- 20
- Standard amino acids, each with its own letterShorter forms save space on labels and certificates.
- 1968
- Year tentative one-letter rules were published
- N to C
- The direction a sequence is read, left to right
The short answer
What do the letters in a peptide sequence mean?
A peptide sequence is a row of letters. Each letter stands for one amino acid, and the row lists them in the order they are linked. A peptide is a short chain of amino acids, so the row is the chain written out.
You read it from left to right. The left end is called the N-terminus and the right end is the C-terminus. The first letter is the first amino acid in the chain.
The letter system was set out as tentative rules by a nomenclature commission in 1968 [1]. One short lesson lets you read it.
You will meet this on product pages, on certificates and in lab notes. A sequence is the quickest way to say exactly which peptide a vial holds. Two products can have similar names and different sequences, so compare the letters, not the names.
Reading order
Which way do you read it?
Start at the left. The left end, the N-terminus, is the end that carries a free amine group. The right end, the C-terminus, carries a free acid group. These are textbook terms, and they only tell you which end is which.
Direction matters. The same letters in the reverse order are a different peptide. A row that reads GAK is not the same chain as KAG, even though both use the same three letters.
Counting the letters gives the length. A row of ten letters is a ten-amino-acid chain. That number should match the length stated on the product page and the certificate.
It is a quick check you can do with no equipment. If a page says a peptide is fifteen amino acids long and the row has fourteen letters, something is off. Ask the supplier before you go further.
- 01
Find the left end
The first letter is the N-terminus.
- 02
Read to the right
Each letter is the next amino acid in the chain.
- 03
Stop at the last letter
That is the C-terminus.
- 04
Count the letters
The count is the length of the peptide.
The alphabet
Where do the letters come from?
Each of the 20 standard amino acids has a full name, a three-letter short form and a one-letter code. Many codes are simply the first letter of the name. G is glycine and A is alanine.
Others are not. K is lysine, W is tryptophan and F is phenylalanine. Nobody guesses these. You look them up, the same way you look up a country code.
The commission followed up with recommended symbols for amino acid derivatives and peptides in a report published in 1972 [2]. Shared symbols let labs write the same chain the same way.
Two letters are easy to confuse on a first read. I is isoleucine and L is leucine. Check each letter slowly. A single swapped letter names a different peptide.
- 1968
Tentative rules for a one-letter notation are published.
- 1972
Recommended symbols for amino acid derivatives and peptides are published.
A few codes to know
| Letter | Short form | Amino acid |
|---|---|---|
| G | Gly | Glycine |
| A | Ala | Alanine |
| K | Lys | Lysine |
| W | Trp | Tryptophan |
| F | Phe | Phenylalanine |
| E | Glu | Glutamic acid |
| D | Asp | Aspartic acid |
| Q | Gln | Glutamine |
What is missing
What can a row of letters leave out?
Plain letters name the amino acids and their order. They do not show everything about a molecule. Two peptides can share the same letters and still differ at the ends.
One review lists changes that can happen to a peptide after it is made: acetylation at the N-terminus, amidation at the C-terminus, and the formation of an N-terminal pyroglutamyl residue [3]. A plain row of capitals shows none of these.
Cyclic peptides and non-standard residues need more than letters. A 2026 paper uses a notation called HELM, which records which building block sits where and how the parts connect [4]. You do not need to read HELM. It shows that a simple row of letters has limits.
That is why a good label states the ends and the shape in words next to the sequence. If it is silent, treat the row as incomplete and ask.
- Does the N-terminus carry an acetyl group?
- Does the C-terminus carry an amide group?
- Is the peptide linear or cyclic?
- Are any residues non-standard?
Checking it
Does a matching sequence prove what is in the vial?
No. A sequence on a label is a statement. The test result is what checks it. A certificate of analysis lists the measured mass and the purity from the lab. The certificate of analysis page shows where to look.
Here is why a mass reading has limits. Leucine and isoleucine are two different amino acids with identical molecular masses [5]. A swap between them is hard to see on a plain mass reading.
A 2026 study built a two-step fragmentation method to tell the two apart in peptides, including linear and cyclic ones [5]. The authors describe it for use in quality control of leucine and isoleucine isomer impurities.
So the useful question is what method confirmed the sequence. A certificate that states mass alone confirms the total, not the order. The quality page explains how testing is handled.
Your checklist
What should you check on a label?
Before you rely on a sequence, run through a short list. It takes a minute. The supplier checklist has the wider version.
If a term on a certificate is unfamiliar, the glossary explains it in plain words. If you want to see how a lab handles a sample, the lab page walks through it.
- The sequence is written in capital letters, N-terminus on the left
- The letter count matches the stated length
- End groups such as acetyl or amide are stated, or marked as free
- The certificate gives a measured mass and a purity
- The certificate says how the sequence was confirmed
- The product name on the vial matches the certificate
Questions
Common questions
Does the sequence tell me how pure a peptide is?
No. The sequence says what the chain should be. Purity is a separate measurement on the certificate.
Can a row of letters show every detail of a peptide?
No. It names the amino acids and their order. End groups and ring shapes need to be stated in words.
What is the N-terminus?
It is the left end of the chain, the end with a free amine group. The sequence is read from there.
Can two peptides share the same letters?
Yes. If the order differs, or the ends are modified, they are different molecules.