Every peptide in this catalogue is a chain of amino acids held together by one kind of bond. The 31 residues of NN9535 are joined by 30 of them; the 15 residues of BPC-157 by 14. The bond is an amide, and the chemists who work with it call it the peptide bond.
Three properties of that bond explain a surprising amount of what a buyer sees on a certificate and a catalogue page. Its formation removes water, which fixes the molecular weight. Its planarity, worked out by Pauling and Corey in 1951, fixes the shapes a chain can adopt. And its slow hydrolysis in water is the reason lyophilised material is stored dry and solutions have short lives.
This guide takes the three in turn, works the arithmetic on catalogue molecules, and ends with what a broken bond looks like in the two analytical methods a certificate reports.
Condensation: two amino acids, one bond, one water
An amino acid has an amino group (NH2) and a carboxyl group (COOH). When the carboxyl of one meets the amino of another, the carbon of the carboxyl bonds to the nitrogen of the amino, and the OH from the carboxyl leaves with one H from the amino as a molecule of water. What remains between the two residues is the group C(=O)-NH, an amide. Because this reaction joins amino acids specifically, the amide is called a peptide bond, and the reaction a condensation.
In the laboratory the reaction does not run on its own at a useful rate; the carboxyl must be activated, which is what coupling reagents do in solid-phase synthesis. The thermodynamics are the same either way: each bond formed costs the molecule one water, 18.015 Da by average mass (PubChem CID 962 gives water as 18.015 g/mol).
The consequence is that a peptide of n residues has n minus 1 peptide bonds and has lost n minus 1 waters relative to its constituent free amino acids. That is the whole reason a catalogue molecular weight is less than the sum of amino acid masses, and it is checkable.
The arithmetic on two catalogue peptides
BPC-157 is the ideal test because its 15 residues are all standard amino acids with no modifications: Gly-Glu-Pro-Pro-Pro-Gly-Lys-Pro-Ala-Asp-Asp-Ala-Gly-Leu-Val. Taking average masses of the free amino acids from the Matrix Science reference table (glycine 75.07, glutamic acid 147.13, proline 115.13, lysine 146.19, alanine 89.09, aspartic acid 133.10, leucine 131.17, valine 117.15), the fifteen sum to 1671.77. Fourteen bonds remove 14 × 18.015 = 252.21. The result, 1419.56, matches the catalogue molecular weight of 1419.55 g/mol to the rounding of the last decimal.
KPV, Lys-Pro-Val, gives the same check on a smaller scale: 146.19 + 115.13 + 117.15 = 378.47, less two waters at 36.03, is 342.44 against the catalogue's 342.43 g/mol.
| Peptide | Residues | Peptide bonds | Sum of free amino acids | Water removed | Calculated mass | Catalogue molecular weight |
|---|---|---|---|---|---|---|
| BPC-157 | 15 | 14 | 1671.77 | 14 × 18.015 = 252.21 | 1419.56 | 1419.55 g/mol |
| KPV | 3 | 2 | 378.47 | 2 × 18.015 = 36.03 | 342.44 | 342.43 g/mol |
Peptide bond arithmetic on two unmodified catalogue peptides. Free amino acid masses are average masses from the Matrix Science reference table (residue mass plus 18.015); catalogue figures are from the Restate Health product records.
The same check cannot be run on NN9535 from its letter code alone, and the reason is instructive. Its 31 residues contain Aib at position 2, a residue outside the standard twenty, and lysine 26 carries a side chain of γ-glutamic acid, two AEEA spacer units and a C18 fatty diacid, each joined by amide bonds of exactly the same kind as the backbone. The molecule therefore has 30 backbone peptide bonds and several more amides in the side chain, and the catalogue formula C187H291N45O59 and molecular weight 4113.58 g/mol describe the whole structure. Lau and colleagues' 2015 discovery paper gives those modifications in words. Molecular weight for buyers explains why a certificate must say which mass basis it is reporting before any such figure is compared.
Why the bond is planar
A simple single bond between carbon and nitrogen would rotate freely. The peptide bond does not. The nitrogen's lone pair is shared with the adjacent carbonyl, so the bond is best described as a resonance hybrid between C(=O)-NH and C(-O⁻)=NH⁺. That partial double-bond character shortens the C-N bond, roughly 1.33 Å against about 1.47 Å for an ordinary C-N single bond, and holds the six atoms around it (the carbonyl carbon and oxygen, the nitrogen and its hydrogen, and the two flanking α-carbons) in one plane.
Pauling, Corey and Branson used exactly this constraint in 1951. Starting from the planarity of the amide group and from bond lengths measured in crystals of small amides, they derived the two regular structures a chain of planar units joined at flexible α-carbons can form with good hydrogen bonding, and named them the α-helix and the pleated sheet. The two papers in which they did it, in April and May of that year, are the foundation of structural biology and the direct explanation of why the bond is a rigid unit rather than a hinge.
The chain's flexibility therefore lives entirely in the two bonds either side of each α-carbon, the angles called φ and ψ. Ramachandran, Ramakrishnan and Sasisekharan showed in 1963 that only certain combinations of these two angles are possible without atoms colliding, and the plot of allowed combinations bears Ramachandran's name. For a buyer the practical meaning is that a peptide's backbone is a sequence of rigid plates with limited hinge angles between them, which is why sequence determines shape.
Cis, trans and proline
Because the bond is planar, the two α-carbons on either side can sit on opposite sides of it (trans) or the same side (cis). Trans is overwhelmingly favoured for ordinary residues because the cis arrangement brings the side chains into collision. Proline is the exception. Its side chain loops back onto its own nitrogen, so the nitrogen carries no hydrogen and the two arrangements are much closer in energy. Stewart, Sarkar and Wampler surveyed protein crystal structures in 1990 and found that cis peptide bonds, though rare overall, occur disproportionately before proline residues.
BPC-157 has four prolines in 15 residues, including three in a row. That does not mean its bonds are cis, but it does mean the molecule can exist as slowly interconverting conformers, and that is one of the ordinary reasons a pure peptide can show peak broadening or a small shoulder on HPLC without being impure. A good analyst recognises the pattern; a buyer should at least know it exists before reading a purity figure.
Hydrolysis: the bond coming apart
The reaction that formed the bond runs in reverse in water. A water molecule adds across the C-N bond, the carboxyl and the amino group are regenerated, and the chain is cut into two shorter chains. This is hydrolysis, and it is the basic degradation route for any peptide in solution.
It is slow. Radzicka and Wolfenden measured the uncatalysed rate at neutral pH in 1996 and found half-lives of centuries at 25 °C, which is why proteases, the enzymes that cut peptide bonds, are among the most powerful catalysts known. In a research vial there are no proteases, so simple hydrolysis of an ordinary backbone bond is rarely what degrades material over weeks. What does are the faster side reactions that the backbone and side chains permit: deamidation of asparagine and glutamine, which proceeds through an intermediate that can also cleave the chain; acid-catalysed cleavage at aspartic acid, especially an Asp-Pro bond; oxidation of methionine, cysteine and tryptophan; and aggregation. Manning and colleagues' 2010 review catalogues these and the conditions that accelerate them, and the library's peptide degradation pathways guide sets them out for a buyer.
All of them need water, or need the molecule to be mobile. In lyophilised solid at -20 °C there is almost none of either, which is the entire argument for the storage lines on every catalogue record: keep it dry and cold, and reconstitute only what will be used.
What a broken bond looks like on HPLC
Reversed-phase HPLC separates peptides mainly by hydrophobicity. A cleavage produces two fragments, each shorter than the parent and each carrying a new charged terminus, so both are usually more polar than the intact molecule and elute earlier. On a chromatogram the sign of hydrolysis is therefore new, earlier peaks growing at the expense of the main peak over time, rather than a single late shoulder. HPLC purity explained covers how those peaks are turned into the purity percentage.
The percentage can mislead. If a fragment has lost all its aromatic residues it absorbs weakly at the detector wavelength and is under-counted; if it is very small it may elute with the solvent front and not be counted at all. A purity figure that has not moved is reassuring only if the chromatogram was run under conditions that would have shown the fragments.
What it looks like on mass spectrometry
Mass spectrometry makes the arithmetic visible. If a peptide of mass M is cut at one bond, the two fragments have masses that sum to M plus 18.015, because the water that was removed when the bond formed has been put back. A spectrum of degraded material shows the parent ion losing intensity while new ions appear at masses that pair up in exactly that way, and an analyst can read which bond was cut from which pair appears.
That is also why mass spectrometry is a stronger identity test than a mass alone. Fragmenting the molecule deliberately inside the instrument and reading the ladder of masses establishes the sequence, not just the formula. Mass spectrometry and peptide identity explains the difference between a single intact mass on a certificate and a method that reads the chain.
What this means for a buyer
- The molecular weight on a catalogue page is checkable arithmetic for an unmodified peptide, and the check is residue masses plus one water. Where it cannot be run from the letter code, the molecule is modified and the certificate's structural confirmation carries the weight.
- A planar, rigid bond means sequence fixes shape. The same 15 residues in a different order are a different molecule with an identical mass, which is why identity by mass alone is weaker than identity by sequence.
- The bond's susceptibility to water is the physical reason for every storage line in the catalogue. The cheapest stability measure a laboratory has is to leave material lyophilised until it is needed.
- A broken bond shows up as earlier HPLC peaks and as pairs of masses summing to the parent plus 18.015. A certificate that shows neither, on a lot tested recently, is the evidence the bonds were intact when the material left the supplier.