB recommendations and map one-to-one onto the 20 standard amino acids.

Which amino acid is abbreviated E?

E is glutamic acid, written Glu in three-letter form. Its amide, glutamine, is Gln and Q. Aspartic acid is D (Asp) and asparagine is N (Asn), so each acid and amide pair has its own distinct codes.

In which direction is a peptide sequence written?

From the N-terminus on the left to the C-terminus on the right. The N-terminal residue has the free amino group and the C-terminal residue has the free carboxyl group. Reversing the order gives a different peptide.

What do B, Z and X mean in a sequence?

B means aspartic acid or asparagine (Asx), Z means glutamic acid or glutamine (Glx), and X means an unknown or other amino acid (Xaa). They are used when a sequence is only partly determined. Because X can have other meanings in some settings, define it in your document.

How are modified amino acids shown in peptide names?

D-forms are marked with a D before the symbol, and substitutions in named peptides are shown in square brackets with a position number, as in [D-Tyr 4]. Terminal groups can be shown with H and OH, or with the modifying group in their place. The IUPAC named-peptide recommendations set out each kind of modification.