Cyclic peptides and disulfide bonds explained

What makes a peptide cyclic

A peptide can be cyclised in several ways: by joining its two ends, by linking a side chain to an end, or by linking two side chains. The most common route in naturally occurring hormones is a disulfide bond between two cysteines.

How a disulfide bond works

Cysteine carries a thiol group. When two thiols are oxidised, they join to form a sulfur-sulfur bond. Oxytocin is a nine-residue peptide with a bond between its first and sixth residues, and octreotide has one between its second and seventh.

Because the bond is covalent, it is far stronger than the weak interactions that hold the rest of a peptide's shape in place.

Why researchers care about the ring

A ring restricts how the chain can fold. That can improve receptor binding and selectivity, and it can protect the peptide from enzymes that chew in from the ends.

The trade-off is that the bond itself is a point of chemical vulnerability. Reducing agents can break it, and peptides with more than two cysteines can form the wrong pairing.

What it means for mass and purity

Forming a disulfide bond removes two hydrogen atoms, so the cyclic form is about 2 Da lighter than the open, reduced form. In mass spectrometry that small shift is the signature of a correctly closed ring.

In HPLC, a mixture of open, correctly closed and wrongly paired forms can show up as extra peaks or a broadened peak, which makes the chromatogram worth reading closely.

What to check on a Certificate of Analysis

For a ring-shaped peptide, a CoA is only meaningful if it addresses the ring.

• Is the observed mass compared with the calculated mass of the cyclic form?
• Is the HPLC trace a single clean peak, or are there shoulders that could be isomers?
• Does the sequence on the document show the disulfide pairing where the peptide has one?
• Does the batch number match the vial in your hands?