DTNB Solution (pH 8.0, 10 mM): Thiol Detection Protocol

DTNB Solution (pH 8.0, 10 mM) is a ready-to-use formulation of Ellman's reagent designed for the colorimetric quantification of free thiol (sulfhydryl) groups in proteins, peptides, and small molecules by measuring absorbance at 412 nm.

Reagent

5,5'-Dithiobis(2-nitrobenzoic acid) (DTNB)

Concentration

10 mM

pH

8.0

Detection Wavelength

412 nm

Reaction Product

TNB2− (2-nitro-5-thiobenzoate), yellow chromophore

SKU

D1528838

Principle of the Assay

DTNB (Ellman's reagent) reacts stoichiometrically with free thiol groups (–SH) under mildly alkaline conditions (pH 7.5–8.5). Each mole of thiol reduces one mole of the disulfide bond in DTNB, releasing one mole of the chromophoric mixed disulfide and one mole of TNB²⁻ (2-nitro-5-thiobenzoate). The TNB²⁻ anion is intensely yellow and absorbs maximally at 412 nm, enabling spectrophotometric quantification.

The molar extinction coefficient (ε) of TNB²⁻ is approximately 14,150 M⁻¹cm⁻¹ at 412 nm in aqueous buffer. Beer–Lambert law is applied to calculate thiol concentration from measured absorbance values.

Materials and Equipment

• DTNB Solution (pH 8.0, 10 mM) — SKU D1528838
• Reaction buffer: 0.1 M sodium phosphate or Tris-HCl, pH 8.0
• Thiol-containing sample (protein, peptide, or small molecule)
• L-Cysteine or reduced glutathione (GSH) for standard curve preparation
• UV-Vis spectrophotometer capable of reading at 412 nm
• Cuvettes (path length 1 cm) or 96-well microplate reader
• Micropipettes and tips (appropriate volume range)
• Timer

Step-by-Step Protocol

1. Prepare Standard Curve

Prepare a series of L-cysteine or GSH standards (e.g., 0, 0.05, 0.1, 0.25, 0.5, 1.0 mM) in reaction buffer (pH 8.0). This range may be adjusted based on anticipated sample thiol concentration.

2. Set Up Reactions

For each standard and sample, combine the following in a cuvette or microplate well:

• Reaction buffer: to final volume of 1.0 mL (cuvette) or 200 µL (microplate)
• Sample or standard: 10–100 µL
• DTNB Solution (10 mM): add to a final concentration of 0.5–1.0 mM (e.g., 50–100 µL of 10 mM stock per 1 mL reaction)

Include a reagent blank containing buffer and DTNB but no sample.

3. Incubate

Mix gently and incubate at room temperature (25 °C) for 15–30 minutes, protected from direct light, until colour development is complete.

4. Measure Absorbance

Record absorbance at 412 nm against the reagent blank. For microplate format, read in endpoint mode at 412 nm.

5. Calculate Thiol Concentration

Plot A₄₁₂ versus thiol concentration for standards to generate a linear calibration curve. Determine sample thiol concentration by interpolation. Alternatively, apply Beer–Lambert law directly: C (M) = A₄₁₂ / (ε × l), where ε = 14,150 M⁻¹cm⁻¹ and l = path length in cm.

Troubleshooting and Tips

• Low or no colour development: Confirm sample pH is 7.5–8.5; DTNB reacts poorly below pH 7.0. Ensure sample is not oxidised prior to assay.
• High background absorbance: Check that the reagent blank is properly prepared. Aromatic compounds or reducing agents other than thiols may contribute to background.
• Non-linear standard curve: Dilute high-concentration samples to fall within the linear range of the assay.
• Protein interference: For samples with high protein content, consider deproteinisation (e.g., TCA precipitation) or dilution before assay.
• Storage of DTNB Solution: Store at 2–8 °C, protected from light. Avoid repeated freeze-thaw cycles. Discard if solution appears turbid or discoloured.

Safety and Handling

DTNB is an irritant. Wear appropriate personal protective equipment (PPE) including laboratory coat, nitrile gloves, and eye protection when handling the solution. Avoid inhalation and contact with skin or eyes. Dispose of waste in accordance with local institutional and regulatory guidelines. Refer to the Safety Data Sheet (SDS) supplied with SKU D1528838 for complete hazard information.

Frequently asked questions

What is the working concentration of DTNB in the assay?

The DTNB Solution is supplied at 10 mM (pH 8.0). In a typical assay, it is diluted to a final working concentration of 0.5–1.0 mM in the reaction mixture to ensure sufficient reagent excess over the thiol groups being measured.

Can DTNB Solution be used to detect thiol groups in intact proteins?

Yes, DTNB reacts with accessible (surface-exposed) free cysteine residues in native proteins under mild alkaline conditions. Buried or disulfide-bonded cysteines are generally not detected unless the protein is first denatured and reduced (e.g., with urea plus DTT or TCEP), followed by removal of the reducing agent before the DTNB assay.

Why must the assay be performed at pH 8.0?

The reactivity of thiol groups with DTNB is strongly pH-dependent. At pH 8.0, the thiolate anion (–S−) is the predominant species and is far more nucleophilic than the protonated thiol (–SH), resulting in rapid and quantitative reaction with DTNB. Below pH 7.0, the reaction rate drops significantly, leading to underestimation of thiol content.

Are there any compounds that interfere with the DTNB assay?

Common reducing agents such as DTT, β-mercaptoethanol, and TCEP will react with DTNB and must be removed from samples prior to assay (e.g., by desalting column or dialysis). High concentrations of imidazole, EDTA, or strongly absorbing chromophores at 412 nm can also interfere. Ensure blanks are prepared in matched buffer to control for these effects.

References

1. Ellman GL. Tissue sulfhydryl groups. Archives of Biochemistry and Biophysics. 1959;82(1):70-77. doi:10.1016/0003-9861(59)90090-6
2. Riddles PW, Blakeley RL, Zerner B. Ellman's reagent: 5,5'-dithiobis(2-nitrobenzoic acid)—a reexamination. Analytical Biochemistry. 1979;94(1):75-81. doi:10.1016/0003-2697(79)90792-9
3. Winther JR, Thorpe C. Quantification of thiols and disulfides. Biochimica et Biophysica Acta. 2014;1840(2):838-846. doi:10.1016/j.bbagen.2013.08.022

Products are supplied for research and development use only. Not for use in humans, animals, diagnosis, or therapy.