Protein Chemistry

Lecture 2 Paper 1

🌟 Big Picture: Why Modify Tryptophan?

Proteins are rarely “finished” after ribosomal synthesis. Post-translational modifications (PTMs) are essential for function, regulation, and signaling. Chemists want to recreate, mimic, or extend these PTMs to:

Understand biological function 🧬
Attach probes for imaging 🔬
Improve stability or pharmacology 💊
Introduce entirely new functions 🛠️

Why target tryptophan (Trp)?

🧮 Extremely rare (~1.1% of residues in eukaryotes)
🎯 Often unique per protein → excellent site selectivity
🧪 Contains an indole ring with distinctive reactivity
⚠️ Less reactive than Cys/Lys → historically difficult, but highly rewarding

Older Trp modifications relied on harsh oxidants (e.g., NBS, NPS-Cl), causing side reactions and protein damage. Modern chemistry has transformed this field.

🧱 Section 1: Late-Stage Modification Strategies

Two general approaches exist:

Unnatural amino acids (UAAs) – powerful but require genetic engineering
Natural residue targeting – simpler, more practical

This review focuses on late-stage chemical modification of native Trp residues, especially methods developed in the last decade.

⚙️ Section 2: C–H Activation at the Indole Ring

C–H activation is the most direct way to modify Trp, typically at the C2 position of the indole.

2.1 Palladium-Catalyzed C–H Arylation 🧲

Early Pd methods required high temperature (≈80 °C) → peptide-only
Key innovations:
- Aryl iodides, boronic acids, diaryliodonium salts
- Gradual move to room temperature and aqueous media
Strengths:
- High regioselectivity
- Broad aryl scope
Limitations:
- Still challenging for folded proteins
- Catalyst loading can be high

📌 Takeaway: Excellent for peptide diversification; protein compatibility is improving.

2.2 Ruthenium-Catalyzed Arylation & Alkylation 🔩

Uses Ru(II) (cheaper than Pd)
Requires pyridyl-protected Trp
Enables:
- Trp–Trp ligation
- Solid-phase peptide synthesis adaptations
Drawback: High temperatures (≈120 °C)

2.3 Gold-Catalyzed Alkynylation 🪙

Introduces alkyne handles → click chemistry
Uses hypervalent iodine reagents (TIPS-EBX)
Successfully applied to proteins
Major limitation: very high catalyst loading

2.4 Manganese-Catalyzed Alkynylation & Allylation ⚙️

Uses base metals instead of precious metals
Requires N-protected Trp
Enables:
- Alkynylation
- Allylation
- Peptide stapling 🧷
Still needs elevated temperature (~80 °C)

2.5 Cobalt-Catalyzed Allylation 🧲

Co(III) catalyst
Allylation at C2
Products can undergo secondary reactions (e.g., Alder-ene)
Adds modularity to peptide design

2.6 C–N Bond Formation ✨

N-iodosuccinimide (NIS)–mediated coupling with triazoles
Produces fluorescent peptides 💡
Requires carboxyl protection or strong acids

⚡ Section 3: Radical-Based Trp Modifications

Radical chemistry avoids transition metals and often works under milder, aqueous conditions.

3.1 ABNO-Based Radical Tagging 🧪

Uses keto-ABNO radicals
Selective for exposed Trp residues
Successfully applied to:
- Aβ₁–₄₂
- Lysozyme
- BSA
Structural accessibility matters 🏗️

3.2 Perfluoroalkylation

3.2.1 Trifluoromethylation (–CF₃) ❄️

Enables ¹⁹F NMR and imaging
Uses radical CF₃ sources (e.g., Langlois’ reagent)
Works in neutral aqueous buffer
Challenges:
- Side reactions with Cys, Tyr, His
Solutions:
- Excess Met as a scavenger
Even buried Trp residues can be modified

🚀 Advanced application: ¹⁸F-trifluoromethylation for PET imaging

3.2.2 Other Perfluoroalkylations

Uses perfluoroalkyl iodides
Mostly demonstrated on short peptides

3.3 Photocatalytic Trp Modification 💡

With Photocatalyst

Targets the β-position (not indole!)
Uses Ir-based photoredox catalysts
Compatible with insulin and glucagon
High chemoselectivity due to indole radical cation formation

Without Photocatalyst

UV-B light generates Trp radicals
N-carbamoyl pyridinium salts trap radicals
Extremely fast (≈45 min, 94% conversion)
Applied to native proteins (e.g., lysozyme)

🔥 Section 4: Oxidative Modifications

The indole ring is easily oxidized, but control is difficult.

4.1 Substitution Reactions

Peroxynitrite/CO₂ modifies Trp selectively in some proteins
Cerium(IV) ammonium nitrate (CAN) enables coupling to anilines
Conversions often <50%

4.2 Formation of Kynurenine (Kyn) & N-Formylkynurenine (NFK)

Oxidative cleavage of the indole ring leads to biologically relevant products.

Chlorine dioxide → NFK (but oxidizes Tyr too)
Fenton chemistry → Kyn + NFK (also oxidizes Met)
AAPH oxidation
- Initially non-selective
- Selectivity restored by adding excess Met
- Applied to therapeutic IgG1

🔗 Section 5: N-Terminal Trp Modification (Pictet–Spengler)

Acid-catalyzed condensation between:
- N-terminal Trp
- Aldehyde-containing peptide
Produces tetrahydro-β-carbolines
Useful for:
- Peptide ligation
- Mimicking proline turns
Limited follow-up in later literature

🧩 Section 6: Other Creative Strategies

6.1 Rhodium Carbenoids

React at C2 or indole N
Early versions required very low pH (protein denaturation)
Improved versions operate at pH ~6

6.2 Malondialdehyde (MDA) Condensation

Targets indole nitrogen
Modification is reversible
Requires extremely acidic conditions (≥3 M HCl or 50% TFA)
Limits protein applications

🔮 Section 7: Summary & Outlook

What we’ve learned:

🧠 Trp is a powerful handle for site-selective protein chemistry
🧱 C–H activation dominates peptide modification
⚡ Radical and photochemical methods are protein-friendly
🔥 Oxidation can mimic biological stress but risks side reactions

Future directions:

Lower catalyst loadings
Fully aqueous, room-temperature conditions
Broader substrate scope
Better compatibility with folded proteins and therapeutics

🎯 Bottom line: Tryptophan modification has evolved from harsh oxidation to a versatile, precision toolkit for protein science, chemical biology, and drug development.

Quiz

Score: 0/29 (0%)

Q0. Why is tryptophan considered an attractive target for late-stage protein modification?

It is the most abundant amino acid in proteins

It has high nucleophilicity similar to cysteine

It is low in abundance and contains a uniquely reactive indole ring

It is always exposed on protein surfaces

Q1. Which position on the indole ring of tryptophan is most commonly targeted in C–H activation reactions?

C-3 position

C-2 position

N-1 position

β-position of the side chain

Q2. What is a major limitation of early palladium-catalyzed C–H arylation methods on tryptophan?

Lack of regioselectivity

Requirement for very acidic conditions

High temperatures incompatible with proteins

Inability to modify peptides

Q3. Which aryl donor enabled milder palladium-catalyzed tryptophan arylation without Cu(II) cocatalysts?

Aryl bromides

Aryl boronic acids

[MesPhI]OTf

Aryl chlorides

Q4. What key modification did Ackermann’s group introduce to enable room-temperature Pd-catalyzed arylation?

Use of microwave heating

Replacement of Pd(OAc)2 with Pd(OTs)2(MeCN)2

Addition of strong bases

Use of organic solvents only

Q5. Why does ruthenium-catalyzed C–H activation often require a directing group on tryptophan?

To increase solubility in water

To protect the indole nitrogen

To enable regioselective C–H activation

To prevent oxidation

Q6. What is a key advantage of gold-catalyzed alkynylation of tryptophan?

It exclusively modifies buried residues

It introduces an alkyne handle for click chemistry

It requires no protecting groups

It operates without any catalyst

Q7. Which base metal catalyst was successfully used for C–H alkynylation of protected tryptophan residues?

Iron

Nickel

Manganese

Zinc

Q8. What type of peptide architecture was enabled by manganese-catalyzed allylation of tryptophan?

Linear peptides only

β-sheet-rich peptides

Macrocyclic stapled peptides

Disulfide-bridged peptides

Q9. What is the main benefit of radical-based tryptophan modification compared to metal-catalyzed C–H activation?

Higher regioselectivity

Ability to form C–C bonds only

Avoidance of transition metals and milder conditions

Exclusive modification of buried residues

Q10. The keto-ABNO-based modification of tryptophan preferentially targets exposed residues.

True

False

Q11. Radical trifluoromethylation of tryptophan can be carried out in neutral aqueous buffers.

True

False

Q12. In trifluoromethylation reactions, cysteine residues are completely unreactive and never form side products.

True

False

Q13. Photocatalytic modification reported by Shi et al. targets the β-position of tryptophan rather than the indole ring.

True

False

Q14. The indole nitrogen is oxidized during photocatalytic β-position modification of tryptophan.

True

False

Q15. UV-B-induced modification of tryptophan without a photocatalyst can reach near-complete conversion within an hour.

True

False

Q16. Oxidation-based tryptophan modifications generally suffer from high selectivity and minimal side reactions.

True

False

Q17. Chlorine dioxide oxidation of tryptophan selectively forms N-formylkynurenine without affecting tyrosine residues.

True

False

Q18. Fenton oxidation of proteins can convert tryptophan into kynurenine and N-formylkynurenine.

True

False

Q19. Adding excess methionine can improve selectivity in AAPH-mediated oxidation of tryptophan.

True

False

Q20. The Pictet–Spengler reaction enables ligation at an N-terminal tryptophan residue.

True

False

Q21. Pictet–Spengler-based ligation forms tetrahydro-β-carboline structures.

True

False

Q22. Early rhodium carbenoid-based tryptophan modifications were limited by very low pH requirements.

True

False

Q23. Later improvements in rhodium carbenoid chemistry allowed reactions to proceed at near-neutral pH.

True

False

Q24. Malondialdehyde (MDA) modification of tryptophan is fully irreversible under all conditions.

True

False

Q25. MDA-based modification requires strongly acidic conditions, limiting protein compatibility.

True

False

Q26. C–H activation methods are currently the most versatile strategy for peptide-level tryptophan modification.

True

False

Q27. Radical-based methods generally offer a wider substrate scope than C–H activation methods.

True

False

Q28. The review concludes that future research will likely focus on milder, protein-compatible tryptophan modification strategies.

True

False