Cyclic Peptides: Synthesis Strategies & Design Considerations

Cyclic peptides have become an increasingly important tool in drug discovery, bridging the gap between small molecules and larger biologics. Their specific features offer unique advantages, but also require careful design and synthesis strategies. 

Macrocyclic & Cyclic Peptide Libraries: Formats and Scale 

Advances in synthesis and purification now enable the reliable production of (macro-)cyclic peptide libraries in various formats, from focused sets for target validation to larger collections for early-stage screening.

• Scale: from 1mg to 1g Purity: Crude to 95% 
• Formats: separately as a library or pooled 
• Delivery in individual tubes or tube racks 
• Customized Modifications 

Leveraging extensive expertise, JPT has successfully synthesized thousands of cyclized peptides with high success rates, including challenging sequences. 

Benefits for Choosing JPT 

• Multiple cyclization strategies (head-to-tail, side-chain, linker-based) 
• High sequence fidelity and controlled cyclization efficiency 
• Custom library design to match biological targets 
• Optional modifications (labels, PTMs, non-natural residues) 
• Rigorous QC including HPLC and MS 
• Scalable production from discovery to preclinical research 

Custom Peptide Synthesis

From Linear to Cyclic: Key Synthesis Principles 

Most cyclic peptides are synthesized via solid-phase peptide synthesis (SPPS), starting with a linear precursor. Compared to linear peptides, additional considerations come into play, such as sequence design to favor efficient cyclization and minimize side reactions. Protecting group strategies, resin choice, and cleavage conditions all influence the success of downstream cyclization. 

Methods of Cyclization 

Several cyclization strategies are routinely employed, depending on the intended application: 

Position: 

• Head-to-tail cyclization via amide bond formation 
• Side-chain to side-chain cyclization, often using disulfide bridges or lactam formation 
• Side-chain to backbone cyclization for additional structural rigidity 

Types of Cyclization: 

• Cys-Cys (disulfide) cyclization: the most straightforward and widely used approach to peptide cyclization. Common challenges include dimerization and region-isomer formation in multi-cysteine peptides. These can be minimized under high-dilution conditions, using site-specific orthogonal protection strategies or thermodynamically controlled folding. This way, JPT can synthesize peptides containing up to four disulfide bridges. While disulfide-cyclized peptides are well suited for screening and early optimization, their limited stability under reducing conditions often necessitates subsequent replacement of the disulfide bond with more stable linkages. 
• Amide cyclization: Amide bonds offer high chemical stability and are therefore frequently used. Common formats include head-to-tail cyclization, linking the N- and C-termini, and side-chain-to-side-chain cyclization via lactam formation. As with disulfide cyclization, dimerization must be controlled, typically by high-dilution conditions. In head-to-tail cyclization, slow reaction kinetics can lead to side reactions such as racemization, making the choice of cyclization site, reagent, and conditions critical. 

Each method influences conformational freedom, stability, and biological performance, and can be tailored to the desired screening or optimization strategy. 

Why do we apply Capping to our Synthesis? 

Peptides are synthesized stepwise by attaching one amino acid after the other. Since no chemical reaction yields 100%, each step involves a small percentage of the coupling reaction not working. The residual amino acid may not react with the next one in sequence, thus creating a deletion sequence. These deletion sequences could easily lead to false positive responses in your assays (e.g. presenting as de novo epitopes). 

We at JPT developed a proprietary capping method to reduce said deletion sequences. After each step of adding a protected amino acid to the existing peptide sequence, we apply an extra capping step. This eliminates the chance of free NH3 forming capped truncated peptides. 

Also in cyclic peptide synthesis, selective protection and deprotection steps are critical in ensuring regio- and chemoselective cyclization. Common approaches include orthogonal side-chain protection, N-terminal or C-terminal capping, and temporary masking of reactive functionalities to prevent oligomerization or undesired cross-linking.