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# Bradford Assay Protocol for Protein Quantification

> Published on Vicena | [View on web](https://vicena.ai/wiki/bradford-assay-protocol-for-protein-quantification-eoc2Zw)

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## User

What is the detailed Bradford assay protocol for protein quantification, including BSA standard curve preparation, reagent volumes, incubation time, wavelength, and how to handle samples with detergent?

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## Vicena

The Bradford assay is a rapid colorimetric method for protein quantification based on the binding of Coomassie Brilliant Blue G-250 dye to proteins. When the dye binds to protein in an acidic medium, its absorption maximum shifts from 465 nm (red/brown) to 595 nm (blue).

### 1. Reagents and Materials
*   **Bradford Reagent:** Typically contains Coomassie G-250, ethanol/methanol, and phosphoric acid. It must be equilibrated to room temperature and gently inverted before use.
*   **Protein Standard:** Bovine Serum Albumin (BSA) at a stock concentration of 2.0 mg/mL.
*   **Diluent:** Use the same buffer as your sample (e.g., PBS, Tris, or water) to prepare the standards.

### 2. BSA Standard Curve Preparation
Prepare a series of dilutions from the 2.0 mg/mL BSA stock. The following table is a standard scheme for a working range of 125–1500 µg/mL:

| Vial | Vol. of Diluent (µL) | Vol. of BSA Stock (µL) | Final BSA Conc. (µg/mL) |
| :--- | :--- | :--- | :--- |
| A | 0 | 300 (Stock) | 2000 (Optional high point) |
| B | 125 | 375 (Stock) | 1500 |
| C | 325 | 325 (Stock) | 1000 |
| D | 175 | 175 (Vial B) | 750 |
| E | 325 | 325 (Vial C) | 500 |
| F | 325 | 325 (Vial E) | 250 |
| G | 325 | 325 (Vial F) | 125 |
| Blank | 400 | 0 | 0 |

### 3. Assay Protocols and Reagent Volumes
The volumes depend on whether you are using a spectrophotometer (cuvette) or a microplate reader.

| Protocol Type | Sample Volume | Bradford Reagent | Mixing Method |
| :--- | :--- | :--- | :--- |
| **Standard Cuvette** | 30–50 µL | 1.5 mL | Vortex or invert |
| **Micro Cuvette** | 1.0 mL | 1.0 mL | Vortex or invert |
| **Standard Microplate** | 5–10 µL | 250–300 µL | Plate shaker (30 sec) |
| **Micro Microplate** | 150 µL | 150 µL | Plate shaker (30 sec) |

### 4. Incubation and Measurement
*   **Incubation Time:** 10 minutes at room temperature. The color is typically stable for up to 1 hour, but consistency is key for accuracy.
*   **Wavelength:** Measure absorbance at **595 nm**. (Acceptable range: 575–615 nm, though sensitivity decreases away from 595 nm).
*   **Blanking:** Zero the instrument using the "Blank" (0 µg/mL) sample or water.

### 5. Handling Samples with Detergents
The standard Bradford assay is highly sensitive to interference from detergents (e.g., SDS, Triton X-100, Tween-20), which can cause the reagent to precipitate or change color prematurely.

*   **Compatibility Limits:** Most standard Bradford reagents only tolerate up to **0.1%** of common detergents.
*   **Dilution:** If your protein concentration is high enough, dilute the sample until the detergent concentration is below the interference threshold.
*   **Detergent-Compatible Kits:** Use specialized versions like the [Pierce Detergent Compatible Bradford Assay](https://documents.thermofisher.com/TFS-Assets/LSG/manuals/23246_23246S_deter_compat_bradford_UG.pdf), which can handle up to 1% SDS or 5% Triton X-100.
*   **Alternative Assays:** If detergent levels remain high, consider the **BCA Assay** or **Lowry Assay**, which are more robust against detergents but sensitive to reducing agents.

### 6. Data Analysis
1.  Subtract the average 595 nm reading of the Blank from all other readings.
2.  Plot the blank-corrected absorbance ($y$-axis) vs. concentration ($x$-axis).
3.  Use a **four-parameter logistic (4PL)** or **quadratic (best-fit)** curve rather than a simple linear regression, as the Bradford assay is inherently non-linear over wide ranges.
4.  Calculate the concentration of unknown samples based on the curve and multiply by any dilution factors used.

**References:**
*   [Pierce Bradford Protein Assay Kit User Guide](https://documents.thermofisher.com/TFS-Assets/LSG/manuals/MAN0011181_Coomassie_Bradford_Protein_Asy_UG.pdf)
*   [Bio-Rad Quick Start Bradford Protein Assay Manual](https://www.bio-rad.com/webroot/web/pdf/lsr/literature/4110065A.pdf)
*   [Coomassie Plus (Bradford) Assay Reagent Protocol](https://assets.fishersci.com/TFS-Assets/LSG/manuals/MAN0011344_CoomassiePlus_Bradford_Asy_Reag_UG.pdf)

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