The ABTS free radical scavenging assay is a widely employed spectrophotometric method used to evaluate the antioxidant capacity of both hydrophilic and lipophilic substances. By measuring the ability of samples to scavenge the ABTS radical cation, researchers can quantify total antioxidant activity in food, pharmaceutical, and biological research. This technical article provides a comprehensive overview of the experimental principles, procedural steps, and applications of the ABTS assay.
Technical Overview
The ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)) free radical scavenging assay is a colorimetric method based on the decolorization of the ABTS radical cation. In its oxidized form, the ABTS radical cation exhibits a characteristic blue-green color with strong absorption in the visible spectrum, typically measured at 734 nm. When an antioxidant substance is introduced, it donates an electron or hydrogen atom to the radical, reducing it back to its colorless, non-radical form.
The degree of color suppression is directly proportional to the antioxidant capacity of the sample. The results are typically expressed as a Trolox Equivalent Antioxidant Capacity (TEAC) value, where the antioxidant activity of the sample is compared to a standard curve generated using Trolox, a water-soluble vitamin E analog. This assay is highly valued for its versatility, as it can assess antioxidants in both aqueous and organic solvent systems.
Testing Samples
The ABTS assay is applicable to a diverse range of matrices. Due to the solubility of the ABTS radical in both water and organic solvents, it is suitable for testing a wider variety of samples compared to other assays like DPPH.
Food and Beverages
This category includes fruit juices, tea, coffee, wine, and vegetable extracts. It is also commonly applied to analyze the antioxidant potential of functional foods and dietary supplements.
Pharmaceuticals and Cosmetics
The assay is used to screen herbal medicines, plant extracts for drug development, and cosmetic formulations designed to provide anti-aging or protective effects.
Biological Fluids
In clinical research, the assay is adapted to measure the total antioxidant status in plasma, serum, saliva, and other bodily fluids to assess oxidative stress levels in patients.
Testing Items
The primary output of the ABTS assay is the quantification of radical scavenging activity. The following parameters are typically reported:
- TEAC Value: The Trolox Equivalent Antioxidant Capacity, usually expressed in micromoles of Trolox equivalents per gram or milliliter of sample.
- Percentage Inhibition: The specific percentage of the ABTS radical scavenged by the sample at a given concentration.
- IC50 Value: The concentration of the sample required to scavenge 50% of the initial ABTS radicals, used to compare the potency of different antioxidants.
Testing Methods
The experimental procedure is straightforward and robust, widely adopted in academic and industrial laboratories.
Reagent Preparation
The ABTS radical cation is generated by reacting ABTS stock solution with a strong oxidizing agent, typically potassium persulfate. This reaction mixture is usually allowed to stand in the dark at room temperature for 12 to 16 hours to ensure complete oxidation and radical formation. Immediately before use, the concentrated radical solution is diluted with a buffer (such as PBS) or ethanol to achieve an absorbance of roughly 0.70 at 734 nm.
Measurement Procedure
A specific volume of the diluted ABTS radical solution is mixed with the test sample. The mixture is incubated in the dark for a defined period, typically between 4 to 30 minutes, depending on the specific protocol used. Following incubation, the absorbance is measured at 734 nm using a spectrophotometer or a microplate reader. A blank control is run concurrently using the solvent instead of the sample to establish the initial absorbance.
Data Analysis
The scavenging percentage is calculated using the difference in absorbance between the blank and the sample. A standard curve is constructed using known concentrations of Trolox. The antioxidant capacity of the sample is then interpolated from this curve.
Applications
The ABTS assay serves critical functions across multiple scientific disciplines.
- Food Quality Control: Evaluating the shelf-life and stability of food products by monitoring antioxidant degradation over time.
- Phytochemical Screening: Identifying and isolating bioactive compounds from plant sources with high antioxidant potential for nutraceutical development.
- Clinical Diagnostics: Investigating the relationship between oxidative stress and pathological conditions such as cardiovascular diseases, diabetes, and cancer by monitoring total antioxidant capacity in patients.
FAQ
Below are answers to frequently asked questions regarding the ABTS assay.
What are the advantages of ABTS over the DPPH assay?
While both are radical scavenging assays, ABTS is more versatile because the ABTS radical cation is soluble in both aqueous and organic solvents. This allows for the testing of hydrophilic antioxidants, which the DPPH assay cannot measure effectively due to the insolubility of the DPPH radical in water.
Why is potassium persulfate used?
Potassium persulfate acts as an oxidizing agent to convert the ABTS molecule into the ABTS radical cation. This process ensures that the reagent is in its active, colored form capable of reacting with antioxidants.
How should the ABTS working solution be stored?
The stock solution of the ABTS radical cation is relatively stable if stored in the dark at room temperature. However, it is recommended to prepare it fresh or check the absorbance before each use to ensure it meets the required optical density parameters.