Cyclic Peptides: Methods and Applications
Peptide cyclization to form cyclic peptides or cyclized peptides is a frequently used strategy for the development of peptides with enhanced conformational stability compared to their linear analogs. Cyclic peptides have proven to be useful for several applications, including the mimicry of protein secondary structures, such as protein loops, and the optimization of peptide ligands with increased binding potency, selectivity, and enhanced protease stability.
Methods of Peptide Cyclization
Depending on the cyclization site, there are several general methods to synthesize cyclic peptides, including head-to-tail cyclization, side-chain-to-side-chain, head-to-side-chain, and side-chain-to-tail cyclization. Among these methods, the two former methods are the most frequently utilized. While head-to-tail cycles are usually formed by amide bond formation, side-chain-to-side-chain cycles are most often synthesized via Cys-Cys or amide bond formation within a cyclic peptide.
Head-to-Tail Cyclization
Head-to-tail cyclization involves the formation of an amide bond between the N-terminus and the C-terminus of a linear peptide, resulting in a cyclic peptide with enhanced stability. This method is often used to produce cyclic peptide libraries for screening and drug development.
Side-Chain-to-Side-Chain Cyclization
Side-chain-to-side-chain cyclization involves the formation of a bond between the side chains of two amino acids within a peptide. This method can be achieved through various chemistries, including disulfide bond formation and amide bond formation.
Other Cyclization Methods
In addition to disulfide and amide cyclizations, there are many other cyclization methods available. Examples include cyclizations via click chemistry (copper-catalyzed azide-alkyne cycloaddition, CuAAC) or cyclizations leading to thioethers, such as by cyclization of cysteine side chains with bromoacetate.
Cys-Cys Cyclization
Cys-Cys cyclization results from the formation of disulfide bridges between cysteine residues of the peptide. This is the most straightforward and frequently used peptide cyclization method. However, potential problems may still have to be overcome. The main problem is dimerization, which is a frequent side reaction during cyclization. This can be minimized by cyclization under high dilution conditions. When a peptide contains multiple cysteine residues, challenges arise due to the large number of possible regioisomers. To cope with this, disulfide bridges can be formed regiospecifically between specific cysteine positions. JPT can synthesize peptides with up to four disulfide bonds in one peptide, employing site-specific orthogonal chemistry or thermodynamic stability methods.
One distinct disadvantage of Cys-Cys cyclized peptides is limited stability, especially against reductive conditions. Therefore, drug discovery projects commonly employ a two-step approach:
- Screening and initial optimization of Cys-Cys cyclized peptides.
- Replacement of the Cys-Cys disulfide by more stable bonds.
Amide Cyclization
Amide bonds, which have the advantage over disulfides that they are chemically more stable, are frequently used for cyclic peptides. The following types of cyclic peptides are most often prepared:
Head-to-Tail Cyclic Peptides
Head-to-tail cyclic peptides are formed by linking the N-terminus and C-terminus of a linear peptide through an amide bond. This method provides enhanced stability and resistance to enzymatic degradation.
Side-Chain-to-Side-Chain Cyclic Peptides
Side-chain-to-side-chain cyclic peptides are formed by linking the side chains of two amino acids within the peptide through an amide bond. This method allows for the creation of peptides with specific conformations and properties.
As with Cys-Cys cyclization, there are some synthetic challenges with amide cyclizations. Dimerization has to be limited, which can be accomplished by working under high dilution conditions. The most important challenge with head-to-tail cyclizations is, however, the fact that peptide cyclization is often slow due to entropic reasons. This can lead to undesired side reactions like racemization or peptide capping by coupling reagents. For effective head-to-tail cyclization, it is therefore critical to choose beforehand:
- The optimal cyclization site
- The optimal cyclization reagent
- The optimal cyclization conditions
Over the last year alone, JPT has used its experience for the successful synthesis of more than a thousand amide cyclic peptides with very high success rates, even for challenging peptides.
Cyclic Peptides by JPT Peptide Technologies
JPT Peptide Technologies is the expert in cyclic peptide synthesis, offering tailored solutions for every application. With over 20 years of experience and a broad range of expertise, JPT has pioneered patented technologies in peptide libraries, pools, and microarrays. Our state-of-the-art laboratories in Berlin, Germany, operate under strict regulations, ensuring the highest quality and reliability in peptide manufacturing.
Whether you need micro-scale synthesis, intermediate peptide synthesis, or high-throughput production of cyclic peptide libraries, JPT is your trusted partner. Our comprehensive range of services and customization options ensures that we can meet the unique requirements of your project, from basic research to advanced drug development.
For more information about our cyclic peptide synthesis services or to discuss your specific needs, please contact us. Our team of experts is ready to assist you in achieving your research and development goals.