Peptide Nomenclature and Sequence Notation: How to Read Ac-, -NH₂, Aib, and Fragment Numbering

A certificate of analysis identifies a peptide by its sequence, and that sequence is written in a compact notation that assumes the reader knows the conventions. Ac-LKKTETQ is a complete and unambiguous chemical description to someone fluent in it, and almost opaque to someone who is not. Because identity confirmation depends on comparing a stated sequence against a measured mass, misreading the notation means being unable to tell whether a lot is what it claims to be.

This page is a reference for that notation: the code systems, the reading order, the prefixes and suffixes that denote terminal chemistry, the non-standard residues that appear in engineered peptides, and the conventions used for fragments and salt forms. It is a documentation-literacy guide for laboratory work and contains no handling, preparation, or use instructions of any kind.

Why the notation matters

Two peptides can differ by a single notational element and be genuinely different molecules with different masses. An acetyl prefix adds roughly 42 daltons. A C-terminal amide instead of a free acid subtracts about one dalton. A fragment designation of 17-23 versus 1-43 is the difference between a seven-residue peptide and a whole protein. None of these differences are cosmetic, and each of them shows up in a mass spectrum.

This is why a certificate should be read as a chemical statement rather than a label. The certificate-of-analysis guide covers what each field on the report means; this page covers how to parse the identity string itself.

The two code systems

The twenty standard amino acids each have a three-letter code and a one-letter code. Three-letter codes are more legible and are usual in prose and on specification documents; one-letter codes are compact and are usual for longer sequences and in databases. They carry identical information.

A few of the one-letter assignments are not first-initial and are worth memorising because they are the ones most often misread: K is lysine, R is arginine, W is tryptophan, Y is tyrosine, Q is glutamine, N is asparagine, and E is glutamic acid. The pairs that cause the most trouble are aspartic acid (D) against asparagine (N), and glutamic acid (E) against glutamine (Q) — in each case an acid and its amide, differing by about one dalton, which is precisely the kind of difference that matters when comparing a sequence against a measured mass.

Worked example: the BPC-157 sequence written in three-letter code as Gly-Glu-Pro-Pro-Pro-Gly-Lys-Pro-Ala-Asp-Asp-Ala-Gly-Leu-Val is GEPPPGKPADDAGLV in one-letter code — fifteen residues, hence “pentadecapeptide.” Counting characters in the one-letter form is the quickest way to confirm a stated chain length.

Reading direction: N-terminus to C-terminus

By universal convention a peptide sequence is written from the N-terminus on the left to the C-terminus on the right. The N-terminus is the end bearing the free amino group; the C-terminus is the end bearing the carboxyl group. This is the same direction in which the chain is described biologically, and the same direction in which residue numbering runs.

It is, incidentally, the opposite of the order in which the molecule is usually built. Solid-phase peptide synthesis assembles chains from the C-terminus backward, so the first residue coupled is the last one written. That mismatch is worth holding in mind when reasoning about which residues are most exposed to accumulated coupling errors — a point relevant to the impurity profiles described in Peptide Impurities Explained.

Terminal modifications: Ac- and -NH₂

The two ends of a chain are frequently modified, and the notation for this sits outside the residue string as a prefix or suffix.

Ac- at the start means the N-terminus is acetylated. An acetyl group caps the free amino group, replacing a hydrogen with CH3CO–. This adds approximately 42 daltons and removes a positive charge that the free amine would otherwise carry. Acetylation is often used to make a synthetic fragment resemble its context within a parent protein, or to reduce susceptibility to degradation from that end.

-NH2 at the end means the C-terminus is amidated. The terminal carboxylic acid –COOH is replaced by a carboxamide –C(O)NH2. Because an –OH is exchanged for an –NH2, the amidated form is about one dalton lighter than the corresponding free acid, and it loses the negative charge the free carboxylate would carry at neutral pH. Where no suffix is given, the free acid is normally implied.

These are not optional annotations. Ac-LKKTETQ and LKKTETQ are different molecules with masses differing by roughly 42 daltons, which is why the TB-500 designation specifies the acetylated form — and why a mass spectrum is the test that distinguishes them. Similarly, the amidated C-terminus of a compound such as tirzepatide is part of its defined structure, not a detail to be omitted.

Non-coded and D-residues

Engineered peptides routinely contain residues that are not among the twenty standard amino acids. These have no one-letter code and are written by abbreviation inside the sequence.

Aib is α-aminoisobutyric acid, also written as 2-aminoisobutyric acid or α-methylalanine. Structurally it is alanine carrying a second methyl group on the α-carbon. That extra methyl makes the α-carbon quaternary, which removes the α-hydrogen and sharply restricts the conformations the backbone can adopt — Aib is consequently a strong promoter of helical structure. It also confers resistance to enzymatic cleavage, because proteases recognise the standard backbone geometry that Aib disrupts. Aib substitution is a common design choice in engineered incretin-class peptides; the GLP-2 TZ and GLP-3 RT pages both note it.

D- prefixes denote stereochemistry. The standard amino acids of biology are the L-enantiomers. A D- prefix (as in D-Phe) marks the mirror-image configuration at the α-carbon. A D-residue has exactly the same mass as its L-counterpart, so mass spectrometry alone cannot distinguish them — and yet the two are different molecules with different three-dimensional shapes and different susceptibility to proteolysis. This is a clear illustration of why identity by mass is necessary but not sufficient, a theme developed in HPLC vs mass spectrometry.

Other conventions appear as needed: a lower-case d is sometimes used for the same purpose, unusual residues may be spelled out, and side-chain conjugations — such as a fatty acid attached to a specific lysine — are typically described in prose alongside the sequence rather than encoded within it.

Fragment designations and residue numbering

When a synthetic peptide corresponds to part of a larger natural sequence, the convention is to give the parent name with the residue range in parentheses or after a comma: GRF(1-29), or “residues 17–23.” Numbering starts at 1 at the parent's N-terminus.

Two things follow. First, the range tells you the chain length directly: 1-29 is twenty-nine residues, 17-23 is seven. Second, and more importantly, a fragment is not its parent. The same numbering convention that connects them also separates them, and conflating the two is the single most common identity error in this field. TB-500 corresponds to residues 17–23 of a 43-residue parent protein: seven residues against forty-three, a mass difference of more than fivefold. Findings reported for the intact protein describe the intact protein.

Fragment names also drift. The same 29-residue analog discussed on the CJC-1295 / Ipamorelin page appears in the literature and in vendor listings under more than one name, which is exactly why the molecular weight on a certificate is a more dependable identifier than the product name.

Salt and counter-ion notation

Synthetic peptides are usually isolated as salts rather than as free bases or free acids, and the counter-ion is often stated after the sequence — for instance as an acetate or trifluoroacetate (TFA) salt. The counter-ion comes from the purification chemistry: reverse-phase HPLC mobile phases commonly contain an acid that pairs with basic side chains, and unless a deliberate salt-exchange step follows, that is the form in which the peptide is recovered.

The practical consequence is mass that is not peptide. Counter-ions associate with basic residues, so sequences rich in lysine, arginine, or histidine carry proportionally more of them. Together with residual water this is why the mass of powder in a vial exceeds the mass of peptide in it, and why the net peptide content figure exists at all — the subject of Net Peptide Content vs. Purity. A sequence with several basic residues, such as the doubly-lysine-containing heptapeptide discussed above, illustrates the effect clearly.

Monoisotopic versus average mass

One last convention causes recurring confusion when a certificate figure is checked against a reference value. There are two legitimate ways to state a molecular mass.

Monoisotopic mass is calculated using the single most abundant isotope of each element — carbon-12, hydrogen-1, and so on. It corresponds to what a high-resolution mass spectrometer measures for the lightest isotopic peak, and it is the figure normally quoted on analytical reports for small and medium peptides.

Average mass is calculated from the natural isotopic abundance-weighted average of each element, and is the figure typically given in catalogues and reference tables.

For a small peptide the two are very close; as chains lengthen the gap widens, because there are more carbon atoms each contributing a small probability of being carbon-13. A modest discrepancy between a certificate value and a textbook value is therefore usually a difference of convention rather than a failed identification — but establishing which convention each figure uses is a necessary step before concluding anything from the comparison.

For larger peptides there is a further wrinkle. Electrospray ionisation produces multiply-charged ions, so the instrument reports a series of mass-to-charge values that software deconvolutes into a single neutral mass. A report that shows a coherent charge-state series alongside the deconvoluted result is more informative than a bare number, as noted on the GLP-3 RT page.

FAQ

What does Ac- mean in front of a peptide sequence? It means the N-terminus is acetylated: an acetyl group caps the free amino group at the left-hand end of the sequence. This adds approximately 42 daltons and removes the positive charge the free amine would carry. It is part of the molecule's definition, so Ac-LKKTETQ and LKKTETQ are different compounds with different masses, distinguishable by mass spectrometry.

What does -NH2 at the end of a sequence mean? It means the C-terminus is amidated — the terminal carboxylic acid is replaced by a carboxamide. Because an -OH is exchanged for an -NH2, the amidated form is about one dalton lighter than the free acid and loses the negative charge a free carboxylate would carry at neutral pH. When no suffix is given, the free acid is normally implied.

What is Aib in a peptide sequence? Aib is alpha-aminoisobutyric acid, a non-coded amino acid that is structurally alanine with a second methyl group on the alpha-carbon. The extra methyl makes that carbon quaternary, restricting backbone conformation and promoting helical structure, and it confers resistance to enzymatic cleavage because proteases recognise the standard backbone geometry that Aib disrupts. It is a common design element in engineered incretin-class peptides.

Why can mass spectrometry not distinguish a D-amino acid from an L-amino acid? Because they are mirror images with identical atomic composition and therefore identical mass. Stereochemistry changes three-dimensional shape and biological susceptibility to proteases, but not molecular weight. This is one reason identity by mass alone is necessary but not sufficient, and why chromatographic behaviour and orthogonal methods matter.

What does a designation like residues 17-23 tell you? It identifies which part of a larger parent sequence the synthetic peptide corresponds to, with numbering starting at 1 from the parent's N-terminus. It also gives the chain length directly — residues 17 to 23 is seven residues. Most importantly it signals that the material is a fragment and not the parent molecule, so results reported for the full-length protein do not automatically describe the fragment.

References

• Bachem — Quality Control of Amino Acids and Peptides: A Guide
• Sigma-Aldrich — Peptide Stability
• Daicel Pharma Standards — Impurity Profiling in Peptide Synthesis