Peptide macrocyclization refers to the process of looping a linear peptide chain back onto itself to form a ring structure. In nature, macrocyclic peptides, including the antibiotics Vancomycin and Cyclosporine, occur. Macrocyclization in peptides is a frequently employed strategy. Our peptide chemists now routinely use macrocyclization to connect the peptide head to the tail, or side chains to enhance the stability, selectivity, and affinity of selected peptides.
Macrocyclic peptides have enhanced proteolytic stability. By connecting the N-terminus to the C-terminus of a peptide, the peptide becomes much harder to digest by cellular enzymes. Also, a locked, cyclic shape is more selective, fitting into a specific receptor like a key in a lock. Cyclization enables hiding polar groups, allowing the resulting peptide to slip through cell membranes more easily than its linear cousins.
Macrocyclization Strategies
Head-to-Tail Cyclization
Head-to-tail cyclization is the most common form, where the N-terminus, the head, reacts with the C-terminus, the tail, to form an amide peptide bond.
Peptide bond formation
Side-Chain-to-Side-Chain Conjugation
Cysteine amino acid residues, when incorporated into linear peptides, allow linking these side chains via disulfide bridges formed between two Cysteine residues. This strategy is also known as the sulfur-mediated cyclization strategy.
Peptide native chemical ligation of two linear peptides
Peptide cyclization via native chemical ligation (NCL)
Classical Staudinger Reaction
Staudinger Cyclization Reaction
"Click" Chemistry and Stapling
Modern peptide scientists now use synthetic tricks like Peptide Stapling, in which a hydrocarbon bridge is "clicked" across a peptide turn, usually in an alpha-helix peptide, to prevent unfolding.
Cu(I) based click reaction
The copper catalyzed 1,3-dipolar cycloadditon of an azide to an alkyne to create 1,2,3-triazoles is a Huisgen [3 + 2] cycloaddition reaction.
Cu-free click reaction of azides and DIFOs
The 1,3-dipolar cycloaddition of cyclooctynes with azides, also called “copper-free click chemistry”, is a bioorthogonal reaction with widespread applications in biological discovery.
References
Bechtler C, Lamers C. Macrocyclization strategies for cyclic peptides and peptidomimetics. RSC Med Chem. 2021 Jun 29;12(8):1325-1351. [PMC]
Baskin JM, Prescher JA, Laughlin ST, Agard NJ, Chang PV, Miller IA, Lo A, Codelli JA, Bertozzi CR. Copper-free click chemistry for dynamic in vivo imaging. Proc Natl Acad Sci U S A. 2007 Oct 23;104(43):16793-7. [PMC]
Chang PV, Prescher JA, Sletten EM, Baskin JM, Miller IA, Agard NJ, Lo A, Bertozzi CR. Copper-free click chemistry in living animals. Proc Natl Acad Sci U S A. 2010 Feb 2;107(5):1821-6. [PMC]
Click-chemistry-a-review
Fang P, Pang W-K, Xuan S, Chan W-L, & Leung K C-F; 2014. Recent advances in peptide macrocyclization strategies. Chem. Soc. Rev., 2024, 53, 11725-11771. [RSC]
Gordon CG, Mackey JL, Jewett JC, Sletten EM, Houk KN, Bertozzi CR. Reactivity of biarylazacyclooctynones in copper-free click chemistry. J Am Chem Soc. 2012 Jun 6;134(22):9199-208. [PubMed] [PMC]
White, C., Yudin, A. Contemporary strategies for peptide macrocyclization. Nature Chem 3, 509–524 (2011). [Nchem]