Bradford Assay Discussion Examples | How to Determine Protein Concentration and Key Precautions

What Is the Bradford Assay?

The Bradford assay is a method for determining protein concentration by using changes in absorbance caused by the binding of proteins to Coomassie Brilliant Blue G-250 dye.
Because it can be measured in a relatively short time and is easy to perform, it is widely used in student experiments and laboratories.
In many cases, absorbance near 595 nm is measured, and the concentration of an unknown sample is determined using a calibration curve prepared from a standard protein.

In the Bradford assay, when the dye binds to proteins, the blue color becomes stronger and the absorbance increases.
However, because the ease with which the dye binds varies depending on the type of protein and its amino acid composition, differences in properties between the standard protein and the unknown sample may become a source of error.

Color-Development Principle of the Bradford Assay

Coomassie Brilliant Blue G-250 changes its absorption spectrum when it binds to proteins.
In particular, interactions with basic amino acid residues and hydrophobic regions of proteins stabilize the blue form, increasing absorbance near 595 nm.
Therefore, the increase in absorbance can be used as an indicator of the amount of protein.

However, not all proteins bind to the dye with the same strength.
The intensity of color development may vary depending on the amount of amino acid residues such as arginine, the three-dimensional structure of the protein, and its denaturation state.
This is an important precaution in the Bradford assay.

Main Items to Include in the Results

In the results of the Bradford assay, organize the standard protein concentrations, absorbance values, calibration-curve equation, R², absorbance of the unknown sample, dilution factor, and calculated protein concentration.
In a report, it should be clear that the measured absorbance itself is not the final concentration and that the concentration was determined using the calibration curve and dilution factor.

Main Items to Include in the Results

• Standard protein used
• Concentration series of the standard protein
• Absorbance of each standard solution
• Blank absorbance
• Absorbance after blank correction
• Calibration-curve equation
• Coefficient of determination R²
• Absorbance of the unknown sample
• Dilution factor
• Protein concentration of the diluted sample
• Protein concentration of the original sample
• Mean of repeated measurements
• Variation and standard deviation
• Sources of error and points for improvement

Reference Experimental Values and Examples of Protein Concentration Calculations Using the Bradford Assay

Here, reference experimental values are organized for discussing protein concentration measurement by the Bradford assay, including a calibration curve for standard solutions, absorbance of unknown samples, dilution-factor correction, handling of values outside the measurement range, and the effects of interfering substances.

In the Bradford assay, absorption changes when Coomassie Brilliant Blue dye binds to proteins, and absorbance near 595 nm increases.
A calibration curve is prepared using a standard protein, and the protein concentration is determined by applying the absorbance of an unknown sample to the calibration curve.

Reference Experimental Conditions

Example Measurements of Standard Solutions

The following is a reference example in which the relationship between protein concentration and A595 was measured using BSA standard solutions.

A595 increases as the protein concentration increases.
In the Bradford assay, a calibration curve is prepared using the relationship between the absorbance of the standard solutions and protein concentration within the measurement range.

Calibration-Curve Data After Blank Correction

If the blank A595 is 0.050, the blank is subtracted from the mean absorbance of each standard solution for correction.

Blank correction subtracts the color of the reagent itself and the effects of the cuvette, making it easier to handle absorbance changes derived from the protein.

Example Calibration-Curve Equation

Suppose the following calibration curve is obtained from the blank-corrected standard-solution data.

A595 = 0.00076 × Protein concentration (μg/mL) + 0.002

Rearranging this equation, the protein concentration of an unknown sample can be determined as follows.

Protein concentration (μg/mL) = (A595 − 0.002) ÷ 0.00076

Within the range where the calibration curve has good linearity, the unknown sample concentration is determined by substituting its absorbance into this equation.

Example Calculation of an Unknown Sample Concentration

Consider a case in which Unknown Sample X was diluted 10-fold and the blank-corrected A595 was 0.382.

Diluted concentration = (0.382 − 0.002) ÷ 0.00076 = 500 μg/mL

Because this is the concentration after 10-fold dilution, the concentration of the original sample is as follows.

Original concentration = 500 × 10 = 5000 μg/mL = 5.0 mg/mL

Therefore, the protein concentration of Unknown Sample X is determined to be 5.0 mg/mL.

Example Measurements of Unknown Samples

Unknown Sample Z has a corrected A595 of 0.660 and is on the high-concentration side of the calibration-curve range.
For more accurate measurement, it is better to dilute it further so that its absorbance falls near the middle of the calibration curve.

Effect of an Incorrect Dilution Factor

In the Bradford assay, unknown samples are often diluted before measurement, so the dilution factor must be reflected in the concentration calculation.

Errors in recording the dilution factor directly affect the final concentration.
In a report, it is necessary to clearly state whether the measured concentration is for the “diluted sample” or the “original sample.”

Examples of Measurements Outside the Calibration-Curve Range

If the absorbance of an unknown sample lies outside the calibration-curve range, determining the concentration directly may result in a large error.

Extrapolating values outside the calibration-curve range may cause the protein concentration to be estimated incorrectly by a large amount.

Changes in Absorbance With Reaction Time

The Bradford reaction develops color relatively quickly, but differences in measurement timing may cause differences in absorbance.

If the reaction times differ between the standard solutions and unknown samples, their correspondence with the calibration curve shifts.
It is important to keep the time from reagent addition to measurement consistent for all samples.

Example of Variation in Repeated Measurements

In the Bradford assay, absorbance may vary because of pipetting operations, insufficient mixing, bubbles, or contamination of cuvettes or plates.

If only one value deviates greatly, bubbles, insufficient mixing, pipetting errors, or contamination of the cuvette should be suspected.
If an outlier is excluded, it is desirable to clearly state the reason and perform a remeasurement.

Effects of Interfering Substances

The Bradford assay is relatively simple, but it may be affected by surfactants, high concentrations of salts, and strong buffer components.

When interfering substances are present, possible responses include preparing the standard solutions under the same buffer conditions, diluting the sample, or comparing the results with another protein-quantification method.

Effect of Differences in Standard Protein

In the Bradford assay, the ease of dye binding differs depending on the type of protein.
Therefore, the estimated concentration may change depending on the standard protein used.

The concentration obtained by the Bradford assay is a value converted relative to the standard protein used.
Expressing it as a BSA-equivalent concentration can help avoid misunderstanding.

Example Comparison With the Lowry Method and BCA Assay

Concentrations may differ slightly depending on the method.
This is because each measurement method makes use of different properties of proteins.

Examples of Measurement-Result Evaluation

Example Correction Using a Sample Blank

If the sample itself has color or turbidity, a sample blank without Bradford reagent may be measured and used for correction.

In colored samples, failure to subtract the sample blank may cause the protein concentration to be overestimated.

Example of How to Write the Results

A calibration curve for the Bradford assay was prepared using BSA standard solutions.
The blank A595 was 0.050, and this value was subtracted from the absorbance of each standard solution for correction.
As a result, A595 increased with increasing protein concentration in the range of 0–1000 μg/mL.
The calibration curve was expressed as A595 = 0.00076 × Concentration + 0.002.

When Unknown Sample X was diluted 10-fold and measured, the measured A595 was 0.432.
The blank-corrected A595 was 0.382, and substitution into the calibration curve gave a diluted concentration of 500 μg/mL.
Therefore, the original sample concentration was 500 × 10 = 5000 μg/mL, or 5.0 mg/mL.

For Unknown Sample Z, the corrected A595 was 0.660, placing it on the high-concentration side of the calibration curve.
In such a case, although the value is within the measurement range, it may begin to deviate from linearity.
To obtain a more accurate value, the unknown sample should be diluted further and remeasured so that the absorbance falls near the middle of the calibration curve.

Points for Connecting the Results to the Discussion

In a discussion of the Bradford assay, it is important not only to calculate the concentration from the calibration curve but also to consider the measurement range, dilution factor, interfering substances, and differences among standard proteins.

• Has a calibration curve been prepared using a standard protein?
• Has blank correction been performed to subtract absorbance originating from the reagent?
• Can the absorbance of the unknown sample be substituted into the calibration curve to determine the diluted concentration?
• Has the original sample concentration been restored by multiplying by the dilution factor?
• Have values outside the calibration-curve range been avoided rather than forcibly extrapolated?
• Has the reaction time been kept consistent between standard solutions and unknown samples?
• Can variation caused by bubbles, insufficient mixing, and pipetting errors be discussed?
• Can the possibility that surfactants such as SDS and high salt concentrations affect the measurement be explained?
• Is it understood that the concentration is a BSA-equivalent concentration, and can differences in color development among protein types be discussed?
• Can it be explained that a sample blank may be necessary for colored samples?

Example Discussion

In this experiment, the protein concentration in an unknown sample was measured using the Bradford assay.
When a calibration curve was prepared using BSA standard solutions, A595 increased as the protein concentration increased.
This occurred because Coomassie Brilliant Blue dye in the Bradford reagent bound to proteins, increasing absorbance near 595 nm.
The calibration-curve equation was A595 = 0.00076 × Concentration + 0.002.

For Unknown Sample X, the blank-corrected A595 after 10-fold dilution was 0.382.
Substituting this value into the calibration curve gave a protein concentration of 500 μg/mL after dilution.
Because the original sample had been diluted 10-fold, the final protein concentration was determined to be 5.0 mg/mL.
In this way, in the Bradford assay, the value obtained from the calibration curve is the concentration after dilution, so the dilution factor must always be corrected for.

On the other hand, for samples whose absorbance is close to the high-concentration side of the calibration curve, the concentration may be overestimated or underestimated.
This is because in the Bradford assay, the relationship between absorbance and concentration may deviate from linearity above a certain range.
When the absorbance is high, as in Unknown Sample Z, a more reliable concentration can be obtained by further diluting the sample and measuring it near the middle of the calibration curve.

Possible sources of error include errors in preparation of standard solutions, variation in pipetting, insufficient mixing with the reagent, bubbles, and differences in reaction time.
In particular, if bubbles are present in a microplate or cuvette, absorbance may become abnormally high.
In addition, if the reaction times after adding Bradford reagent differ between standard solutions and unknown samples, correspondence with the calibration curve shifts.
Therefore, it is important to standardize measurement procedures and reaction times.

Furthermore, the Bradford assay may be affected by surfactants such as SDS, high salt concentrations, and the color or turbidity of the sample itself.
In colored samples, a sample blank may be necessary to subtract the absorbance of the sample itself.
In addition, the concentration obtained by the Bradford assay is a value converted relative to the standard protein used, so in this case it must be interpreted as a BSA-equivalent concentration.

Summary

In the Bradford assay, a calibration curve is prepared using a standard protein, and the protein concentration is determined from the A595 of an unknown sample.
Blank correction, correction for the dilution factor, and measurement within the calibration-curve range are important.

This reference example covered BSA standard solutions, calibration curves, concentration calculations for unknown samples, dilution factors, handling of values outside the measurement range, reaction time, variation in repeated measurements, interfering substances, differences among standard proteins, and sample-blank correction.
In a report, it is useful to clearly show the concentration-calculation process and discuss it in relation to the measurement conditions and sources of error.

Discussion of a BSA Calibration Curve

In the Bradford assay, BSA is often used as the standard protein.
Several BSA standard solutions of known concentration are prepared, and the absorbance of each is measured to create a calibration curve.
Within the range in which the calibration curve is close to linear, protein concentration can be determined from absorbance.

However, the binding affinity of Coomassie dye may differ between BSA and proteins in an unknown sample.
Therefore, it is easier to understand the concentration determined from a BSA calibration curve as a value expressing the proteins in the unknown sample as an equivalent amount of BSA.

Linear Range of the Calibration Curve

In the Bradford assay, the range in which absorbance and protein concentration are proportional is limited.
On the low-concentration side, absorbance is small and is easily affected by the blank and measurement noise.
On the high-concentration side, dye binding or color development may become saturated, making absorbance less proportional to concentration.

Therefore, the absorbance of the unknown sample must fall within the linear range of the calibration curve.
If it falls outside the range, the basic response is to change the dilution factor and remeasure.
Determining concentration by extrapolation results in larger errors.

How to Determine the Concentration of an Unknown Sample

The protein concentration of an unknown sample is determined by substituting the absorbance of the unknown sample into the calibration-curve equation.
If the calibration-curve equation is y = ax + b, the absorbance of the unknown sample is substituted for y, and the protein concentration is calculated as x.
The value obtained at this stage is usually the concentration of the diluted sample used for measurement.

To determine the concentration of the original sample, multiply by the dilution factor.
For example, if a 10-fold diluted sample was measured, the value obtained from the calibration curve is multiplied by 10 to obtain the original sample concentration.
Forgetting to apply the dilution factor is a common calculation error in Bradford-assay reports.

Original sample concentration = Concentration determined from calibration curve × Dilution factor

Precautions for Blank Correction

In the Bradford assay, a blank is measured to subtract absorbance originating from the reagent itself and the solvent.
A sample containing no protein but with Bradford reagent added is often used as the blank.
Absorbance after blank correction is used for preparing the calibration curve and calculating unknown-sample concentrations.

If blank correction is insufficient, absorbance not originating from proteins is also included, and the concentration may be overestimated.
Particularly for low-concentration samples, the relative effect of the blank becomes larger.

Discussion When Absorbance Is Too High

If the absorbance of an unknown sample is too high, the protein concentration may exceed the linear range of the calibration curve.
Determining the concentration under this condition requires extrapolation and lowers reliability.
At high concentrations, there is also a possibility that the dye cannot bind sufficiently or that the linearity of absorbance measurement decreases.

If the absorbance is too high, the unknown sample is diluted further and remeasured so that it falls within the calibration-curve range.
In a report, rather than ending with “the concentration was high because the absorbance was too high,” it is useful to explain the relationship with the calibration-curve range.

Discussion When Absorbance Is Too Low

If the absorbance of an unknown sample is too low, possible causes include a low protein concentration, excessive dilution of the sample, insufficient color-development time, deterioration of the reagent, or precipitation of proteins.
At low absorbance values, the relative effects of the blank and measurement noise become larger, reducing the reliability of the concentration calculation.

In this case, possible improvements include reducing the dilution factor, increasing the sample amount, or checking the measurement conditions.

Effect of Surfactants

In the Bradford assay, surfactants may act as interfering substances.
If a sample contains a surfactant such as SDS, it may affect the interaction between Coomassie dye and proteins or the state of the dye, causing absorbance to fail to accurately reflect the actual amount of protein.

Because protein extracts may contain surfactants, it is important to check the composition of the sample buffer when performing the Bradford assay.
Preparing the standard protein under the same buffer conditions as the unknown sample makes it easier to standardize the effect of interference.

Effects of Reducing Agents, Salts, and Buffer Components

The Bradford assay may be less affected by reducing agents than the BCA or Lowry methods, but this does not mean that components in the sample have no effect at all.
High concentrations of salts, strong surfactants, extreme pH, and organic solvents may affect the state of the dye and proteins.

If the sample buffer and standard-solution conditions differ greatly, absorbance may differ even at the same protein concentration.
Therefore, it is important to prepare the standard solutions in the same buffer as the unknown sample or to select a quantification method with less interference.

Differences in Color Development Depending on Protein Type

In the Bradford assay, the intensity of color development may differ depending on the type of protein.
Coomassie dye interacts mainly with basic amino acid residues and hydrophobic regions, so differences in amino acid composition and protein structure may cause different absorbance values even at the same mass concentration.

Therefore, when BSA is used as the standard protein, the concentration of the unknown sample should strictly be interpreted as a “BSA-equivalent” concentration.
If the target protein is clearly known, preparing a calibration curve using that protein as the standard may provide a more accurate result.

Precautions Regarding Reaction Time

The Bradford assay develops color relatively quickly, but it is important to keep the reaction time consistent among samples.
If the time between adding Bradford reagent and measuring absorbance differs between the standard solutions and unknown samples, the degree of color development changes and absorbance may differ.

In addition, if the time before measurement is too long, changes in color stability, precipitation, or background may become a problem.
It is important to perform the measurement within the reaction time specified in the laboratory manual.

Errors Caused by Pipetting

In the Bradford assay, standard solutions, unknown samples, and Bradford reagent must be dispensed accurately.
Errors in micropipette operation cause deviations in protein concentration or reagent amount and affect absorbance.
Particularly during preparation of a calibration curve, errors in the dilution series of standard solutions shift the entire regression line.

Bubbles in the tip, liquid remaining on the tip, differences in plunger position, and insufficient mixing can all cause variation.
Because small volumes are handled, even small volume errors can have a large effect.

Errors Caused by Insufficient Mixing

If the sample and reagent are not sufficiently mixed after adding Bradford reagent, color development does not proceed uniformly.
When measuring with a microplate, insufficient mixing in the well or bubbles may affect absorbance.
When a cuvette is used, insufficient mixing may cause the color intensity in the measured portion not to represent the sample as a whole.

Contamination or Bubbles in Cuvettes and Plates

In absorbance measurements, contamination, scratches, fingerprints, and bubbles on cuvettes or microplates affect measured values.
If contamination or bubbles are present in the light path, light is scattered or blocked and absorbance may appear higher than the actual value.
In microplates, contamination or bubbles at the bottom of wells also require attention.

Before measurement, it is important to check for bubbles and wipe the outside of the cuvette clean.
If a measurement is performed with bubbles present, the absorbance may vary greatly even for the same sample.

Causes of Overestimating Protein Concentration

Causes of overestimating protein concentration in the Bradford assay include insufficient blank correction, contamination of cuvettes or plates, bubbles, turbidity of the sample, interference from surfactants, and errors in calculating the dilution factor.
If absorbance becomes high because of factors other than proteins, the concentration determined from the calibration curve also becomes high.

Causes of Underestimating Protein Concentration

Causes of underestimating protein concentration include excessive dilution of the sample, insufficient color-development time, deterioration of Bradford reagent, protein precipitation, contamination with substances that interfere with dye binding, and differences in color response between the unknown sample and standard protein.
If proteins have precipitated, the amount of soluble protein in the measurement solution decreases and absorbance becomes lower.

When Calibration-Curve Points Vary

If the measurement points on the calibration curve vary, possible causes include errors in dilution of the standard solutions, pipetting errors, insufficient mixing, differences in color-development time, bubbles, and contamination of plates or cuvettes.
If an outlier is excluded, a reason based on the operation record or observations is required.

Variation in Repeated Measurements

If measurements of the same unknown sample vary among repeated measurements, possible causes include errors in dispensing volume, insufficient mixing, bubbles, differences among wells, and differences in reaction time.
The smaller the variation among repeated measurements, the higher the reproducibility of the measurement can be judged to be.

Discussing the cause of large variation in addition to the mean value makes the report more persuasive.
If one measured value deviates greatly, records of bubbles or dispensing errors should be checked.

Differences Between the Bradford Assay and Other Quantification Methods

The Bradford assay can be performed quickly and is relatively simple, but attention must be paid to differences in color development among protein types and the effects of surfactants.
The BCA and Lowry methods use different color-development principles and therefore differ in their sensitivity to interfering substances and measurement ranges.
The UV absorption method requires no reagent, but it is affected by nucleic-acid contamination and differences in aromatic amino acid content.

When comparing the Bradford assay with other methods in a report, it is useful to briefly describe its advantages and limitations.
For example, it can be summarized as being suitable for measuring many samples in a short time, while being subject to errors caused by sample components and protein type.

When the Results Can Be Considered Good

Results of the Bradford assay can be considered good when the calibration curve shows good linearity, the absorbance of the unknown sample falls within the calibration-curve range, and variation among repeated measurements is small.
It is also important that blank correction be appropriate, the dilution factor be calculated correctly, and the sample contain few interfering substances.

Example Discussion When the Experiment Did Not Go Well

If the Bradford assay does not go well, possible causes are considered from results such as a non-linear calibration curve, an unknown sample outside the range, variation in absorbance, a high blank, or standard-solution points that deviate.
It is easier to organize the discussion by separately considering operational errors, sample components, protein type, surfactants, color-development time, and the condition of the measuring equipment.

How to Write Points for Improvement

In a discussion of the Bradford assay, including points for improvement in addition to sources of error makes the report easier to organize.
Improvements are easier to organize by dividing them into preparation of the calibration curve, sample dilution, color reaction, absorbance measurement, and confirmation of sample components.

Improvements to Calibration-Curve Preparation

• Prepare BSA standard solutions accurately
• Prepare the dilution series carefully
• Mix each standard solution thoroughly
• Confirm the linear range of the calibration curve
• If an outlier appears, check the operation record

Improvements to Unknown-Sample Measurement

• Dilute unknown samples so that they fall within the calibration-curve range
• Correctly reflect the dilution factor in the calculation
• Check the sample for turbidity or precipitates
• Check for interfering substances such as surfactants
• Match the buffer conditions of the standard solutions and unknown samples

Improvements to Operation and Measurement

• Use the micropipette accurately
• Avoid bubbles in the tip
• Mix thoroughly after adding Bradford reagent
• Keep the reaction time consistent among samples
• Perform blank correction appropriately
• Avoid contamination of cuvettes or plates
• Perform multiple measurements and use the mean value

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of the Bradford assay, simply writing that “the concentration was determined from the calibration curve” or “the absorbance was high” results in a superficial discussion.
Relating the color-development principle, linear range of the calibration curve, dilution factor, interfering substances, and operational errors produces a more persuasive discussion.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of the Bradford assay.
Adjust the necessary parts according to your own experimental results.

• In the Bradford assay, absorbance near 595 nm increases when Coomassie Brilliant Blue G-250 dye binds to proteins.
• The protein concentration of the unknown sample was calculated using a calibration curve prepared from BSA standard solutions.
• Because the absorbance of the unknown sample was within the calibration-curve range, relatively reliable quantification without extrapolation is considered to have been possible.
• Because the unknown sample was diluted before measurement, the dilution factor was taken into account when determining the original sample concentration.
• In the high-concentration range, saturation of dye binding or color development may cause the proportional relationship between absorbance and concentration to break down.
• Surfactants may affect color development in the Bradford assay and may increase or decrease absorbance.
• Because BSA and proteins in the unknown sample may differ in dye-binding properties, the determined concentration may include errors caused by differences in protein type.
• If blank correction is insufficient, absorbance originating from the reagent may be included and the concentration may be overestimated.
• If reaction time or mixing conditions differ among samples, the progress of color development changes and variation in absorbance occurs.
• Bubbles in cuvettes or wells interfere with light transmission and may cause absorbance to appear high.

Points to Check When Discussing the Bradford Assay

Checking the following points before writing the report makes the discussion easier to write.

• Have you explained the color-development principle of the Bradford assay?
• Have you stated the measurement wavelength?
• Have you written the type of standard protein used?
• Have you shown the equation and R² of the BSA calibration curve?
• Have you checked the linear range of the calibration curve?
• Have you checked whether the absorbance of the unknown sample falls within the range?
• Have you correctly reflected the dilution factor?
• Have you performed blank correction?
• Have you considered interfering substances such as surfactants?
• Have you discussed differences in color development between BSA and the unknown protein?
• Have you considered pipetting and insufficient mixing as sources of error?
• Do the points for improvement correspond to the sources of error?

Summary

The Bradford assay is a protein-quantification method that uses the change in absorbance near 595 nm caused by the binding of Coomassie Brilliant Blue G-250 dye to proteins.
A calibration curve is prepared using a standard protein such as BSA, and the protein concentration is determined by substituting the absorbance of an unknown sample into the calibration curve.
If the unknown sample is diluted before measurement, the dilution factor must always be taken into account when determining the concentration of the original sample.

In the Bradford assay, it is important to measure within the linear range of the calibration curve.
If the absorbance of the unknown sample is too high, it should be diluted, and if it is too low, the measurement conditions should be reviewed.
In addition, surfactants, buffer components, bubbles, contamination of cuvettes or plates, differences in color-development time, and pipetting errors affect the results.

In a report, rather than simply writing that “the concentration was determined by the Bradford assay,” discuss the color-development principle, BSA calibration curve, absorbance range, dilution factor, blank correction, interfering substances, and differences in color development among protein types in relation to one another.
A more persuasive discussion of the Bradford assay can be produced by explaining which factors may cause the concentration to be estimated too high or too low.