2-Mercaptoethanol
[CAS 60-24-2]

Identification

• Classification: Pharmaceutical intermediate >> Heterocyclic compound intermediate >> Quinoline compound
• Name: 2-Mercaptoethanol
• Synonyms: Thioglycol; 2-Hydroxy-1-ethanethiol
• Molecular Formula: C₂H₆OS
• Molecular Weight: 78.13
• CAS Registry Number: 60-24-2
• EC Number: 200-464-6
• SMILES: C(CS)O

Properties

• Density: 1.1±0.1 g/cm³ Calc.^*, 1.115 g/mL (Expl.)
• Melting point: -100 °C (Expl.)
• Boiling point: 157.0 °C 760 mmHg (Calc.)^*, 157 °C (Expl.)
• Flash point: 73.9 °C (Calc.)^*, 73 °C (Expl.)
• Index of refraction: 1.479 (Calc.)^*, 1.5 (Expl.)

^* Calculated using Advanced Chemistry Development (ACD/Labs) Software.

Safety Data

• Hazard Symbols: GHS05;GHS06;GHS07;GHS08;GHS09 Danger
• Risk Statements: H301-H302-H310-H311-H314-H315-H317-H318-H331-H332-H361-H373-H400-H410-H411
• Safety Statements: P203-P260-P261-P262-P264-P264+P265-P270-P271-P272-P273-P280-P301+P316-P301+P317-P301+P330+P331-P302+P352-P302+P361+P354-P304+P340-P305+P354+P338-P316-P317-P318-P319-P321-P330-P332+P317-P333+P317-P361+P364-P362+P364-P363-P391-P403+P233-P405-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Chronic hazardous to the aquatic environment	Aquatic Chronic	2	H411
Skin irritation	Skin Irrit.	2	H315
Serious eye damage	Eye Dam.	1	H318
Acute toxicity	Acute Tox.	3	H331
Acute hazardous to the aquatic environment	Aquatic Acute	1	H400
Acute toxicity	Acute Tox.	3	H311
Acute toxicity	Acute Tox.	3	H301
Specific target organ toxicity - repeated exposure	STOT RE	2	H373
Acute toxicity	Acute Tox.	4	H302
Acute toxicity	Acute Tox.	2	H310
Skin sensitization	Skin Sens.	1A	H317
Skin corrosion	Skin Corr.	1B	H314
Acute toxicity	Acute Tox.	4	H332
Chronic hazardous to the aquatic environment	Aquatic Chronic	1	H410
Reproductive toxicity	Repr.	2	H361
Skin sensitization	Skin Sens.	1	H317
Reproductive toxicity	Repr.	2	H361f
Specific target organ toxicity - single exposure	STOT SE	3	H336
Specific target organ toxicity - single exposure	STOT SE	3	H335
Acute toxicity	Acute Tox.	2	H330
Eye irritation	Eye Irrit.	2	H319
Specific target organ toxicity - single exposure	STOT SE	2	H371

• Transport Information: UN 2966

chemBlink Chemical Story

2-Mercaptoethanol, CAS 60-24-2, is a small sulfur-containing organic compound with the formula HOCH₂CH₂SH and a molecular weight of 78.14. It contains both an alcohol group and a thiol group, but it is the sulfur-containing -SH group that gives the compound its best-known laboratory role. Usually called 2-mercaptoethanol or beta-mercaptoethanol, it is one of the classic reducing agents used to break disulfide bonds in proteins.

Proteins are chains of amino acids, but a newly synthesized chain does not normally remain stretched out like a piece of string. It folds into a three-dimensional structure held together by many interactions. Hydrogen bonding, electrostatic attraction, hydrophobic effects, and other noncovalent forces all contribute, but some proteins have an additional structural connection that is much stronger: the disulfide bond.

Disulfide bonds form when two cysteine residues become linked through their sulfur atoms:

Protein-S-S-Protein

The bond may connect two regions of the same polypeptide chain or join separate protein chains. In either case, it acts rather like a molecular cross-link. Such bonds are especially common in proteins that function outside cells, where the oxidizing environment favors their formation. They can make protein structures substantially more resistant to unfolding.

This stability becomes a problem when a biochemist deliberately wants to take a protein apart.

A familiar example is sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The purpose of this technique is usually to separate polypeptides primarily according to their molecular size. Sodium dodecyl sulfate binds to proteins, disrupts many of the noncovalent interactions responsible for folding, and gives the resulting chains a strongly negative character. Heating the sample further assists denaturation.

But sodium dodecyl sulfate does not efficiently solve every structural problem. A covalent disulfide bond can continue holding two parts of a protein together even after many noncovalent interactions have been disrupted. This is where 2-mercaptoethanol enters the sample buffer.

Its thiol participates in thiol-disulfide exchange and reducing chemistry that converts protein disulfide bonds into free sulfhydryl groups. In simplified form:

Protein-S-S-Protein + reducing agent → 2 Protein-SH

As the protein disulfide is reduced, 2-mercaptoethanol itself becomes oxidized, commonly forming a disulfide-linked product. The important result for the protein is that the covalent sulfur bridge has been opened.

This distinction explains why a reducing sample buffer can reveal information that a nonreducing electrophoresis experiment cannot. Suppose a protein consists of two polypeptide chains connected by disulfide bonds. Under nonreducing conditions, the chains may remain linked and migrate together. Add 2-mercaptoethanol, and the disulfide bridges are broken, allowing the individual chains to separate. Comparing reducing and nonreducing gels can therefore provide clues about whether protein subunits are connected through disulfide bonds.

Intramolecular disulfide bonds matter as well. If a disulfide bridge connects two parts of the same polypeptide, breaking it allows the chain to unfold more completely. This helps make electrophoretic migration depend more consistently on polypeptide length rather than on residual three-dimensional structure.

The chemistry is possible because thiols are unusually useful participants in reversible oxidation-reduction reactions. Two thiol groups can be oxidized to a disulfide, while a disulfide can be reduced back to thiols. Biology itself makes extensive use of this sulfur chemistry. Cysteine residues form structural disulfides in proteins, while cellular systems based on compounds such as glutathione and thioredoxin regulate the oxidation state of protein thiols.

2-Mercaptoethanol gives the experimenter a simple chemical way to push this balance toward the reduced state. It is therefore used not only during protein electrophoresis but also in laboratory solutions where maintaining sulfhydryl groups in reduced form is important.

The compound has another characteristic familiar to generations of laboratory workers: its strong and unpleasant odor. Thiols are famous for powerful odors even at relatively low concentrations. 2-Mercaptoethanol also presents significant health hazards and can be toxic through several routes of exposure, so its volatility and toxicity require appropriate containment and handling procedures. Its usefulness as a biochemical reagent should not obscure the need for careful laboratory practice.

Alternative reducing agents are available. Dithiothreitol is widely used and has favorable reducing properties, while newer odorless or less volatile reagents are useful in some applications. Nevertheless, 2-mercaptoethanol remains deeply associated with molecular biology and protein chemistry because it is effective, soluble in water, inexpensive, and historically embedded in many established experimental protocols.

Its importance also illustrates a useful principle of biochemical experimentation. To understand a complicated biological structure, scientists often deliberately destroy selected parts of it. Detergent disrupts one class of interactions, heat assists unfolding, and a reducing agent attacks another type of connection. By removing these structural features in a controlled way, the underlying polypeptide chains become easier to measure and compare.

A protein sample prepared for reducing electrophoresis is therefore being chemically dismantled before it ever enters the gel. Sodium dodecyl sulfate attacks much of the noncovalent architecture, while 2-mercaptoethanol opens sulfur bridges that would otherwise survive.

Few molecules demonstrate this idea more clearly than HOCH₂CH₂SH. It has only two carbon atoms and one sulfur atom, yet that sulfur gives it the ability to undo covalent connections that help hold much larger biological molecules together. In countless laboratories, 2-mercaptoethanol has served as a tiny molecular tool for taking proteins apart so that scientists can understand how they were put together.

References

1. PubChem. 2-Mercaptoethanol, CID 1567, CAS 60-24-2. Molecular formula, molecular weight, properties, uses, and safety information.

2. Nelson, D. L.; Cox, M. M. Lehninger Principles of Biochemistry. Protein structure, cysteine disulfide bonds, and biological redox chemistry.

3. Laemmli, U. K. (1970). "Cleavage of structural proteins during the assembly of the head of bacteriophage T4." Nature, 227, 680-685.

4. Standard biochemical literature and reagent documentation describing 2-mercaptoethanol as a reducing agent for protein disulfide bonds in polyacrylamide gel electrophoresis.