DCM — Dichloromethane¶
Resin Swelling, Wash, and Cleavage Cocktail Solvent¶
Introduction¶
Dichloromethane (DCM, methylene chloride) is the second most frequently used solvent in solid-phase peptide synthesis, after DMF. Its unique combination of properties — exceptional swelling of polystyrene-divinylbenzene (PS-DVB) resins, low viscosity for efficient washing, volatility for rapid solvent removal, and chemical inertness under SPPS conditions — makes it indispensable for several key operations. DCM excels as a pre-swelling solvent before the first coupling, as an intermediate wash solvent to bridge between DMF and other solvents, and as a carrier or diluent in cleavage cocktails. Its lower polarity compared to DMF also makes it the preferred medium for coupling reactions involving hydrophobic sequences or when aggregation must be reduced.
Chemical and Physical Properties¶
| Property | Value |
|---|---|
| IUPAC name | Dichloromethane |
| CAS number | 75-09-2 |
| Molecular formula | CH₂Cl₂ |
| Molecular weight | 84.93 g/mol |
| Boiling point | 39.6 °C (at 1 atm) |
| Melting point | −96.7 °C |
| Density | 1.326 g/mL (20 °C) |
| Dielectric constant (ε) | 9.08 (25 °C) |
| Dipole moment | 1.60 D |
| Viscosity | 0.413 cP (25 °C) |
| Refractive index | 1.423 (nD²⁰) |
| Flash point | None (non-flammable in closed cup test) |
| Vapour pressure | 436 mmHg (25 °C) |
| Vapour density | 2.93 × air (heavier than air) |
| Solubility in water | 13 g/L (20 °C) |
| Solubility of water in DCM | 1.5 g/L (20 °C) |
| Electrical conductivity | Very low (4.3 × 10⁻¹¹ S/m) |
| Evaporation rate (BuAc=1) | 14.5 (very fast) |
Role in SPPS¶
Resin Swelling¶
DCM produces the highest swelling volume for standard polystyrene-based SPPS resins, which is critical for achieving optimal reaction kinetics in SPPS:
| Resin Type | Swelling in DCM (mL/g) | Swelling in DMF (mL/g) | Ratio (DCM/DMF) |
|---|---|---|---|
| Wang resin (100–200 mesh) | 6–8 | 4–6 | 1.4–1.6× |
| Rink Amide (100–200 mesh) | 7–9 | 5–7 | 1.3–1.5× |
| 2-Cl-Trt (100–200 mesh) | 6–8 | 4–6 | 1.3–1.6× |
| PAM resin (Boc chemistry) | 6–8 | 4–5 | 1.5–1.7× |
| Sieber resin | 6–8 | 4–6 | 1.4–1.5× |
Pre-swelling protocol: Suspend dry resin in DCM (10 mL/g) for 15–30 min before the first DMF coupling. The swollen resin provides maximum solvent-accessible surface area for the initial, most critical coupling step. DCM is then exchanged for DMF through a series of DCM → DMF/DCM 1:1 → DMF washes (3 × 10 seconds each).
Washing¶
DCM serves as an intermediate wash solvent between DMF-based coupling and deprotection cycles:
| Wash Step | Solvent | Volume | Duration | Purpose |
|---|---|---|---|---|
| After coupling | DMF | 5 × 5–10 mL/g | 30 s each | Remove excess reagents |
| Transition | DMF → DMF/DCM → DCM | 3 × 5 mL/g | 15 s each | Gradient polarity change |
| After Fmoc deprotection | DMF | 5 × 5–10 mL/g | 30 s each | Remove piperidine/DMF |
| Final resin wash | DCM then MeOH | 3 × 5 mL/g each | 30 s each | Prepare for drying/storage |
| Before HF cleavage (Boc) | DCM | 3 × 10 mL/g | 30 s each | Remove residual DMF and acetic acid |
Cleavage Cocktail Component¶
DCM is used as a diluent in cleavage cocktails, primarily in two scenarios:
| Application | DCM Concentration | Purpose |
|---|---|---|
| Boc deprotection wash | 50% TFA/DCM | Carrier for acidic deprotection cocktail |
| Protected peptide cleavage (Sieber/2-Cl-Trt) | 95–99% DCM + 1–5% TFA | Mild acidolysis retaining side-chain PGs |
| Pre-HF resin swelling (Boc SPPS) | 100% DCM | Ensure HF access to all peptide-resin bonds |
| HF cleavage cocktail diluent | 10% DCM + 10% p-cresol + 80% HF | Diluent/viscosity modifier for HF |
Aggregation Reduction¶
DCM's lower polarity (ε = 9.08 vs. 36.7 for DMF) helps disrupt peptide chain aggregation during difficult couplings. For sequences with high aggregation propensity (β-sheet-prone, hydrophobic, or long sequences), coupling in DMF/DCM mixtures (1:1 to 1:3, v/v) can improve yields by:
- Reducing inter-chain hydrogen bonding through lower solvent polarity
- Improving resin solvation in hydrophobic domains
- Increasing diffusion rates of activated amino acids (lower viscosity)
Chlorinated Solvent Handling — Special Considerations¶
DCM requires distinct handling protocols compared to non-chlorinated SPPS solvents:
| Property | Handling Implication |
|---|---|
| Heavier than air | Vapour accumulates in low-lying areas (under fume hood counters, pits) — cannot be detected by standard overhead vapour monitors |
| Low boiling point (39.6 °C) | Evaporates rapidly at room temperature; 15 min open exposure evaporates 70% of a 10 cm² surface |
| PVC incompatibility | DCM dissolves PVC tubing, gloves, and container linings — use HDPE, PTFE, or glass |
| Light sensitivity | DCM slowly photolyses to HCl and phosgene under intense UV/sunlight — store in amber or opaque containers |
| Phase behaviour | DCM forms a dense lower layer with water — organic layer is always the bottom phase |
Safety Data¶
Health Hazards¶
| Hazard | Classification | Workplace Limit | Chronic Effect |
|---|---|---|---|
| Carcinogenicity | H350 (suspected of causing cancer — IARC Group 2A) | TWA: 25 ppm (8 h); STEL: 50 ppm (15 min) | Increased risk of brain, lung, and liver cancer |
| Acute toxicity (inhalation) | H332 (harmful if inhaled) | 25 ppm TWA | Drowsiness, dizziness, impaired coordination |
| Skin irritation | H315 (causes skin irritation) | — | Defatting of skin, dermatitis with repeated exposure |
| Eye irritation | H319 (causes serious eye irritation) | — | Reversible; conjunctival redness |
| CNS depression | H336 (may cause drowsiness/dizziness) | Effects detectable from 200 ppm | Narcotic effect at high concentrations |
| Metabolite | — | — | Metabolised to CO (detectable in exhaled breath) |
Metabolic carboxyhaemoglobin: DCM is metabolised in the liver to carbon monoxide, producing measurable increases in blood COHb levels. After exposure to 100 ppm for 2 hours, COHb levels rise to 3–4%. Individuals with cardiovascular conditions are particularly susceptible to this effect.
Chemical Hazards¶
| Scenario | Reaction | Risk |
|---|---|---|
| DCM + strong base (NaOH, KOH) | Hydrolysis to formaldehyde + HCl | Exothermic; generates reactive intermediates |
| DCM + strong oxidisers (HNO₃, perchloric acid) | Potentially explosive oxidation | May produce phosgene (COCl₂) |
| DCM + TFA (under basic conditions) | Explosive diazomethane-like intermediates | Extremely hazardous — see TFA waste disposal note |
| DCM + Al/Li metal | Alkylation of metal surface | Fire risk with finely divided metals |
| DCM + dimethyl sulfoxide (DMSO) with TFA | Pummerer rearrangement | Exothermic; produces toxic gases |
| DCM photolysis | HCl + phosgene + chlorine | Occurs in strong UV light; avoid exposure |
Handling Best Practices¶
- Fume hood mandatory — DCM evaporates too rapidly for any open-bench work
- Nitril or neoprene gloves — butyl rubber offers slightly better breakthrough time (>30 min); natural rubber is rapidly dissolved
- No PVC equipment — DCM dissolves PVC in seconds; use HDPE, PTFE, or glass containers
- Container sealing — Vapour emissions from unsealed containers exceed OSHA PEL within minutes in a standard laboratory
- Carbon monoxide monitoring — Consider CO monitoring for personnel handling DCM daily (>2 h/day) at scale
- Avoid heating — Heating DCM accelerates vapourisation and decomposition; use sealed systems with condensers if heating is required
Waste Disposal¶
- Collect as halogenated solvent waste — never mix with acetone or non-halogenated waste
- Environmental regulation: DCM is an ozone-depletion substance precursor under the Montreal Protocol (Class III ODS in Europe)
- Disposition: Incineration at licensed hazardous waste facility with HCl scrubber
- Ship in clearly labelled, sealed HDPE containers; fill level ≤80% (vapour pressure)
🔗 Related: DMF | Resin Loading | SPPS Process | TFA | Scavenger Selection Guide | Custom Peptide Synthesis — Quality Standards