Disulphide bridges

This is a relatively straightforward operation, where two cysteines are oxidised under carefully controlled conditions to form a disulphide bridge.
These peptides are stable at neutral to low pH but can be reversibly reduced at higher pH.

Cyclic thioether

These are formed when an activated amino function reacts with the side chain of a Cys residue to create a thioether. Advantages are the ease of formation and their pH stability.

These are highly suited to making structurally restrained peptide libraries.

Cyclic through a peptide bond.

Much depends on the natural folding of the peptide in the solvent used for the cyclisation reaction.This process is more problematic than the disulphide or thioether route.