Peptide Cyclization Methods: Disulfide, Lactam & Click Chemistry

Why Cyclization Matters in Peptide Research

Linear peptides, despite their remarkable target selectivity, face inherent limitations that restrict their utility as research tools: rapid proteolytic degradation, high conformational flexibility, and poor membrane permeability. Cyclization—the formation of a covalent bond that connects two points within a peptide chain to create a macrocyclic ring—addresses all three of these challenges simultaneously.

By constraining the peptide backbone into a defined three-dimensional shape, cyclization reduces the entropic penalty of target binding, enhances resistance to exopeptidases (which require a free terminus to initiate cleavage), and can shield polar amide bonds from solvent, improving passive membrane permeability (White & Yudin, 2011). These properties have made cyclic peptides one of the fastest-growing modalities in molecular research, with over 40 cyclic peptide-derived compounds currently approved for various applications and many more under active investigation (Ji et al., 2024).

This guide provides a comprehensive overview of the major cyclization strategies available to researchers, from classical disulfide bridging to cutting-edge enzymatic and display-based approaches.

Classification of Cyclic Peptides

Before examining individual techniques, it is useful to understand the four fundamental cyclization topologies:

• •Head-to-tail (backbone) cyclization: An amide bond links the N-terminal amine to the C-terminal carboxylate, creating a homodetic macrocycle with no free termini.
• •Side-chain-to-side-chain cyclization: A covalent bridge connects two amino acid side chains (e.g., disulfide between two cysteines, lactam between lysine and glutamic acid).
• •Head/tail-to-side-chain cyclization: One terminus is linked to an internal side chain, producing a lariat-shaped topology.
• •Backbone-to-backbone (non-amide) cyclization: Non-natural linkages such as triazoles, thioethers, or hydrocarbon staples replace the traditional amide bond.

Each topology confers distinct structural and functional properties. Head-to-tail cyclization eliminates both termini, maximizing proteolytic stability. Side-chain bridges can locally constrain secondary structures like alpha-helices or beta-turns without altering the backbone sequence. The choice of cyclization strategy depends on the desired ring size, the target interaction surface, and the synthetic accessibility of the macrocycle (Bechtler & Sedlmayer, 2021).

Disulfide Bridge Formation

Disulfide bonds between two cysteine residues represent the most ancient and widely studied peptide cyclization strategy. Found throughout nature—in hormones like oxytocin and somatostatin, in venom peptides like conotoxins, and in plant-derived cyclotides—disulfide bridges provide a reversible, redox-sensitive constraint that can stabilize complex three-dimensional folds.

Mechanism and Methods

The disulfide bond forms through oxidation of two cysteine thiol (-SH) groups to yield a -S-S- linkage. In laboratory settings, this can be achieved through several approaches:

• •Air oxidation: Dissolving the reduced peptide in aqueous buffer at mildly alkaline pH (7.5–8.5) and allowing atmospheric oxygen to drive disulfide formation. This is the simplest method but offers limited control over regioselectivity in peptides with multiple cysteines.
• •Oxidizing agents: Reagents such as iodine, DMSO, or potassium ferricyanide (K3[Fe(CN)6]) provide more controlled oxidation kinetics. Iodine in methanol/water mixtures is particularly effective for rapid, clean cyclization.
• •On-resin oxidation: Performing disulfide formation while the peptide remains attached to the solid-phase synthesis resin, using reagents like thallium(III) trifluoroacetate or iodine, can improve yields for certain sequences.

Advantages and Limitations

Disulfide-cyclized peptides benefit from straightforward incorporation of natural cysteine residues via standard SPPS protocols. The resulting bridge is approximately 2.05 Angstroms in length and introduces a well-characterized geometric constraint. However, the reversibility that makes disulfide bonds biologically elegant also limits their stability: reducing environments (e.g., high intracellular glutathione concentrations) can cleave the bond, and the -S-S- linkage is susceptible to disulfide shuffling in the presence of free thiols.

For researchers requiring greater stability, the disulfide bridge can serve as a starting point for further modification—replacement with thioether, selenoether, or dicarba linkages that maintain similar geometry while eliminating redox sensitivity.

Lactam Bridge Cyclization

Lactam (amide bond) cyclization between the side chains of a dibasic and a diacidic amino acid—most commonly lysine (Lys) and glutamic acid (Glu) or aspartic acid (Asp)—provides a stable, irreversible constraint that is widely used to stabilize alpha-helical conformations.

Synthetic Approach

Lactam cyclization is typically performed using orthogonal protecting group strategies during SPPS. The key requirement is that the side-chain amine of Lys and the side-chain carboxylate of Glu/Asp must be selectively deprotected while all other side chains remain protected. Common orthogonal pairs include:

• •Alloc/Allyl: The allyloxycarbonyl group on Lys and allyl ester on Glu/Asp, both removed by palladium(0)-catalyzed deprotection.
• •ivDde/OPp: The 1-(4,4-dimethyl-2,6-dioxocyclohex-1-ylidene)-3-methylbutyl group on Lys removed by hydrazine, paired with appropriate Glu/Asp protection.

After selective deprotection, on-resin cyclization is achieved using standard coupling reagents (HATU, HBTU, or PyBOP) with a suitable base (Posada et al., 2022). The reaction typically requires extended coupling times (12–24 hours) due to the conformational constraints of on-resin macrolactamization.

Research Applications

Lactam bridges positioned at i, i+4 spacing effectively stabilize single turns of an alpha-helix, mimicking protein-protein interaction interfaces. This makes them particularly valuable in research on helix-mediated signaling pathways. The lactam bridge shares conceptual ground with hydrocarbon stapling technology, though it uses naturally occurring amino acid residues rather than synthetic olefin-bearing side chains.

Thioether (Lanthipeptide) Cyclization

Thioether bonds—formed between a cysteine thiol and a dehydroalanine (Dha) or chloroacetyl electrophile—provide exceptionally stable, non-reducible macrocyclic constraints. This chemistry is inspired by the biosynthesis of lantibiotics, a class of ribosomally synthesized and post-translationally modified peptides (RiPPs) produced by bacteria.

Chemical Thioether Formation

The most common laboratory approach involves reacting a cysteine thiol with a chloroacetyl group installed at the peptide N-terminus or on a side chain. The reaction proceeds readily in aqueous buffer at mildly basic pH (7.0–8.0), forming a stable thioether linkage. This approach is highly chemoselective, as the chloroacetyl group preferentially reacts with the more nucleophilic thiol over amines at neutral pH.

Advantages for Research

Thioether-cyclized peptides combine the metabolic stability of a non-reducible bond with straightforward synthetic access. The thioether linkage is resistant to both oxidizing and reducing environments, making these macrocycles suitable for investigations in complex biological matrices. The lanthipeptide scaffold has become a popular platform for library-based screening approaches, as the cyclization chemistry is compatible with genetically encoded peptide libraries expressed in bacterial systems.

Click Chemistry: CuAAC and Strain-Promoted Approaches

The copper-catalyzed azide-alkyne cycloaddition (CuAAC), widely known as 'click chemistry,' has become a powerful tool for peptide cyclization since its adaptation to macrocycle synthesis in the early 2000s. The reaction between an azide and a terminal alkyne, catalyzed by Cu(I), forms a 1,4-disubstituted 1,2,3-triazole that serves as a stable, biocompatible linkage.

Implementation

For peptide cyclization, an azide-bearing amino acid (e.g., azidoalanine, azidolysine) and a propargyl-modified residue are incorporated at defined positions during SPPS. After cleavage and global deprotection, cyclization is performed in solution using CuSO4/sodium ascorbate as the catalytic system. The reaction proceeds in aqueous conditions at room temperature, achieving high yields even at the millimolar concentrations where conventional head-to-tail cyclization often produces oligomeric byproducts.

Strain-promoted azide-alkyne cycloaddition (SPAAC) eliminates the need for copper catalysis by using cyclooctyne-modified residues, making it compatible with copper-sensitive applications and enabling cyclization in the presence of other functional groups.

The Triazole as a Pharmacophore

The resulting triazole ring is not merely a structural linker—it serves as a bioisostere of the amide bond, with similar size, dipole moment, and hydrogen-bonding capacity. This property means that click-cyclized peptides can maintain or even enhance the binding properties of their lactam-cyclized counterparts while offering superior metabolic stability (Bechtler & Sedlmayer, 2021).

Head-to-Tail Macrolactamization

Head-to-tail cyclization, forming an amide bond between the N-terminus and C-terminus of a linear peptide precursor, produces the most natural macrocyclic topology—exemplified by cyclosporine A, the landmark 11-residue cyclic peptide immunosuppressant first isolated from Tolypocladium inflatum (Freeman, 1987).

Synthetic Challenges

Head-to-tail cyclization is notoriously challenging because it requires the activated C-terminal carboxylate to react with the distal N-terminal amine in an intramolecular fashion, competing against intermolecular oligomerization. Key strategies to favor macrocyclization include:

• •High-dilution conditions: Performing the reaction at low peptide concentrations (typically 0.5–2 mM) to minimize intermolecular reactions. This is the classical Ruggli-Ziegler dilution principle.
• •Turn-inducing residues: Incorporating D-amino acids, proline, or N-methylated residues that pre-organize the linear precursor into a conformation favorable for cyclization.
• •Pseudoproline dipeptides: Temporary backbone protection that introduces a cis-amide bond geometry, reducing the effective ring size and promoting cyclization.
• •C-terminal activation: Using coupling reagents like HATU, DMTMM, or DEPBT that minimize epimerization at the C-terminal residue during activation.

On-Resin vs. Solution-Phase Cyclization

Two complementary strategies exist for head-to-tail cyclization. In the solution-phase approach, the linear peptide is first assembled on resin via SPPS, cleaved, globally deprotected, and then cyclized in solution at high dilution. In the on-resin approach, the peptide is anchored through a side-chain functional group (e.g., via the side chain of Asp, Glu, or Lys to a specialized resin), allowing the head and tail to be free for cyclization while still attached to the solid support. On-resin cyclization benefits from the pseudo-dilution effect of the solid support, which inherently favors intramolecular reactions.

Enzymatic Cyclization: Nature's Macrocyclization Machinery

Perhaps the most exciting frontier in peptide cyclization is the use of enzymes that have evolved to catalyze macrocycle formation with extraordinary efficiency and selectivity.

Sortase A

Sortase A from Staphylococcus aureus recognizes the LPXTG motif (where X is any amino acid) and cleaves between the threonine and glycine residues, forming a thioester intermediate with the active-site cysteine. This intermediate can then be captured by an N-terminal oligoglycine nucleophile, forming a native peptide bond. When both the LPXTG motif and the oligoglycine nucleophile are present within the same peptide, sortase catalyzes head-to-tail cyclization. While widely used due to commercial availability, sortase A requires relatively high enzyme concentrations and extended reaction times (Schmohl & Schwarzer, 2014).

Butelase 1

Butelase 1, an Asx-specific ligase isolated from the tropical plant Clitoria ternatea, represents a dramatic improvement over sortase-mediated cyclization. This enzyme achieves cyclization rates approximately 20,000-fold faster than sortase A, with catalytic efficiencies (kcat/Km) in the range of 10^6 M^-1 s^-1. Butelase 1 requires only an Asn-His-Val tripeptide recognition motif at the C-terminus, cleaving after asparagine and ligating the resulting N-terminus to complete the macrocycle (Nguyen et al., 2016).

PatG and Other Cyanobactin Macrocyclases

The cyanobactin biosynthetic pathway employs PatG, a subtilisin-like serine protease that catalyzes macrocyclization of ribosomally synthesized peptides. PatG recognizes a conserved AYDG motif and can cyclize diverse peptide sequences, making it a versatile tool for generating libraries of natural product-like macrocycles. Related enzymes from other cyanobactin pathways expand the available recognition motifs and substrate scope.

Display-Based Screening of Cyclic Peptide Libraries

Modern cyclic peptide research increasingly relies on powerful combinatorial screening technologies that can evaluate millions to trillions of macrocyclic candidates simultaneously.

Phage Display

Phage display of cyclic peptide libraries, pioneered using disulfide-constrained CX7C libraries displayed on M13 bacteriophage, remains a workhorse technology for identifying cyclic peptide binders. Recent advances have expanded the cyclization chemistries available for phage-displayed libraries, including thioether formation via chloroacetyl-cysteine reaction and aldehyde-mediated cyclization bridging lysine and cysteine side chains (Wang et al., 2024). The MOrPH-PhD platform further extends phage display capabilities by incorporating non-canonical amino acids for posttranslational macrocyclization (Owens et al., 2022).

mRNA Display and the RaPID System

The Random non-standard Peptide Integrated Discovery (RaPID) system, developed by Hiroaki Suga and colleagues, combines mRNA display with genetic code reprogramming to access vast libraries (>10^12 members) of macrocyclic peptides containing both natural and non-canonical amino acids. The system uses a reconstituted E. coli translation apparatus (PURE system) in which the genetic code is reprogrammed to incorporate D-amino acids, N-methylated amino acids, and other non-natural building blocks. Cyclization typically occurs through spontaneous thioether formation between an N-terminal chloroacetyl group and an internal cysteine (Goto et al., 2011; Passioura et al., 2021).

The RaPID system has proven exceptionally productive, generating first-in-class cyclic peptide ligands for diverse protein targets and serving as the discovery engine behind several compounds now advancing through investigation pipelines (You et al., 2024).

SICLOPPS: Genetically Encoded Cyclic Libraries

Split-Intein Circular Ligation of Peptides and Proteins (SICLOPPS) offers a complementary, cell-based approach to cyclic peptide screening. In this system, a target peptide sequence is genetically fused between two halves of a split intein. Upon expression in E. coli, the split intein undergoes protein trans-splicing, producing a head-to-tail cyclized peptide inside living cells. This enables direct functional screening using bacterial genetic selection—cells harboring bioactive cyclic peptides that inhibit a target pathway can be selected using growth-based or reporter-based assays (Tavassoli, 2017).

Enhancing Cyclic Peptide Properties Through Post-Cyclization Modification

Cyclization alone does not guarantee optimal research tool properties. Several additional modifications are commonly employed to further enhance cyclic peptide characteristics:

N-Methylation

Selective N-methylation of backbone amide bonds within cyclic peptides can dramatically improve membrane permeability by reducing the number of hydrogen bond donors and favoring intramolecular hydrogen bonding. This strategy is exemplified by cyclosporine A, in which seven of eleven backbone amides are N-methylated, enabling the molecule to achieve oral bioavailability—a remarkable feat for a peptide of its molecular weight (~1,203 Da) (Li et al., 2021; Wang & Bhatt, 2016).

D-Amino Acid Incorporation

Replacing selected L-amino acids with their D-enantiomers can serve dual purposes: inducing conformational turns that facilitate cyclization (particularly valuable for head-to-tail macrolactamization) and enhancing proteolytic stability by disrupting protease recognition motifs.

Bicyclic and Multicyclic Architectures

Introducing multiple cyclization points—for example, combining a head-to-tail amide bond with internal disulfide bridges or thioether staples—creates bicyclic or polycyclic architectures with even greater conformational rigidity. The cyclotide scaffold, featuring a cyclic cystine knot (CCK) motif with three interlocking disulfide bonds within a head-to-tail backbone, represents nature's most stable peptide fold and serves as an inspiration for engineered multicyclic frameworks.

Practical Considerations for Researchers

Choosing a Cyclization Strategy

The optimal cyclization approach depends on several experimental factors:

Quality Assessment

Confirming successful cyclization requires rigorous analytical characterization. HPLC analysis typically reveals a distinct shift in retention time between linear and cyclic forms. Mass spectrometry should show a loss of 18 Da (water) for lactam cyclization or 2 Da (hydrogen) for disulfide formation. Tandem MS/MS sequencing and NMR spectroscopy provide definitive structural confirmation. These data should be reflected in the Certificate of Analysis for any research-grade cyclic peptide.

Stability Considerations

Cyclic peptides benefit from the same storage best practices as their linear counterparts—lyophilized storage at -20 degrees Celsius or below, protection from moisture, and reconstitution in appropriate solvents immediately before use. However, their enhanced proteolytic stability means they are generally more forgiving of brief exposure to ambient conditions. Researchers should remain mindful of degradation pathways specific to the cyclization chemistry employed—for example, disulfide-containing macrocycles require protection from reducing agents.

The Future of Cyclic Peptide Research

The cyclic peptide field is experiencing a period of unprecedented growth, driven by converging advances in synthesis, screening, and computational design. Machine learning approaches are being applied to predict cyclic peptide conformations and membrane permeability, while automated flow chemistry platforms are accelerating the synthesis and optimization cycle. The integration of non-canonical amino acids, cell-permeable scaffolds, and targeted delivery strategies is expanding the accessible target space from extracellular receptors to intracellular protein-protein interactions that were previously considered beyond peptide reach.

As screening platforms like RaPID continue to mature and enzymatic cyclization tools become more widely accessible, the barrier to generating high-quality cyclic peptide research tools continues to decline. For researchers entering this space, understanding the fundamental cyclization techniques outlined in this guide provides the foundation for selecting the right macrocyclization strategy for any given research question.

Research Tools

Scientists sourcing peptides for laboratory research can browse verified peptide suppliers and use the price comparison tool to evaluate options across vendors.

References

1. White CJ, Yudin AK. Contemporary strategies for peptide macrocyclization. Nat Chem. 2011;3(7):509-524. PMID: 21697871

2. Ji X, et al. Cyclic Peptides for Drug Development. Angew Chem Int Ed Engl. 2024;63(3):e202308251. PMID: 37870189

3. Bechtler C, Lamber C. Macrocyclization strategies for cyclic peptides and peptidomimetics. RSC Med Chem. 2021;12(8):1325-1351. PMID: 34447937

4. Posada L, et al. Three Methods for Peptide Cyclization Via Lactamization. Methods Mol Biol. 2022;2371:87-104. PMID: 34596840

5. Nguyen GKT, et al. Butelase 1: A Versatile Ligase for Peptide and Protein Macrocyclization. J Am Chem Soc. 2015;137(49):15398-15401. PMID: 26633100

6. Schmohl L, Schwarzer D. Enzyme-catalyzed peptide cyclization. Curr Opin Chem Biol. 2014;22:16-23. PMID: 29249237

7. Wang C, et al. A facile strategy for the construction of a phage display cyclic peptide library. Nat Commun. 2024;15:5577. PMID: 39092106

8. Owens AE, et al. MOrPH-PhD: A Phage Display System for Genetically Encoded Macrocyclic Peptides. Methods Mol Biol. 2022;2371:225-253. PMID: 34596853

9. Goto Y, et al. In vitro selection of unnatural cyclic peptide libraries via mRNA display. Methods Mol Biol. 2012;805:349-357. PMID: 22094817

10. Passioura T, et al. The RaPID Platform for the Discovery of Pseudo-Natural Macrocyclic Peptides. Acc Chem Res. 2021;54(18):3662-3672. PMID: 34505781

11. You S, et al. The coming of age of cyclic peptide drugs: an update on discovery technologies. Expert Opin Drug Discov. 2024;19(8):961-973. PMID: 38872502

12. Tavassoli A. Genetic Selections with SICLOPPS Libraries. Methods Mol Biol. 2019;2001:383-396. PMID: 31134578

13. Li Y, et al. Improvement on Permeability of Cyclic Peptide/Peptidomimetic: Backbone N-Methylation as A Useful Tool. Mar Drugs. 2021;19(6):311. PMID: 34072121

14. Wang CK, Craik DJ. Cyclic peptide oral bioavailability: Lessons from the past. Biopolymers. 2016;106(6):901-909. PMID: 27178381

15. Qian Z, et al. Discovery and Mechanism of Highly Efficient Cyclic Cell-Penetrating Peptides. Biochemistry. 2016;55(18):2601-2612. PMID: 27089101

16. Freeman DJ. Cyclosporine: structure, pharmacokinetics, and therapeutic drug monitoring. Ther Drug Monit. 1987;9(3):356-363. PMID: 3322675

Frequently Asked Questions: Peptide Cyclization Research

Why do cyclic peptides often have better oral bioavailability than linear peptides?

Linear peptides are rapidly degraded by proteases in the GI tract and fail to cross the intestinal epithelium due to their conformational flexibility and polar surface area. Cyclization restricts the peptide backbone, reducing proteolytic susceptibility by preventing enzyme recognition of the cleavage site geometry. Additionally, N-methylation of backbone amides — frequently combined with cyclization — reduces the number of hydrogen-bond donors, enabling passive membrane permeability through a "chameleonic" mechanism where the peptide adopts compact conformations in hydrophobic environments. Cyclosporin A (cyclosporine) is the canonical example: a cyclic N-methylated undecapeptide with ~30% oral bioavailability.

What is the difference between head-to-tail cyclization and side-chain-to-tail cyclization?

Head-to-tail (N-to-C) cyclization connects the peptide's N-terminus directly to its C-terminus via an amide bond, forming a homodetic cyclic peptide. This requires on-resin or solution-phase cyclization after chain assembly and avoids modifying side chains. Side-chain-to-tail cyclization connects a side-chain nucleophile (typically Lys ε-amine or Asp/Glu carboxyl) to either terminus, introducing structural diversity and leaving one terminus free for further derivatization. Cyclic RGD peptides used in integrin research are commonly synthesized this way (e.g., c(RGDfK) with Lys ε-amine to C-terminus lactam).

What is a disulfide bridge and how is it reduced for research use?

A disulfide bridge forms when two cysteine residues are oxidized, creating a covalent S-S bond that locks the peptide into a particular conformation. Natural disulfide-rich peptides (conotoxins, defensins) rely on these bridges for receptor selectivity. For research, disulfide-bridged peptides are supplied oxidized (reduced peptide + air oxidation during formulation) and should be kept oxidized during storage. To study the reduced (linear) form, treat with dithiothreitol (DTT, 10 mM) or tris(2-carboxyethyl)phosphine (TCEP, 1 mM) immediately before use. Note: some "cyclic" peptide labels in supplier catalogs refer to disulfide cyclization even when the backbone is technically linear.

How does ring size affect biological activity in peptide research?

Ring size determines conformational constraint, flexibility, and solubility. Very small rings (4-6 amino acids) are highly constrained, potentially too rigid for productive receptor interaction. Optimal rings for most receptor-targeting research peptides fall in the 5-12 amino acid range, where the ring adopts stable secondary structures (β-turns, γ-turns) that present key pharmacophore elements. Larger rings (12-20+ amino acids) behave more like semi-flexible scaffolds and can accommodate complex binding epitopes, as seen in macrocyclic natural products. Ring size also affects solubility: large hydrophobic cycles often require co-solvent systems (DMSO/water).

Can cyclic peptides be screened in standard cell-based assays?

Yes, but with considerations. Cyclic peptides with good aqueous solubility can be added directly to cell culture media. For hydrophobic cycles, prepare a stock solution in DMSO (≤10 mM) and dilute to <0.1% DMSO final concentration to avoid cytotoxicity. Disulfide-bridged peptides should not be added to cell culture medium containing strong reducing agents (e.g., high-concentration β-mercaptoethanol). For peptides studied for cell-penetrating properties, confirm uptake using fluorescent-labeled analogs (FITC-conjugated) before interpreting intracellular activity data.

What are the key analytical challenges for characterizing cyclic peptides?

Cyclic peptides present distinct analytical challenges compared to linear peptides: (1) Sequencing: Edman degradation requires a free N-terminus and fails on true head-to-tail cycles — MS/MS fragmentation is required. Cyclic peptide MS/MS spectra show ring-opening fragmentation patterns that differ from linear peptide b/y-ion series. (2) Purity assessment: RP-HPLC remains the standard method, but cyclic peptides may co-elute with synthetic byproducts that differ only in cyclization efficiency. (3) Quantification: Extinction coefficients may differ from linear precursors if cyclization shifts chromophore environment (e.g., for Tyr/Trp-containing cycles). Request mass-confirmed COA for cyclic peptide research materials. See Peptide Purity Testing Guide for HPLC/MS methodology.

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Further Reading:

• •Peptide Bioconjugation Strategies: PEGylation, Lipidation, and Beyond for Half-Life Extension
• •RGD Peptides: Integrin-Binding Motifs Driving Biomaterials, Targeted Delivery, and Cell Adhesion Research
• •Stapled Peptides: Engineering Conformationally Constrained α-Helical Peptides for Research
• •How Peptides Are Synthesized: Solid-Phase vs Solution-Phase Methods Explained
• •Reconstitution Calculator
• •Peptide Stack Builder

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Cyclic Peptides on Peptides.SO: Compound Research Pages

Several well-characterized research peptides studied in the cyclization literature are available through Peptides.SO's verified supplier network. The following compound pages include live cross-supplier pricing and documentation:

Hexarelin — Synthetic Cyclic Hexapeptide GHS

Hexarelin (His-D-2MeTrp-Ala-Trp-D-Phe-Lys-NH2) is a conformationally constrained GHRP-class secretagogue. Its D-amino acid substitutions at positions 2 and 5 enforce a specific backbone geometry that resists proteolytic degradation — a practical demonstration of conformational stabilization via non-natural amino acid incorporation discussed in this guide.

Live pricing: Hexarelin compound page — 50+ listings, $6.00–$350+/mg from named suppliers including Pure Peptides UK, Peptides Source, Ruo Bio, Top Peptides, and Raw Amino.

SS-31 (Elamipretide) — Synthetic Tetrapeptide with Mitochondrial Targeting

SS-31 (D-Arg-Dmt-Lys-Phe-NH2) uses D-amino acids and a C-terminal amide to create a constrained structure optimized for mitochondrial inner membrane association. The Dmt residue (2'6'-dimethyltyrosine) is a non-natural aromatic residue that contributes to both structural rigidity and ROS scavenging.

Live pricing: SS-31/Elamipretide compound page — 59 listings, $2.10–$116/mg from NUPEPS Peptides, HK Peptides Worldwide, Strate Labs, and Pure Peptides UK.

Semax and Selank — Conformationally Defined Short Peptides

Semax (Met-Glu-His-Phe-Pro-Gly-Pro, MW ~822 Da) and Selank (Thr-Lys-Pro-Arg-Pro-Gly-Pro, MW 751.9 Da) are both short linear peptides whose conformational behavior in solution is relevant to their receptor binding — the proline-rich C-terminal extensions in both were engineered to influence backbone flexibility in a predictable way.

• •Semax compound page — 151+ listings from $1.33/mg
• •Selank compound page — 145+ listings from $1.83/mg

Research Tools

• •Reconstitution Calculator — concentration calculations for cyclic peptide stock solutions
• •Peptide Stack Builder — plan multi-compound research protocols
• •Peptide Purity Testing: HPLC & Mass Spec — identity confirmation for conformationally complex peptides
• •How to Read a Peptide COA — evaluating mass spec data for D-amino acid-containing peptides

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For research use only. Cyclic peptide handling follows the same RUO framework as all research-grade compounds on this platform.