Short answer: A cyclic peptide is a peptide whose chain is closed into a ring. The ring can be formed with several kinds of chemical bond, including amide, lactone, ether, thioether and disulfide bonds [1]. When an amide bond joins the amino and carboxyl termini, the result is a head-to-tail (N-to-C) cyclic peptide [1]. Compared with linear peptides, rings are more rigid, and because a head-to-tail ring has no free ends it resists exopeptidases [1].
This article is educational chemistry for laboratory readers. It does not describe effects in people or animals and is not guidance for human or animal use.
What makes a peptide "cyclic"
A linear peptide has a free amino group at one end and a free carboxyl group at the other (see our N-terminus vs C-terminus guide). In a cyclic peptide, a covalent bond closes the chain into a ring [1]. One review lists amide, lactone, ether, thioether and disulfide bonds as ways that ring can be built [1].
The main families are:
- Head-to-tail (backbone) cyclic peptides: an amide bond between the amino and carboxyl termini closes the ring [1]. In a true N-to-C cyclic peptide there is only one possible cyclization, so the ring structure is straightforward [1]. This backbone, or homodetic, cyclization is described as a simple but powerful modification [6].
- Side-chain and disulfide-bridged rings: when the ring is not N-to-C, more varied ring structures become possible, including bicyclic and tricyclic peptides [1]. Human guanylin, for example, has four cysteines that form two intramolecular disulfide bonds, making it a bicyclic peptide [1]. (See our disulfide bond guide.)
- Stapled peptides: "stapling" means joining two parts of a molecule with a cross-linker, and the term tends to be used for any cyclic peptide that is not N-to-C [1]. Cross-linking methods include disulfide formation, olefin, cysteine-alkylation and biaryl cross-linking [1].
- Polycyclic and higher-order systems: beyond simple monocyclic peptides, chemists have built polycyclic peptides and even higher-order cyclic, spherical and cylindrical peptide systems [3].
Why ring closure changes a peptide
A review of cyclic peptides lists three properties that set them apart from linear counterparts [1]:
- Rigidity: the ring makes the molecule more rigid, which is linked to higher binding affinity and selectivity through a smaller entropy penalty in the Gibbs free energy equation [1].
- Resistance to exopeptidases: cyclic peptides lack amino and carboxyl ends, so exopeptidases that trim from the ends cannot act on them [1]. A rigid ring can also block endopeptidases if it keeps the enzyme away from the cleavage site [1].
- Membrane permeability: some cyclic peptides show better membrane permeability [1].
How chemists close the ring
The core problem: entropy
Cyclic peptides have been difficult, and often impossible, to prepare with traditional synthetic methods [2]. For the ring to close, the activated peptide has to adopt an entropically disfavored pre-cyclization conformation before it can form the product [2]. Avoiding side products during cyclization remains a major challenge [4].
Head-to-tail lactamization in solution
One common route builds the linear chain by standard solid-phase peptide synthesis (SPPS), keeps the side chains protected, and closes the ring in solution [6]. Such linear peptides are suited to active-ester formation at high dilution to form the macrocycle [6]. (For background on SPPS, see our peptide synthesis guide.)
A 2022 study of this approach shows what can go wrong and how conditions matter [6]:
- Coupling reagent: the authors used DEPBT, a reagent described as superior at avoiding epimerization, with 2 to 3 equivalents of DEPBT and 10 equivalents of triethylamine [6].
- Solvent: at 2 mM, cyclization of one precursor reached 39% (by LC–MS) after 13 hours in DMF but 89% in DMSO; the reaction failed in acetonitrile alone because of solubility [6].
- Oligomers: short rings were the hardest. The pentapeptide precursor CTWAR was the most prone to epimerization and oligomerization, side products that are difficult to detect and remove [6]. Swapping one residue changed the outcome sharply: with cysteine as the second residue of the linear RCTWA precursor, the cyclic pentamer outweighed the doubled cyclodecamer side product by more than 99:1, but replacing that cysteine with proline flipped the ratio to less than 1:99 [6].
- Ring size: the standardized conditions delivered 7- and 8-residue cyclic peptides, while 5-residue rings were less predictable because of competing cyclodecamer formation [6].
- Deprotection: side-chain protecting groups were removed afterwards with a cocktail of triisopropylsilane, dithiothreitol, trifluoroacetic acid and water [6]. (See our trifluoroacetic acid guide.)
Site-selective and newer methods
Because side products lower yields, chemists have developed site-selective cyclization chemistry, which reduces side products and gives a greater yield of the desired macrocycle [4]. One example, the CyClick method, forms an imine between an aldehyde and the N-terminus and then traps it irreversibly through attack by the second amide nitrogen [4].
Other strategies surveyed in a 2024 review include chemical routes such as native chemical ligation and transition-metal catalysis, biological routes such as post-translational enzymatic modification and genetic code reprogramming, and supramolecular routes such as metal-directed folding and noncovalent self-assembly [3]. Multicomponent reactions have also become cyclization tools; a review notes that they can cyclize peptides through their termini or side chains and add diversity at the ring-forming group itself, not just the amino acid sequence [5].
Library methods
Cyclic peptides have also been generated through phage display, split-intein circular ligation of peptides and proteins (SICLOPPS), mRNA display, combinatorial chemistry and de novo synthesis [1]. In early phage display libraries, a -Cys-X1-X2-X3-X4-X5-X6-Cys- sequence formed a ring through a disulfide bond between the two cysteines, with roughly 10⁹ (about a billion) different sequences in a library [1]. Adding a chemical cross-linker means only three cysteines are needed to form a bicyclic peptide [1].
Three well-studied natural examples
Gramicidin S
Gramicidin S is produced by the bacterium Aneurinibacillus migulanus and was first discovered in 1941 [7]. It is a cyclic, C2-symmetric decapeptide with the sequence cyclo(Pro-D-Phe-Leu-Orn-Val)2 [7]. Its structure combines two classic secondary-structure elements: the two Pro-D-Phe dipeptides form two type II β-turns, and the two Leu-Orn-Val stretches form an antiparallel β-sheet [7]. Modification strategies have included non-natural amino acids, changes to ring size, and edits to the β-turn and β-strand regions [7]. (See our beta sheet and D-amino acids guides.)
Cyclosporin A
Cyclosporin A (CsA) is a cyclic undecapeptide with a molecular weight of 1,202 [8]. Seven of its 11 residues are N-methylated [8]. In crystal structures it adopts two forms: an open form with no intramolecular hydrogen bonds and polar groups exposed to solvent, and a closed form, seen in chloroform, with four intramolecular hydrogen bonds and the polar groups facing inward [8]. The peptide bond between two of its N-methylated residues is trans (180°) in the open form and cis (0°) in the closed form [8].
N-methylation removes the hydrogen-bonding ability of backbone amide groups [8]. To study how it shapes ring conformation, one group synthesized a complete library of N-methylated cyclic alanine pentapeptides, which gave insight into how N-methylation modulates cyclic peptide conformation [9].
Cyclotides
Cyclotides are plant micro-proteins about 30 to 40 residues long, with a head-to-tail cyclized backbone further stabilized by three disulfide bonds arranged as a cystine knot [10]. All naturally occurring cyclotides contain between 27 and 37 amino acids, six of which are cysteines [10]. The disulfides Cys I–Cys IV and Cys II–Cys V form a ladder, and Cys III–Cys VI threads through it [10]. This cyclic cystine knot (CCK) makes the backbone very rigid and compact and is responsible for high stability to thermal, chemical and proteolytic degradation [10].
The first cyclotide, kalata B1, was found in the plant Oldenlandia affinis (coffee family, Rubiaceae) [11]. It stayed folded even after being extracted with boiling water [10]. A survey of more than 200 Rubiaceae species reported 22 potential cyclotide-producing species, and one estimate puts the number of undiscovered cyclotides in Rubiaceae at 50,000 or more, making cyclotides by far the largest family of cyclic proteins [11].
Analyzing cyclic peptides
Cyclic peptides need some extra care in mass spectrometry. In tandem MS, a cyclic peptide undergoes quasi-statistical ring opening that gives several linear intermediate ions, ultimately producing many more fragments than a linear peptide does [6]. Even so, the 2022 study observed a large proportion of the possible b and y fragments, enough for confident sequence confirmation [6].
Oligomers are another trap. The doubled-ring cyclodecamer side products showed up at the same apparent m/z as the target because of a tendency toward doubly charged states, and isotope-pattern analysis, which shows 0.5 Da spacing for a doubly charged ion instead of 1 Da, was used to tell them apart [6]. Standard HPLC methods could not separate the components well enough to measure the ratio, so the authors used nanoflow UHPLC–HRMS [6]. (See our mass spectrometry peptide testing and HPLC peptide testing guides.)
Cyclic peptides in the wider field
Over 40 cyclic peptide drugs were in clinical use as of 2017, and the large majority of approved cyclic peptides are derived from natural products [12]. Rational design and in vitro evolution techniques now let researchers develop cyclic peptide ligands de novo [12]. Peptide macrocycles have also found uses well beyond medicine, in nanomaterials [2] and functional biomaterials [3].
FAQ
What is a cyclic peptide?
A peptide whose chain is closed into a ring by a covalent bond such as an amide, lactone, ether, thioether or disulfide [1].
What is head-to-tail cyclization?
Forming an amide bond between the peptide's amino and carboxyl termini, giving an N-to-C cyclic peptide [1].
Why are cyclic peptides more stable than linear ones?
They lack free ends, so exopeptidases cannot act on them, and a rigid ring can also block endopeptidases from reaching cleavage sites [1].
What is a stapled peptide?
A peptide in which two parts of the chain are joined by a cross-linker; the term is often used for any cyclic peptide that is not N-to-C [1].
Why is peptide cyclization hard?
The activated linear peptide must adopt an entropically disfavored conformation before the ring can close [2], and side products such as oligomers compete with the desired ring [4][6].
How are cyclic peptides sequenced by MS?
Tandem MS opens the ring at many points, producing more fragments than a linear peptide, which software can annotate to confirm the sequence [6].
Bottom line
Cyclic peptides close their chains with an amide, disulfide, thioether or other bond [1]. The ring adds rigidity and removes the free ends that exopeptidases need [1], but closing it fights entropy [2] and invites oligomer and epimer side products, so reagents, solvent and sequence all matter [6]. Natural examples such as gramicidin S [7], cyclosporin A [8] and the cyclotides [10] show how turns, N-methylation and knotted disulfides shape ring structure, and careful MS is needed to confirm that the ring, not a doubled oligomer, was made [6].
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