This protocol details the complete solid-phase peptide synthesis (SPPS) workflow using Fmoc (9-fluorenylmethyloxycarbonyl) chemistry, the most widely adopted strategy for laboratory and pilot-scale peptide production.

1. Pre-Synthesis Planning

Sequence Analysis

Before synthesis begins, the target sequence must be evaluated for:

• Coupling difficulty: Aggregation-prone sequences (poly-Ala, poly-Val, poly-Ile) require microwave assistance or pseudoproline dipeptides
• Oxidation-sensitive residues: Met and Cys may require temporary protection
• Aspartimide formation: Asp-Gly sequences are prone to aspartimide rearrangement at elevated temperatures
• Racemization risk: C-terminal Fmoc-Gly is racemization-free; C-terminal Fmoc-Ser/Thr/His require caution

Resin Selection

Resin Type	Loading (mmol/g)	C-Terminal	Best For
Wang	0.3–1.0	Free acid	Standard peptides
Rink Amide MBHA	0.3–0.7	Amide	C-terminal amide
2-Chlorotrityl Cl	0.8–1.4	Free acid	Fragments, sensitive sequences
HMPB-ChemMatrix	0.2–0.4	Free acid	Long peptides (>30 residues)
Sieber Amide	0.2–0.6	Amide	Acid-labile protecting groups

Resin swelling: Wash resin with DMF (3 × 1 min) before first coupling. Allow 15–20 min swelling time for polystyrene resins; ChemMatrix resins swell in aqueous solutions.

Coupling Strategy

For standard sequences, double-coupling with HATU/DIPEA is recommended. For difficult sequences:

1. Standard: HATU (2 eq), DIPEA (4 eq), 20 min
2. Difficult: HATU (3 eq), DIPEA (6 eq), 2 × 30 min
3. Very difficult: DIC/Oxyma Pure (3 eq each), 60 min, 70°C
4. Microwave-assisted: DIC/HATU, 75°C, 5 min per coupling

2. Fmoc Deprotection

Reagent Preparation

Reagent	Concentration	Volume per wash
Piperidine	20% (v/v) in DMF	10 mL/g resin
DMF (wash)	100%	10 mL/g resin

Deprotection Protocol

1. Drain树脂 (resin) completely from storage solvent
2. Add 20% piperidine/DMF (10 mL/g resin)
3. Stir gently or shake for 2 min (first deprotection)
4. Drain and discard filtrate
5. Add fresh 20% piperidine/DMF
6. Stir for 8 min (second deprotection, ensures complete Fmoc removal)
7. Drain and save filtrate for Fmoc monitoring

Fmoc Monitoring (UV Absorbance)

The dibenzofulvene-piperidine adduct absorbs at 301 nm (ε = 7,800 M⁻¹cm⁻¹):

1. Dilute 100 µL deprotection filtrate to 3 mL with DMF
2. Measure absorbance at 301 nm
3. Calculate coupling yield: Expected = 1 – (A_measured / A_theoretical × loading)
4. Acceptable: >95% per coupling; <90% indicates problem

Washing After Deprotection

1. DMF wash: 3 × 1 min (10 mL/g resin)
2. Verify complete deprotection: no yellow color in final wash

3. Amino Acid Coupling

Pre-Activation Protocol (HATU)

1. Dissolve Fmoc-amino acid (2.0 eq relative to resin loading) in DMF
2. Add HATU (1.9 eq)
3. Add DIPEA (4.0 eq) — solution should become faintly yellow
4. Allow pre-activation for 1–2 min at room temperature
5. Add to drained resin immediately

Coupling Conditions

Parameter	Standard	Difficult	Microwave
Fmoc-AA eq	2.0	3.0	3.0
HATU eq	1.9	2.9	2.9
DIPEA eq	4.0	6.0	6.0
Time	20 min	2 × 30 min	5 min
Temperature	25°C	25°C	70°C
Solvent	DMF	DMF/NMP	DMF

Coupling Completion Test (Ninhydrin/Kaiser Test)

1. Take 3–5 resin beads from reaction
2. Add 100 µL each: 5% ninhydrin in EtOH, 80% phenol in EtOH, 0.1 M KCN in pyridine
3. Heat at 100°C for 5 min
4. Blue/green = free amine (incomplete coupling)
5. Yellow/colorless = coupling complete
6. For secondary amines (Pro, N-methyl): use chloranil test (blue = incomplete)

4. Iterative Coupling-Deprotection Cycles

Complete Cycle (1 residue)

Step	Reagent	Time	Wash
1. Deprotection	20% piperidine/DMF	2 + 8 min	DMF (3×)
2. Coupling	Fmoc-AA/HATU/DIPEA	20 min	DMF (3×)
3. Capping (optional)	Ac₂O/DIPEA	5 min	DMF (3×)
4. Repeat from Step 1	—	—	—

Capping (Optional)

Capping with acetic anhydride (5% v/v) and DIPEA (10% v/v) in DMF for 5 min terminates truncated sequences. This simplifies purification but reduces overall yield by 5–15%.

5. Cleavage and Global Deprotection

Cleavage Cocktail (Reagent K)

Component	Amount	Purpose
TFA	82.5 mL	Solvent/cleavage agent
Thioanisole	5.0 mL	Scavenger (cation scavenger)
EDT	2.5 mL	Scavenger (for Cys protection)
m-Cresol	5.0 mL	Scavenger
H₂O	5.0 mL	Scavenger (for tBu-based groups)

Alternative cocktails:

• TFA/TIS/H₂O (95:2.5:2.5): Standard cleavage for most sequences
• TFA/TIS/EDT/H₂O (92.5:2.5:2.5:2.5): For sequences with Cys, Met, Trp
• Low-TFA (70% TFA): For acid-sensitive sequences

Cleavage Protocol

1. Wash resin with DCM (3 × 1 min) then MeOH (1 × 1 min)
2. Dry resin under vacuum for 5 min
3. Add cleavage cocktail (10–15 mL/g resin)
4. Stir at room temperature for 2–3 hours (standard) or 4–6 hours (long peptides)
5. For very long peptides (>50 residues): extend to 6–8 hours
6. Filter resin; collect filtrate
7. Wash resin with TFA (2 × 2 mL/g resin), combine washes

Precipitation

1. Add cold diethyl ether (10 volumes) to cleavage filtrate
2. Vortex and cool at -20°C for 30 min
3. Centrifuge at 4,000 × g for 10 min
4. Discard supernatant
5. Repeat ether wash 2×
6. Dissolve pellet in 50% MeOH/H₂O or 0.1% TFA/H₂O for purification

6. Quality Control During Synthesis

In-Process Controls

Test	When	Acceptance
Fmoc UV monitoring	Every deprotection	>95% deprotection
Kaiser test	After each coupling	Complete coupling
LC-MS (aliquot)	After 10 residues	Correct mass, no deletion
Full LC-MS	Post-cleavage	Single major product

Common Failures

• Deletion sequences: Incomplete coupling (check Kaiser test, increase coupling time)
• Truncated sequences: Difficult couplings (use microwave, pseudoproline dipeptides)
• Oxidized products: Met/Cys oxidation (add EDT, work under N₂)
• Aspartimide: Asp-Gly sequences (reduce temperature, use dipeptide building blocks)

7. Purification Overview

After cleavage, crude peptide is purified by preparative RP-HPLC:

1. Column: C18, 250 × 21.2 mm, 5 µm, 100 Å
2. Mobile phase A: 0.1% TFA in H₂O
3. Mobile phase B: 0.1% TFA in MeCN
4. Gradient: 10–90% B over 40–60 min
5. Flow rate: 10–20 mL/min
6. Detection: UV at 220 nm

Fraction collection: Collect fractions corresponding to the major peak; analyze by analytical LC-MS. Pool fractions >95% purity. Repeat purification if needed.

8. Lyophilization

1. Combine purified fractions
2. Flash-freeze in liquid nitrogen
3. Lyophilize for 24–48 hours
4. Store dried peptide at -20°C (short-term) or -80°C (long-term)

References

1. Chan, W.C., White, P.D. (Eds.). Fmoc Solid Phase Peptide Synthesis: A Practical Approach. Oxford University Press, 2000.
2. Bodanszky, M., Bodanszky, A. The Practice of Peptide Synthesis. Springer-Verlag, 1994.
3. Fields, G.B., et al. “Synthesis of peptides and proteins.” Methods in Enzymology 289 (1997): 1–87.
4. Cemillán, J.A., et al. “Microwave-assisted peptide synthesis.” Nature Protocols 1 (2006): 1953–1958.

Further Reading

• SPPS Troubleshooting — Common problems and solutions
• Solid-Phase Synthesis Overview — Theory and principles
• Purification Methods — Detailed HPLC protocols
• Mass Spectrometry — Analytical characterization