A pH-Tunable Thiourea Bridge Opens New Doors in Cyclic Peptide Drug Discovery

Rethinking Peptide Cyclization Chemistry

Cyclic peptides bridge the gap between large biologics and small molecules. They combine high target selectivity with good cell permeability potential. However, making the closed ring structure is historically difficult. Most traditional approaches rely on disulfide bonds or cysteine-cysteine cross-linking. These methods limit structural scaffold diversity. Phage biocompatibility is also reduced by them.

A 2026 study in Science Advances by Bai et al. presents a new method. The team used a bifunctional cross-linker called 1-(bromomethyl)-4-isothiocyanatobenzene (pNCSBnBr). This molecule contains two orthogonal reactive handles. A benzylic bromide sits at one end. It alkylates cysteine thiols very efficiently. An isothiocyanate group is on the other end. This group forms a thiourea bridge with an amine, regulated strictly by pH. The linker reacts selectively with the N-terminal -amine at pH 6.5 because it is more nucleophilic. This produces head-to-side chain macrocycles. When the buffer is shifted to pH 9.5, lysine -amines deprotonate and react. This second condition yields side chain-to-side chain cyclic structures.

This pH-dependent selectivity acts as the foundation of the platform. One single linker produces two different cyclic shapes. A thiourea moiety gets embedded right into the peptide backbone. This specific pharmacophore already exists in approved medicines like enzalutamide and thioacetazone. Two genetically encoded cyclic peptide libraries were built with this chemistry. The researchers found strong binders for 3 separate targets: cyclophilin D (Cyp D), Keap1, and MDM2.

Core Results The Cyclization Concept

Fig 1 shows the core concept. Panel A displays the reaction of the pNCSBnBr cross-linker with a linear peptide that has an N-terminal amine and a cysteine thiol. The benzylic bromide alkylates the cysteine sulfur at pH 6.5. This creates a proximity effect. Because of this, the isothiocyanate group quickly reacts with the nearby N-terminal amine. A head-to-side chain ring forms with a thiourea junction. The exact same cross-linker targets the -amine of a lysine residue if the pH is raised to 9.5. A side chain-to-side chain cyclic architecture is generated.

Image: https://ecdn6.globalso.com/upload/p/1443/image_other/2026-06/fig-1-the-construction-of-thiourea-containing-cyclic-peptides-using-one-pot-cyclization.jpg

Reaction order is critical here. If the initial cysteine alkylation does not anchor the linker, the isothiocyanate group fails to react efficiently in water because it is too hydrophobic. Selective cyclization requires this proximity-driven mechanism in biological settings. Panel B displays how this chemistry integrates into phage display. The workflow requires treating the phage library with the cross-linker. Then, magnetic beads are used to pan against biotinylated targets. Amplification and next-generation sequencing (NGS) follow. This workflow moves the organic chemistry step directly into a high-throughput screening application.

pH Governs Selectivity

Image: https://ecdn6.globalso.com/upload/p/1443/image_other/2026-06/fig-2-one-step-strategy-for-selectively-forming-the-cyclic-versions-of-model-peptides.jpg

Chemical proof of concept was demonstrated in Fig 2 using two model linear peptides. Pep 1 (AVGSLQRGC) contains a free N-terminal amine and a C-terminal cysteine. This serves as the head-to-side chain substrate. Panel A tracks cyclization kinetics using HPLC and mass spectrometry under various acetonitrile levels. Yields improve when organic solvent is decreased. The reaction achieves 93% conversion in pure aqueous PBS at pH 6.5 after 12 hours at 37 degrees C. The observed mass is 1036.48 Da, matching the calculated mass of the thiourea-bridged cyclic product.

Pep 2 (KSFELYWDGLC) was prepared to check potential cross-reactivity. Its N-terminus is acetylated, so only a lysine -amine and a cysteine thiol are free. Panel B shows that the lysine -amine is totally unreactive at pH 6.5. Only linear alkylated intermediate is detected. However, cyclization goes to completion within 3 hours at pH 9.5. The measured mass of 1548.66 Da confirms the side chain-to-side chain macrocycle. There is no cross-reactivity between the two amines under these separate conditions. This sharp pH-switching makes the method highly practical. Controlling buffer pH allows researchers to enforce one specific topology during library construction.

On-Phage Validation and Biocompatibility

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Many cyclization methods fail when transitioning from isolated peptides to functional phage particles. Fig 3 evaluates this step. Panel A shows that pNCSBnBr successfully modifies a model peptide attached to the N1-N2 domain of a disulfide-free pIII coat protein. A mass increase of +147 Da occurs, which matches linker insertion and cyclization. Thiourea linkage between the Ala N-terminus and the Cys C carbon was verified using 2D NMR on a cleaved peptide sample.

Panel B illustrates pull-down recovery. Adding a biotinylated version of the linker (N-biotin) to phage at 400 M allows capture of around 1011 phage particles on streptavidin beads. The control without biotin yields only about 104 particles, showing surface modification is highly selective.

Infectivity results for both phage libraries are given in Panel C. For the N(12)-library cyclized at pH 6.5, phage titers stay completely stable at linker concentrations up to 600 M. This proves the reaction has great biocompatibility. The K(7)-library, treated at pH 9.5, shows a different pattern. Phage titers decrease 200-fold at 250 M of linker. The basic conditions are harsher on the phage, but screening remains viable at lower concentrations. The authors document this trade-off between reaction rate and phage survival directly.

Three Targets, Two Libraries

Image: https://ecdn6.globalso.com/upload/p/1443/image_other/2026-06/fig-4-the-panning-of-two-types-of-thiourea-containing-cyclic-peptides-toward-several-therapeutically-relevant-proteins.jpg

Screening results against the three protein targets are compiled in Fig 4. Panel A shows NGS data from panning the N(12)-library against Cyp D. The most enriched sequence, CD1 (AHVTPGFMRLQGSC), makes up 7.4% of total reads. It binds Cyp D with a Kd value of 0.74 plus-minus 0.03 M. Panel D shows this cyclic variant binds over 37-fold stronger than the linear peptide D1, which has a Kd of 27.74 M. This binding difference highlights the structural rigidity provided by the thiourea linkage. Functional inhibition is evaluated in Panel E. CD1 blocks the peptidyl-prolyl isomerase function of Cyp D with an IC50 of 4.14 M, whereas the uncyclized linear control lacks measurable activity.

Panel B shows the selected sequences from the K(7)-library for MDM2 and Keap1. The primary Keap1 binder is CK1 (KSLRSLQSC), showing 13.9% sequence abundance and a strong Kd of 46.12 plus-minus 2.59 nM. The MDM2 ligand, named CM1, displays moderate binding in the micromolar range. Panel C displays the enrichment profile during Keap1 selection. Sequence diversity drops across 3 selection rounds into a few dominant sequences. A serine-rich motif is highly conserved in the sequence logo.

Unpacking CK1's Keap

1 Interaction

Image: https://ecdn6.globalso.com/upload/p/1443/image_other/2026-06/fig-5-schematic-representation-of-the-predictive-binding-pattern-of-ck1-with-keap1-and-the-affinity-identification-of-ck1-with-keap1-or-keap1-mutant.jpg

Fig 5 focuses on CK1, the strongest binder found in the experiments. Panel A displays the BLI binding data, which fits a 1:1 interaction model with a Kd of 46.12 nM. The structural mechanism is explored in Panel B. Molecular dynamics simulations show that CK1 interacts with Arg380, Arg415, and Arg483 residues inside the Keap1 Kelch domain. The natural substrate Nrf2 targets this exact same arginine cluster. Remarkably, CK1 interacts here without using the traditional ETGE acidic sequence motif found in all other Keap1 peptides. This proves that the thiourea structural frame enforces a shape that mimics natural binding interactions with a completely unique amino acid sequence.

Panels C and D test these specific contact points. Changing Arg380, Arg415, or Arg483 to alanine using single mutations decreases CK1 binding affinity by over 10-fold. Similarly, changing Ser5 to alanine on CK1 decreases affinity by more than 300-fold. Panel E provides functional verification. In a competitive fluorescence polarization assay, CK1 displaces a labeled Nrf2(69-84) peptide from Keap1. The measured Ki value is 317 plus-minus 38.6 nM, confirming real protein-protein interaction inhibition. The compound also exhibits good serum stability. It remains intact during 12 hours of incubation in calf serum with minimal breakdown.

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