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Short answer
B-mercaptoethanol (2-mercaptoethanol, β-mercaptoethanol, or BME) is a clear, organosulfur liquid used in biochemistry primarily as a reducing agent. It cleaves covalent protein disulfide bonds through intermolecular thiol-disulfide exchange, denaturing protein structures for techniques like SDS-PAGE. Because BME is highly toxic, volatile, and fatal upon skin contact, researchers must handle it inside a certified chemical fume hood.
Definition and scope
B-mercaptoethanol is a bifunctional organic molecule featuring an ethyl alcohol backbone and a terminal sulfhydryl (thiol) group. Its chemical formula is C2H6OS, structural formula HOCH2CH2SH, and molecular weight 78.13 g/mol. In research laboratories, pure BME is supplied neat at approximately 14.3 M with a density of 1.114 g/cm³ at 20 °C and a boiling point of 157 °C to 158 °C.
What is another name for 2-mercaptoethanol?
2-Mercaptoethanol has several common synonyms. Its systematic IUPAC name is 2-sulfanylethan-1-ol. In protocols, it is referenced as β-mercaptoethanol (beta-mercaptoethanol), BME, 2-ME, thioethylene glycol, and 2-hydroxyethyl mercaptan. Chemical suppliers typically label it as 2-mercaptoethanol for electrophoresis or molecular biology.
What is the CAS number for beta-mercaptoethanol?
The Chemical Abstracts Service (CAS) registry number for beta-mercaptoethanol is 60-24-2, and its PubChem Compound ID is CID 1567. Researchers should confirm CAS 60-24-2 on safety data sheets (SDS) to prevent confusion with other volatile mercaptans.
How it works: mechanism of disulfide bond reduction
Proteins maintain folded structures through covalent disulfide bonds (–S–S–) between cysteine residues. B-mercaptoethanol cleaves these bridges via bimolecular nucleophilic substitution (SN2) thiol-disulfide exchange:
- Thiolate activation: At neutral to alkaline pH, BME deprotonates to generate active nucleophilic thiolate anions (HOCH2CH2S⁻).
- First nucleophilic attack: The thiolate anion attacks a protein disulfide bond (P1–S–S–P2), releasing one free cysteine thiolate (P2–S⁻) and forming a mixed disulfide intermediate (P1–S–S–CH2CH2OH).
- Second nucleophilic attack: A second BME molecule attacks the mixed intermediate, releasing the remaining cysteine (P1–SH) and producing an oxidized dimer (HOCH2CH2–S–S–CH2CH2OH).
Because the second step requires an intermolecular collision between two separate BME molecules, the reaction depends on chemical equilibrium. Experimental protocols require a substantial molar excess of BME to drive reduction toward completion and prevent re-oxidation.
What does BME do to proteins?
BME cleaves intra-chain and inter-chain disulfide bridges across protein structures. This reduction linearizes polypeptide chains and disassembles multi-subunit complexes held together by covalent linkages, such as antibody heavy and light chains. When combined with heat and sodium dodecyl sulfate (SDS), BME allows proteins to migrate through polyacrylamide gels strictly according to their primary molecular mass.
Types and related reducing agents
Laboratories select among three primary reducing agents for protein biochemistry: 2-mercaptoethanol (BME), dithiothreitol (DTT), and tris(2-carboxyethyl)phosphine (TCEP). While all three cleave disulfide bonds, their kinetics and operational limits differ.
| Feature | 2-Mercaptoethanol (BME) | Dithiothreitol (DTT) | TCEP-HCl |
|---|---|---|---|
| Chemical class | Monothiol | Vicinal dithiol | Alkyl phosphine |
| Reduction mechanism | Two-molecule attack | Intramolecular cyclization | Nucleophilic attack and hydrolysis |
| Optimal pH range | pH 7.0–9.0 (optimal > 7.5) | pH 7.0–9.0 (inactive below 7.0) | pH 1.5–8.5 (acid-active) |
| Air stability | Moderate in stock; oxidizes in buffer | Poor; oxidizes rapidly in air | High; stable in aqueous solution |
| Odor profile | Strong, penetrating thiol stench | Moderate thiol odor | Odorless |
| Thiol labeling | Incompatible; quenches maleimides | Incompatible; must be removed | Compatible; no free sulfhydryls |
DTT provides higher reducing potency than BME because its intramolecular cyclization forms an energetically favored cyclic disulfide. TCEP offers an odorless alternative that functions across acidic pH values and does not interfere with maleimide labeling.
Where it matters for research laboratories and supply decisions
BME is widely stocked in biological research facilities because of its low cost, liquid stability, and broad inclusion in standard operating procedures.
SDS-PAGE sample preparation
In denaturing electrophoresis, researchers add 2% to 5% v/v (280 mM to 700 mM) BME to 2× or 4× Laemmli sample buffer. This high concentration ensures that disulfide bridges dissociate completely before electrophoresis. For full protocol parameters, review our guide to SDS-PAGE protein gel electrophoresis.
Protein refolding
Recombinant proteins extracted from bacterial inclusion bodies require strong denaturants and reducing agents to unfold completely. Diluting or dialyzing denatured intermediates into redox buffers with controlled BME concentrations allows target proteins to re-establish native disulfide bonds. For practical refolding workflows, consult our guide to protein refolding.
Cell culture supplementation
In immunology workflows, BME is added to cell culture media (such as RPMI 1640) at 50 µM to 55 µM. At these micromolar levels, BME acts as an extracellular antioxidant, promoting cystine uptake and supporting glutathione synthesis in sensitive primary lymphocytes. When formulating media stocks, see our overview of buffer and solution preparation.
Assay compatibility
Because BME reduces metal ions, it interferes with colorimetric assays that rely on copper reduction. Standard bicinchoninic acid assays cannot tolerate BME concentrations above 1 mM (0.01% v/v). For dilution requirements and compatibility thresholds, consult our guide to BCA protein assay standard curve calculation.
Chemical hazards, toxicity, and safe handling
B-mercaptoethanol presents severe toxicological risks that demand strict laboratory controls and personal protective equipment.
Is beta-mercaptoethanol toxic to humans?
Yes. Beta-mercaptoethanol is acutely toxic across all primary exposure routes. According to manufacturer safety data sheets (SDS), BME carries the signal word Danger and the following GHS classifications:
- H310: Fatal in contact with skin (Category 2). BME rapidly penetrates dermal layers into systemic circulation.
- H301 + H331: Toxic if swallowed or inhaled (Category 3). Vapors cause respiratory tract irritation and nausea.
- H318: Causes serious eye damage (Category 1). Splashes cause severe corneal burns.
- H315 + H317: Causes skin irritation and allergic skin reaction (Category 2 / Category 1A).
- H373: May cause organ damage (liver, heart) through prolonged ingestion.
- H410: Very toxic to aquatic life with long-lasting effects.
Exposure limits and ventilation controls
OSHA has not established a Permissible Exposure Limit (PEL) under 29 CFR 1910.1000. However, the American Industrial Hygiene Association (AIHA) specifies a Workplace Environmental Exposure Level (WEEL) of 0.2 ppm (8-hour TWA) with a Skin notation, confirming significant dermal toxicity. Because rapid olfactory fatigue prevents using smell to gauge exposure, all dispensing and heating must occur inside a certified chemical fume hood with ~100 fpm face velocity. Work at least 6 inches inside the hood with sash lowered.
Personal protective equipment and waste disposal
Personnel handling BME should employ double nitrile gloving, ANSI Z87.1 chemical splash goggles, and buttoned lab coats. Replace outer gloves immediately if splashing occurs.
Never dispose of BME down laboratory sinks. Collect BME liquids in designated hazardous waste containers. For benchtop spills, treat dilute aqueous spills under fume hood exhaust with 10% household bleach (sodium hypochlorite). Bleach oxidizes volatile thiols into non-volatile, odorless water-soluble sulfonates, extinguishing the foul odor prior to waste absorption.
Common misunderstandings
- BME and DTT can be exchanged at equal concentrations: BME requires 100 mM to 700 mM working concentrations due to its intermolecular mechanism, whereas DTT functions at 5 mM to 20 mM.
- Absence of smell confirms safety: Thiol compounds cause rapid olfactory adaptation. Fume hood containment must be verified through airflow monitors, not human perception.
- Freezing sample buffers preserves BME indefinitely: Dilute BME in aqueous buffer volatilizes and oxidizes over repeat freeze-thaw cycles. Fresh addition prior to heating yields more reproducible electrophoretic band separation.
- BME is compatible with thiol-reactive labels: Residual BME quenches maleimides and iodoacetamides. BME must be removed by buffer exchange prior to chemical conjugation.
Frequently asked questions
What does BME do to proteins?
BME cleaves covalent disulfide bonds between cysteine residues. This disrupts tertiary and quaternary folding, facilitating complete polypeptide denaturation and subunit dissociation for molecular analyses like SDS-PAGE.
What is another name for 2-mercaptoethanol?
Common synonyms include β-mercaptoethanol, beta-mercaptoethanol, BME, 2-ME, 2-sulfanylethan-1-ol, and thioethylene glycol.
Is beta-mercaptoethanol toxic to humans?
Yes. Beta-mercaptoethanol is classified as fatal in contact with skin (GHS Category 2, H310) and toxic if swallowed or inhaled (Category 3). It causes serious eye damage and skin sensitization. Manipulations must be conducted in a chemical fume hood using double nitrile gloves.
What is the CAS number for beta-mercaptoethanol?
The CAS number for beta-mercaptoethanol is 60-24-2, with PubChem Compound ID CID 1567.
Sources
- National Center for Biotechnology Information (NCBI). PubChem Compound Summary for CID 1567, 2-Mercaptoethanol. https://pubchem.ncbi.nlm.nih.gov/compound/2-Mercaptoethanol. Checked 2026-11-07.
- Sigma-Aldrich / MilliporeSigma. Safety Data Sheet for 2-Mercaptoethanol, BioReagent for Molecular Biology (CAS 60-24-2). https://www.sigmaaldrich.com/. Checked 2026-11-07.
- Occupational Safety and Health Administration (OSHA). Chemical Database: 2-Mercaptoethanol. AIHA WEEL 8-hour TWA 0.2 ppm (Skin). https://www.osha.gov/chemicaldata/. Checked 2026-11-07.
- University of California, Davis Safety Services. Chemical Safety and Thiol Management: 2-Mercaptoethanol Handling and Spill Response. https://safetyservices.ucdavis.edu/. Checked 2026-11-07.
