1. Rob Brown Special Topic – 25/01/112
Peptide Coupling
Rob Brown
Special Topic – 25/01/112

2. Peptide Coupling The Amide Bond – Biological Importance
Key chemical bond in proteins:
Enzymes (biological catalysis)
Structural and mechanical function
Cell signalling
Immune response
Cell adhesion
- Amide-containing pharmaceuticals:

3. Peptide Coupling The Peptide Bond – Biological Synthesis
- Ribosomal Protein Translation
- DNA → RNA → Peptides/Proteins
-Chemical Synthesis? - Solid Phase
- Solution Phase

4. Peptide Coupling The Peptide Bond – Chemical Synthesis Problem:
Solution:

5. Peptide Coupling
The Peptide Bond – Chemical Synthesis

6. Peptide Coupling
Chemical Synthesis – Racemization

7. Peptide Coupling
Chemical Synthesis – Racemization

8. Peptide Coupling
Chemical Synthesis – Racemization

9. Peptide Coupling
Chemical Synthesis – Racemization

10. Peptide Coupling
Chemical Synthesis – Common Side Reactions

11. Peptide Coupling Coupling reagent comparison
Published epimerisation tests:

12. Peptide Coupling
Carbodiimide coupling

13. Peptide Coupling
Carbodiimide coupling

14. Peptide Coupling
Carbodiimide coupling

15. Peptide Coupling
Carbodiimide coupling

16. Peptide Coupling
Carbodiimide coupling

17. Peptide Coupling
Carbodiimide coupling

18. Peptide Coupling
Carbodiimide reagents:
Additives:

19. Peptide Coupling
HOXt-based coupling reagents

20. Peptide Coupling
HOXt-based coupling reagents

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HOXt-based coupling reagents

22. Peptide Coupling
HOXt-uronium/aminium salts

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HOXt-uronium/aminium salts

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HOXt-phosphonium salts

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HOXt-immonium salts

26. Peptide Coupling
Other HOXt-derived reagents

27. Peptide Coupling Acid Halide-generating reagents
Harsh activation method
Lack of functional group compatibility
Works well with hindered amino acids
Racemization a major problem
e.g.

28. Peptide Coupling
Acid Halide-generating reagents: Triazines

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Acid Halide-generating reagents: Halo-uronium & phosphonium salts

30. Peptide Coupling
Pentafluorophenol-based reagents

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HODhbt-reagents

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HOSu-reagents

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Phosphorus-reagents

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Mixed carbonic anhydrides

35. Peptide Coupling
Polymer-supported reagents

36. Peptide Coupling
Newer approaches to amide bond fomation

37. Peptide Coupling Conclusions
Most reagents are not broadly applicable, relatively narrow uses.
Specialized, expensive reagents work well for SPPS
-High yields required, by products can be washed out
-Simpler, cheaper all-purpose reagents better for solution phase:
- HATU & HBTU – general and excellent reactivity.
- HATU favoured for quick coupling times
- DIC/HOBt still among the best combinations
- Substitute EDC when inseparable by-products become problematic
- For hindered couplings: PyBrop for N-Me amino acids
PyBop for hindered tertiary amino acid couplings
Triazines for hindered quaternary amino acids
- PS-IIDQ and PS-Mukaiyama reagents best for library synthesis
-Finally, keep it simple, don’t be mislead by needlessly expensive reagents
Main refs: E.Valeur, M. Bradley; Chem. Soc. Rev., 2009, 38,
A. El-Faham, F. Albericio; Chem. Rev., 2011, 111,