How do you calculate protein concentration from absorbance?

To calculate protein concentration using the Beer-Lambert law, divide the absorbance at 280 nm by the product of the molar extinction coefficient and the path length (typically 1 cm). For a protein with an extinction coefficient of 1 per mg/mL, concentration in mg/mL equals the A280 reading directly, multiplied by the dilution factor. For colorimetric assays such as Bradford, use the equation from your standard curve: concentration equals absorbance minus the y-intercept, divided by the slope.

What is the A280 method for protein concentration?

The A280 method measures protein concentration by reading UV absorbance at 280 nm. Aromatic amino acids, particularly tryptophan and tyrosine, absorb strongly at this wavelength. The concentration is calculated using Beer-Lambert: A = epsilon times c times l, where epsilon is the molar extinction coefficient, c is concentration, and l is path length. This method is fast and non-destructive but requires knowing the protein's extinction coefficient for accurate results.

What is the Bradford assay?

The Bradford assay is a colorimetric protein quantification method where Coomassie Brilliant Blue G-250 dye binds to proteins and shifts its absorbance from 465 nm to 595 nm. The colour change is proportional to protein concentration. A standard curve using known BSA concentrations is prepared, and the unknown protein concentration is read from the curve. The Bradford assay is sensitive down to about 1 microgram/mL but shows protein-to-protein variability.

What is the BCA assay for protein concentration?

The BCA (bicinchoninic acid) assay is a colorimetric method where peptide bonds reduce Cu2+ to Cu+ ions in an alkaline solution, and the Cu+ ions chelate with BCA reagent to produce a purple colour with absorbance at 562 nm. It is less susceptible to protein-to-protein variation than Bradford and is compatible with many detergents. Sensitivity is down to about 0.5 micrograms/mL. Heat is required for the colour development step (37 to 60 degrees Celsius for 30 to 60 minutes).

What is a good protein concentration for most experiments?

For SDS-PAGE loading, a concentration of 0.5 to 2 mg/mL allows loading 10 to 40 micrograms per lane in a standard 10 to 20 microlitre volume. For Western blotting, 0.1 to 1 mg/mL is typically sufficient. Enzyme kinetics assays often require 0.1 to 10 micrograms/mL, and structural biology techniques such as cryo-EM may require 0.5 to 5 mg/mL. The optimal concentration depends on the downstream application.

What is the extinction coefficient of BSA?

Bovine serum albumin (BSA) has a molar extinction coefficient of approximately 43,824 M-1 cm-1 at 280 nm, corresponding to about 0.667 per mg/mL using the molecular weight of 66,463 Da. This makes BSA suitable as an A280 standard for comparison. BSA is also the most commonly used standard protein for Bradford and BCA calibration curves because it is inexpensive, stable, and widely available.

How do I measure protein concentration without a spectrophotometer?

Without a spectrophotometer, protein concentration can be estimated by SDS-PAGE comparison against a known standard, dot blot assay, or turbidimetric estimation. However, these methods are less accurate than spectrophotometric assays. Fluorescence-based methods using dyes such as NanoOrange or SYPRO Orange provide high sensitivity quantification on instruments with fluorescence capability and do not require a UV spectrophotometer.

Why is purity ratio 260/280 used for proteins?

The 260/280 absorbance ratio is used to assess protein purity because nucleic acids absorb strongly at 260 nm while proteins absorb at 280 nm. A pure protein preparation should have a 260/280 ratio of approximately 0.57. A ratio above 0.6 suggests nucleic acid contamination, which will cause the protein concentration to be overestimated by the A280 method. For highly pure proteins, the ratio provides a quick contamination check without additional analysis.

What is the detection limit of the Bradford assay?

The standard Bradford assay has a detection range of approximately 1 to 25 micrograms/mL using a 1 mL reaction volume. The micro-Bradford assay, using a smaller sample volume, extends detection down to about 0.1 to 1 microgram/mL. The linear range must be verified with your own standard curve, as the Bradford assay shows a non-linear response at higher concentrations and the curve should be fitted with a polynomial rather than a simple linear regression above 10 micrograms/mL.

Can I use absorbance to calculate protein concentration without a standard curve?

Yes, for the A280 method if you know the molar extinction coefficient of your specific protein, which can be calculated from the amino acid sequence using tools such as ProtParam on ExPASy. The formula is: concentration in mg/mL equals A280 divided by (extinction coefficient divided by molecular weight). If the extinction coefficient is unknown, a general approximation of 1 per mg/mL per A280 unit can be used, but this introduces significant error for proteins with unusually high or low tryptophan and tyrosine content.

Protein Concentration Calculator – A280 and Bradford

What Is the Protein Concentration Calculator?

The Protein Concentration Calculator determines the concentration of a protein solution from a spectrophotometric absorbance reading, using either the Beer-Lambert law for direct UV absorbance at 280 nm or a standard curve for colorimetric assays such as Bradford and BCA. Molecular biologists, biochemists, and protein chemists use it to figure out how much protein is present in a sample before carrying out downstream experiments that require a precise protein input. According to the NCBI Molecular Cloning manual, accurate protein quantification is a prerequisite for reproducible biochemical experiments, as loading errors in gel electrophoresis or incorrect enzyme concentrations in kinetic assays can invalidate entire datasets.

Protein concentration can be measured by several methods, each with different sensitivity ranges, interferences, and compatibility with common laboratory reagents. The A280 direct UV method is fast and non-destructive but requires knowing the protein's molar extinction coefficient. Colorimetric assays such as Bradford (595 nm) and BCA (562 nm) use a standard curve and are more universally applicable to unknown proteins, though they consume the sample and may be interfered with by detergents or reducing agents. Given that the choice of method affects accuracy, the calculator supports all three approaches so you can work out the concentration directly from your assay output.

The Beer-Lambert Law and the A280 Method

The A280 method is based on the Beer-Lambert law: absorbance equals the molar extinction coefficient times the concentration times the path length (A = ε × c × l). For a standard 1 cm path length cuvette, concentration in mg/mL equals absorbance at 280 nm divided by the specific extinction coefficient of the protein in units of mL per mg per cm. Proteins absorb at 280 nm because the aromatic amino acids tryptophan and tyrosine contain pi electron systems that absorb UV light at that wavelength. Proteins with many tryptophan residues have high extinction coefficients; proteins with none (such as collagen) cannot be quantified by A280 at all. The ExPASy ProtParam tool calculates the theoretical extinction coefficient from any amino acid sequence, making it the standard reference for A280-based quantification of characterised proteins.

For unknown proteins where the extinction coefficient is not available, a general approximation of 1.0 per mg/mL per A280 unit is sometimes used, but this introduces significant error. A protein rich in tryptophan residues may have an extinction coefficient of 2.5 or higher, meaning the approximation would underestimate its concentration by 150 percent. That said, the A280 method is non-destructive: the sample can be recovered and used after measurement, which is an important advantage when working with limited or precious protein preparations.

Common Protein Assay Methods Compared

Each quantification method has distinct advantages, linear ranges, and incompatibilities. The table below summarises the key parameters for choosing the right method for your sample type and downstream application.

Method	Wavelength	Linear Range	Key Interferences
A280 (direct UV)	280 nm	0.1 to 2 mg/mL	Nucleic acids, phenol, aromatic compounds
Bradford	595 nm	1 to 25 μg/mL	Detergents (SDS), some basic buffers
BCA	562 nm	0.5 to 2,000 μg/mL	Reducing agents (DTT, beta-ME), chelators (EDTA)
Lowry	750 nm	1 to 100 μg/mL	Many detergents, sucrose, glycerol, Tris

Purity Assessment: The 260/280 Ratio

A spectrophotometer measurement at both 260 nm and 280 nm provides a purity check alongside the concentration. Nucleic acids absorb strongly at 260 nm but proteins absorb at 280 nm. A pure protein sample should have a 260/280 ratio of approximately 0.57. If the ratio is above 0.6, nucleic acid contamination is likely and the A280 reading will overestimate protein concentration. On top of that, the 260/230 ratio (measuring contaminants such as phenol, EDTA, and carbohydrates) should ideally be above 1.5 for a clean sample. The PubMed guide to nucleic acid and protein spectrophotometry covers the interpretation of both purity ratios in detail and their impact on downstream quantification accuracy.

In practice, samples from cell lysates or partial purification steps often contain nucleic acid contamination that artificially inflates the A280 reading. Adding a DNase and RNase digestion step before final quantification, or switching to a colorimetric assay that is not affected by nucleic acid absorbance at 280 nm, gives a more accurate protein concentration in complex mixtures. Given this, the assay method choice should be made after assessing what contaminants are likely present in the sample type being measured.

Accuracy and Limitations

The calculator is mathematically exact for the values entered. Its practical accuracy depends on the quality of the spectrophotometer calibration, the cleanliness of the cuvette or microvolume pedestal, and whether the blank correction was performed correctly using the same buffer as the protein sample. Small errors in blanking are amplified when samples are measured at low absorbance values near the detection limit.

The A280 method is inaccurate for proteins with no tryptophan or tyrosine residues, proteins that aggregate and scatter light, and samples with high nucleic acid contamination. Colorimetric assays are susceptible to protein-to-protein variability because different proteins bind Bradford dye or reduce BCA reagent with different efficiencies. For samples of a known protein type, using a standard curve prepared from the same protein rather than BSA improves accuracy. For absolute quantification in critical experiments, amino acid analysis or quantitative mass spectrometry provides the most accurate determination but requires specialised equipment.

The Most Common Protein Quantification Mistake

The error I encounter most often is blanking the spectrophotometer with water instead of the protein's storage buffer. If the buffer contains imidazole, glycerol, or other UV-absorbing components, blanking with water will include that background absorbance in the protein reading and produce an inflated concentration. With that in mind, always prepare the blank using the same buffer at the same concentration that the protein was eluted or stored in, and measure the blank immediately before the protein sample in the same cuvette or pedestal position. This mistake turns up most often with His-tag purification eluates containing imidazole, where the imidazole absorbance at 280 nm can account for 10 to 30 percent of the total A280 signal in dilute fractions. The Bio-Rad Bradford Protein Assay documentation lists buffer and additive compatibility for all common protein assay formats. With that in mind, our DNA concentration calculator handles the equivalent A260-based quantification when both nucleic acid and protein measurements are needed from the same sample, and the ligation calculator is the natural downstream step once a purified protein-free DNA sample has been confirmed.

Protein Concentration Calculator