Native chemical ligation joins unprotected peptide fragments chemoselectively, and it is the standard route to sequences too long to make by stepwise synthesis.
Key Takeaways
- Solid-phase assembly becomes unreliable beyond roughly fifty residues because crude purity falls and purification yield collapses.
- Classical ligation requires cysteine at the junction, which limits where fragments can be joined.
- Thioester preparation, fragment solubility, and purification of the ligation mixture are the recurring difficulties.
Why stepwise synthesis runs out
Solid-phase assembly becomes unreliable beyond roughly fifty residues because crude purity falls and purification yield collapses. Producing longer chains by stepwise extension is usually impractical rather than merely difficult.
The ligation chemistry
A peptide with a C-terminal thioester reacts with another carrying an N-terminal cysteine, forming an initial thioester-linked intermediate that rearranges to a native amide bond. The reaction is chemoselective and proceeds in aqueous solution without protecting groups.
For related mechanism work, see peptide cyclisation methods.
The cysteine requirement and workarounds
Classical ligation requires cysteine at the junction, which limits where fragments can be joined. Auxiliary groups that are removed after ligation, and cysteine-free variants using alternative chemistry, extend the method to most junctions.
Sequential and convergent strategies
Fragments can be added one at a time or assembled convergently from larger pieces. Convergent assembly is generally more efficient for long targets but requires compatible protecting-group schemes across all fragments.
Practical constraints
Thioester preparation, fragment solubility, and purification of the ligation mixture are the recurring difficulties. The chemistry is well established, but success still depends heavily on fragment design and on handling under inert conditions.
Experimental Conditions and Practical Setup
Ligation is run under inert atmosphere with the thioester fragment handled to limit oxidation, and progress is monitored by LC-MS at intervals rather than judged by a fixed reaction time. Fragment solubility is tested before the reaction, because poor solubility is a more common cause of failure than the ligation chemistry itself.
Ligation planning decisions
| Decision | Constraint | Workaround |
|---|---|---|
| Junction position | Classical method needs cysteine | Auxiliary groups or cysteine-free variants |
| Fragment number | More fragments mean more purifications | Convergent assembly for long targets |
| Thioester preparation | Requires a suitable linker or surrogate | Use an established thioester surrogate |
| Order of assembly | C-to-N or convergent | Convergent usually more efficient |
Practical Notes for the Bench
- Design junction placement early, since cysteine availability constrains it.
- Consider convergent assembly for targets well beyond fifty residues.
- Handle thioester fragments under inert conditions to limit oxidation.
Frequently Asked Questions
Why not just synthesise a long peptide directly?
Because stepwise purity and yield fall sharply beyond roughly fifty residues, making direct synthesis impractical.
Does ligation require cysteine?
Classical native chemical ligation does, though auxiliary and cysteine-free variants now extend the method to most junctions.
What makes ligation projects difficult?
Thioester preparation, fragment solubility, and purification of the reaction mixture, more than the ligation chemistry itself.
Can a protein-size target be made this way?
Yes. That is the purpose of the method. The practical difficulties are fragment preparation, solubility, and purification rather than the ligation chemistry itself.
Related Reading
- peptide cyclisation methods
- difficult peptide sequences
- peptide synthesis scale-up
- food-derived bioactive peptides
- peptide water content
References & Further Reading
- Dawson PE et al. Synthesis of proteins by native chemical ligation. Science. 1994. PubMed 7973629
- Giesler RJ et al. Enhancing native chemical ligation for challenging chemical protein syntheses. Curr Opin Chem Biol. 2020. PubMed 32745915
- Conibear AC et al. Native chemical ligation in protein synthesis and semi-synthesis. Chem Soc Rev. 2018. PubMed 30418441
- Peptide literature search on PubMed
- Full-text archive at PubMed Central
- Peptide research collection at Nature
The notes below reflect common laboratory practice and open literature. They are not clinical recommendations, and peptides discussed are research materials unless stated otherwise.
Reviewed by Dr. Elena Marchetti, Peptide Chemistry & Analytical Characterization.