Cyclic peptides.
Most peptides are open chains with a free start and a free end. A cyclic peptide closes that chain into a ring, end to end or through two side chains. Here are the ways a ring is made, why chemists make them, well-known examples with their PubChem data, and what a ring changes on a lab report.
Four ways to close a chain.
- Head to tail. A peptide bond joins the N-terminal amine to the C-terminal acid, so the ring has no free ends at all. Gramicidin S and cyclosporine are built this way.
- Disulfide. Two cysteine side chains join through an S–S bond. The rest of the chain keeps its ends, as in oxytocin and octreotide. The disulfide guide covers this case in detail.
- Side chain to side chain (lactam). An amide forms between a lysine amine and an aspartate or glutamate acid, closing a ring across the chain.
- Stapled. A hydrocarbon bridge between two non-natural residues locks a helix in place.
Read a ring in a sequence by its notation: cyclo(...) for head to tail, or a note such as cyclic (2→7)-disulfide naming the two cysteines. The sequence notation guide explains the symbols.
Shape, stability and grip.
An open chain flops between many shapes. A ring removes most of them, so the peptide spends its time in one or a few conformations, which is why cyclic peptides often bind their targets more tightly and more selectively than their linear versions. A head-to-tail ring also has no free ends for exopeptidases, the enzymes that trim chains from either terminus, so cyclic peptides usually last longer in enzyme-containing test systems. A few, cyclosporine above all, are unusually able to cross membranes, which has made macrocycles a busy field in drug discovery.
Five well-known rings.
| Compound | Ring | Residues | Formula | MW (g/mol) | PubChem CID |
|---|---|---|---|---|---|
| Cyclosporine | head to tail, 7 N-methylated residues | 11 | C62H111N11O12 | 1202.6 | 5284373 |
| Gramicidin S | head to tail, cyclo(Val-Orn-Leu-D-Phe-Pro)2 | 10 | C60H92N12O10 | 1141.4 | 73357 |
| Octreotide | disulfide, Cys2 to Cys7, with D-Phe and D-Trp | 8 | C49H66N10O10S2 | 1019.2 | 448601 |
| Oxytocin | disulfide, Cys1 to Cys6, amidated C-terminus | 9 | C43H66N12O12S2 | 1007.2 | 439302 |
| Linaclotide | three disulfides | 14 | C59H79N15O21S6 | 1526.8 | 16158208 |
Formulas and weights from PubChem. These are reference compounds for the chemistry, not Helion products.
What a ring changes in the data.
- Mass. Closing a chain head to tail removes one water, 18.02 g/mol, from the linear precursor. A disulfide removes two hydrogens, about 2.02. Mass spectrometry tells the ring from the open chain by exactly that difference.
- Fragmentation. Tandem mass spectrometry reads a linear sequence from one break per fragment. A ring must break twice before a fragment falls away, so cyclic sequences give more complex spectra; see the mass spectrometry guide.
- HPLC. The ring and its open precursor elute at different times, and an incompletely closed batch shows the precursor as a separate peak; the chromatogram guide covers impurity peaks.
The molecular weight calculator gives the open-chain mass; subtract 18.02 for a head-to-tail ring or 2.02 per disulfide.
All linear chains.
Every compound in Helion's catalog is a linear peptide: BPC-157 and TB-500, MOTS-c, SS-31, CJC-1295 and ipamorelin and tesamorelin. GHK-Cu holds its copper ion by coordination, not by a covalent ring. Each compound page lists the sequence, formula and molecular weight, and each lot's lab report confirms identity by mass spectrometry.
Specs and storage by compound.
Questions (6)
What is a cyclic peptide?
A peptide whose chain is closed into a ring, either by a peptide bond between its two ends (head to tail) or by a bond between side chains, most often a disulfide between two cysteines.
Why are cyclic peptides more stable?
A head-to-tail ring has no free ends for exopeptidases to trim, and the ring holds the chain in fewer shapes, which can slow other enzymes too.
Is oxytocin a cyclic peptide?
Yes. A disulfide between cysteine 1 and cysteine 6 closes a six-residue ring, and the last three residues form a tail with an amidated end.
How do you tell a cyclic peptide from a linear one by mass?
A head-to-tail ring weighs 18.02 g/mol less than its linear precursor because one water is lost; each disulfide lowers the mass by about 2.02.
What does cyclo() mean in a sequence?
It marks a head-to-tail ring: the residues inside the parentheses are joined in a loop, with the last one bonded back to the first.
Are any Helion compounds cyclic?
No. All of them are linear chains. GHK-Cu binds copper by coordination rather than a covalent ring.
Published studies (4)
- Cyclic Peptides for Drug Development
- Macrocyclic Peptides as Drug Candidates: Recent Progress and Remaining Challenges
- Selection-based discovery of druglike macrocyclic peptides
- Structure of octreotide, a somatostatin analogue
Citations from PubMed for reference. They describe the compound in the literature, not any Helion lot, and nothing here is guidance on use.