A linear peptide is flexible: in solution it adopts many conformations and only some of them fit its receptor. Closing the chain into a ring “locks” the molecule into a shape closer to the active one. The result is usually higher affinity, greater selectivity and more resistance to proteases, which attack open chains more easily.
Cyclisation can be achieved in several ways: a disulfide bridge between two cysteines (as in oxytocin), a lactam bond between an acidic and a basic side chain (as in Melanotan II and bremelanotide) or a direct link between the N and C termini.
Many approved peptide drugs are cyclic. For analysis, the ring changes the mass compared with the linear form (water or hydrogens are lost when the bond forms), a detail that must match the mass spectrometry result.
In peptides with a disulfide bridge, incorrect oxidation during synthesis can produce mispaired bridges or dimers. That is one of the impurities good analytical control has to rule out.
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Definition for educational and scientific purposes. It is not medical advice or a recommendation for use. NeoPeptidos products are sold labelled for research use only (RUO).