Reading Peptide Sequences: Codes, Modification Shorthand and Mass Arithmetic

Sequences run left to right, beginning at the N-terminus and ending at the C-terminus, written in one-letter or three-letter amino-acid codes, with each chemical modification noted as a prefix or suffix attached to the terminus it belongs to. Picking up this notation takes roughly ten minutes and gives you the quickest possible cross-check between a vial and its certificate of analysis: the sequence defines identity, allows the molecular weight to be predicted, and accounts for most of a molecule's behaviour once dissolved. What follows covers both code systems, the direction convention, the modification shorthand that actually appears on research labels, and a worked calculation converting a sequence into a mass you can hold against a COA. Everything described concerns laboratory reference material.

Why the N-to-C direction is not optional

Residues link through a peptide bond created between one residue's carboxyl group and the next residue's amino group. Because that reaction has a direction, each chain ends differently: one end keeps a free amino group (the N-terminus), the other a free carboxyl group (the C-terminus). Convention everywhere puts the N-terminus first. Gly-Glu-Pro and Pro-Glu-Gly are distinct molecules of identical composition and identical mass, and no mass spectrometer will tell them apart, which is why MS identity confirmation normally accompanies a declared sequence instead of replacing one.

Numbering runs the same way. A label reading HGH Fragment 176-191 uses positions counted from the N-terminus of the 191-residue parent protein, so the product is the 16 residues sitting at those positions rather than a purpose-designed sequence.

The two coding systems

Because three-letter codes are legible and unambiguous, they prevail on product labels and certificates. One-letter codes are compact, so they prevail in databases, journals and synthesis order forms. Both notate the same twenty proteinogenic residues.

Leucine and isoleucine are structural isomers and therefore weigh the same. That one fact underlies a familiar annoyance in peptide analytics: routine mass spectrometry cannot separate L from I, so telling them apart depends on the synthesis record and chromatographic behaviour rather than on mass.

Working through an actual label

Our catalogue gives BPC-157 as Gly-Glu-Pro-Pro-Pro-Gly-Lys-Pro-Ala-Asp-Asp-Ala-Gly-Leu-Val, or GEPPPGKPADDAGLV in single letters. That is fifteen residues, which is what "pentadecapeptide" means. The three prolines in a row near the N-terminus impose a rigid bend on the chain, and the two neighbouring aspartates carry negative charge at neutral pH. Both facts are legible directly from the sequence, and both bear on how the peptide behaves in solution.

Shorthand for modifications

Few research peptides are bare, unmodified chains. The notation is consistent once learned.

• A leading Ac- indicates acetylation at the N-terminus: an acetyl group caps the free amino group, adding 42.04 Da and cancelling a positive charge. TB-500 appears as Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln, or Ac-LKKTETQ.
• A trailing -NH2 indicates C-terminal amidation: -NH2 takes the place of the terminal -OH, removing 0.98 Da and a negative charge. Numerous natural signalling peptides are amidated in the body and usually need the amide for receptor activity; oxytocin is written Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH2.
• A D- prefix identifies the D-enantiomer, such as the D-Trp in GHRP-6 (His-D-Trp-Ala-Trp-D-Phe-Lys-NH2). D-residues weigh the same as their L-forms but make poor substrates for mammalian proteases, so chemists place them deliberately to slow degradation.
• Non-standard residues are spelled out. Aib denotes α-aminoisobutyric acid, which stabilises helices and resists proteases, used in Ipamorelin (Aib-His-D-2-Nal-D-Phe-Lys-NH2) and at position 2 of the GLP-1 backbone in semaglutide and tirzepatide. 2-Nal denotes 2-naphthylalanine.
• Brackets and superscripts flag substitutions against a parent sequence; [D-Ala²], for instance, means D-alanine occupies position 2.
• Cyclisation appears as a connecting line, a "cyclo(...)" wrapper or a declared Cys-Cys disulfide. In oxytocin, Cys1 and Cys6 bridge to close a six-residue ring, and that bond is sensitive to reduction, which matters in storage.

From sequence to molecular weight

To get molecular weight, total the residue masses, add 18.02 Da for the water lost when the chain forms, then correct for modifications. Two worked examples using catalogued figures:

1. KPV (Lys-Pro-Val): 128.17 + 97.12 + 99.13 = 324.42; adding 18.02 gives 342.44 Da. Our catalogue shows 342.43, the 0.01 gap being rounding in the residue table.
2. Epitalon (Ala-Glu-Asp-Gly, AEDG): 71.08 + 129.12 + 115.09 + 57.05 = 372.34; adding 18.02 gives 390.36 Da against a catalogued 390.35.
3. Now add modifications. The same tetrapeptide acetylated and amidated would come to 390.36 + 42.04 − 0.98 = 431.42 Da.

Expect agreement to within about 0.1%. A gap of 18 Da normally means the water term was dropped; 42 Da means an acetyl group was overlooked; 1 Da means an amide was missed. Once the molecular weight is trustworthy you can move between mass and moles when preparing buffers, arithmetic covered in molecular weight, moles and molarity and in the molarity calculator.

Catalogue sequences versus certificate data

What a catalogue publishes is the intended sequence. What a COA publishes is measurement: an observed monoisotopic or average mass from MS and a purity value from reversed-phase HPLC. Neither method reads residues one by one unless MS/MS fragmentation data accompany it, which routine commercial certificates rarely include. Read the declared sequence as the manufacturer's claim and the measured mass as the test of that claim.

Frequent misreadings

• Reading from C to N. A reversed sequence is a different molecule, a retro-peptide, of identical mass, so always check that the leftmost residue really is the N-terminus.
• Counting a salt form as part of the sequence. Trifluoroacetate and acetate counter-ions are not residues; they add to the mass of the vial contents but not to the peptide. This is the usual reason a weighed quantity of powder holds less peptide than the label suggests, and why TFA content belongs on the COA.
• Mistaking fragment numbering for fragment length. "176-191" describes 16 residues, not 191.
• Overlooking the amide. A sequence active only when amidated is a different reagent from its free-acid counterpart, despite a difference of just one dalton.
• Treating a blend as one sequence. A co-formulated vial such as the KLOW Blend holds four sequences and has no single molecular weight; see blends vs single vials.

When the sequence, the stated molecular weight and the observed MS mass all line up, there is reasonable evidence the vial holds what the label claims. When they do not, the sequence is nearly always where the explanation is found fastest.