Choosing the right conversion coefficient for A280 protein quantification
The A280 reading of a protein solution remains unchanged, but selecting a different protein type in software changes the displayed concentration. This does not necessarily indicate a measurement problem: the instrument measures absorbance and then converts it to concentration using the selected coefficient. A wrong coefficient can keep results consistently high or low even when repeated readings agree closely.
This article covers direct A280 quantification of a known protein, focusing on the selection and use of extinction coefficients. For lysates of unknown composition or mixed proteins, the coefficient of one protein is not a universal answer.
Why no single coefficient suits every protein
Protein absorption at 280 nm comes mainly from tryptophan and tyrosine, with a smaller contribution from disulfide bonds. Proteins differ in amino acid composition, so solutions at the same 1 mg/mL can have markedly different A280 values. An extinction coefficient describes how strongly a protein absorbs light at the specified wavelength under particular conditions.
Within the applicable measurement range, Beer's law relates absorbance to concentration, optical path and extinction coefficient. A software option of ‘1 Abs = 1 mg/mL’ makes an approximate assumption: at a 1 cm optical path, 1 mg/mL of that protein gives an A280 of 1. It is a rough estimation option, not a property shared by all proteins.
Selecting a BSA or IgG preset therefore specifies a basis for calculation rather than merely telling the instrument that the sample is a protein. For a different recombinant protein, having a BSA standard available does not make the BSA coefficient appropriate.
Check the units before entering a number
Before using a published coefficient, confirm that it applies at 280 nm, identifies the relevant protein and has a clear definition. The three common forms below are not interchangeable entries.
Molar extinction coefficient. Commonly written as ε, this has units of M⁻¹·cm⁻¹, where M denotes mol/L. Dividing absorbance by the product of molar extinction coefficient and optical path gives mol/L. Multiply by molar mass to obtain mass concentration. With molar mass in g/mol, the result is in g/L, numerically equal to mg/mL.
Mass extinction coefficient. This can be expressed in mL·mg⁻¹·cm⁻¹. Dividing absorbance by the product of this coefficient and optical path directly gives mg/mL. Its numerical value equals the molar extinction coefficient divided by molar mass when the units above are used.
E1% and E0.1%. At a 1 cm optical path, these conventionally denote the absorbances of 1% and 0.1% mass-per-volume solutions, respectively. A 1% solution is 10 mg/mL and a 0.1% solution is 1 mg/mL, so E1% equals E0.1% multiplied by 10 for the same protein. E0.1% has the same numerical value as the mass extinction coefficient above, but entries must still follow the software field definition.
For example, E1% of 10 in one source and E0.1% of 1 in another may describe the same absorption strength. Entering 10 directly into a field requiring E0.1% makes the calculated concentration one tenth of the correct value. Do not copy a number without its heading and units.
Some software instead uses the reciprocal of the extinction coefficient as a ‘protein conversion factor’ and multiplies to calculate concentration. Check the field definition and calculation before entering a value; the word ‘coefficient’ alone is insufficient.
Check optical path and dilution together in a worked example
The following values demonstrate a calculation, not experimental data or instrument performance. Assume a purified protein with a molar mass of 50,000 g/mol and a molar extinction coefficient at 280 nm of 50,000 M⁻¹·cm⁻¹. Its mass extinction coefficient is 1.00 mL·mg⁻¹·cm⁻¹, corresponding to E0.1% of 1.00 and E1% of 10.0.
Prepare a total dilution factor of 5 by adding 1 part original sample to 4 parts diluent. After subtraction of a matching blank, the absorbance expressed for a 1 cm optical path is A280 = 0.800. The measured solution concentration is 0.800 mg/mL and the original concentration is 4.00 mg/mL.
Two conversions are easy to repeat accidentally. First, some microvolume instruments report absorbance as a 1 cm equivalent. Use 1 cm with that value; do not divide again by the actual short optical path. Second, if the software has already applied the dilution factor of 5, do not multiply by it again after export.
An incorrect coefficient creates a different deviation. If a protein's true mass extinction coefficient is 0.50 but the calculation uses 1.00, with all other conditions unchanged and no interference, the reported concentration is half the true concentration. Repeated measurement or averaging cannot remove this conversion bias.
Where to find a coefficient that matches the sample
Start with technical information for the specific protein preparation or traceable literature. Check the protein construct, wavelength, units and buffer conditions. Neither literature nor experimentally determined coefficients are independent of conditions: buffer, pH, ionic strength and protein state can affect absorption.
If only the sequence is available, a protein sequence analysis tool can provide an estimate. The input must match the actual sample, including whether tags remain and whether a signal peptide or fragment has been removed. Some tools provide results under different disulfide-bond assumptions. Cystine in the prediction formula refers to pairs of cysteine residues forming disulfide bonds; do not count every cysteine residue as a disulfide bond.
Sequence predictions provide a reasoned starting point but do not automatically account for glycosylation, dye labels or prosthetic groups. Predictions may also be less reliable for proteins lacking tryptophan. For precise quantification, assess the coefficient's suitability and cross-check with a validated independent method rather than adjusting the coefficient to obtain an expected concentration.
A correct coefficient does not establish suitability for A280
A280 does not recognise a ‘target protein’. Nucleic acids, other ultraviolet-absorbing substances and mixed proteins can contribute to the signal; turbidity or particle scattering can also affect the result. Changing a coefficient alone cannot isolate the target protein's contribution in these samples.
For lysates of unknown composition, first decide whether total protein or a specific target protein is required. BCA and Bradford assays suit many total-protein applications, but sample ranges and buffer compatibility must be checked for the particular kit. They do not suit every formulation, and total-protein results are not target-protein concentrations.
Record the protein identity or construct, coefficient source and units, the optical-path basis of the absorbance, dilution factor and final concentration together. Repeat measurements must remain within the method's valid range. Where needed, compare back-calculated results at different dilutions, remembering that agreement alone does not establish that the coefficient is correct. These records help distinguish changes in the sample, measurement and calculation conditions.