Table 2.

Examples of common stapling chemistries to impose helical peptide conformation.

Chemistry	Residues Involved	Compatible Arrangement	Does Stereochemistry of the Staple Handles Need to be Considered?	Comment
alkene ring-closing metathesis	(homo)serine O-allyl ethers [59]	[i, i + 4]	no	all hydrocarbon staple
	α,α-disubstituted residues with olefinic side chains (R or S configuration, 5 or 8 atoms long) [60]	[i, i + 4] and [i, i + 7]	yes (S5/S5 for [i, i + 4], S8/R5 or S5/R8 for [i, i + 7])
lactamisation	lysine and glutamate, or ornithine and aspartate [61]	only compatible with [i, i + 4] arrangement	no	requires extra orthogonal protective groups for amino and carboxy groups for on resin lactamisation
cycloadditions	azide and alkyne group containing residues with 4 + 2 or 4 + 3 methylene units long side chains [62]	[i, i + 4]	no	well-established click reaction (Cu(I)-catalyzed azide-alkyne cycloaddition)
	tetrazole and alkene group containing residues [63]			UV-induced cycloaddition between tetrazoles and alkenes to yield fluorescent pyrazoline tethers
disulfide bridges	thiol group containing residues [64]	[i, i + 7]	yes (combination of D and L-residues)	chronologically the oldest technique, requires acetamidomethyl protecting groups for thiols, staple unstable (prone to reduction)
thioether bridges	cysteine and an alpha-bromo amide group containing residue [65]	[i, i + 3] and [i, i + 4]	no	staple stable, higher helicity achieved with [i, i + 3] arrangement
	two (homo)cysteines + dichloroacetone crosslinker [66]	[i, i + 4]	no	bis-alkylating crosslinker amenable to further derivation via oxime ligation (e.g., fluorophore or biotin coupling)
	cysteines + perfluoroaromatic crosslinker (e.g., hexafluorobenzene) [48]	[i, i + 4]	no	regioselective reaction (para-disubstituted staple) proceeding under mild conditions in high yield even for unprotected peptides