🧫 Protein Concentration Calculator
Convert A₂₈₀ absorbance into protein concentration and molarity for purification and assay workflows.
Use sequence-derived ε or values from literature (typically at 280 nm).
How to Use This Calculator
Measure A₂₈₀ absorbance
Use a spectrophotometer to record absorbance at 280 nm for your diluted protein solution.
Provide extinction coefficient
Obtain ε from literature, sequence-based calculators, or manufacturer datasheets for your protein.
Enter molecular weight
Input the protein molecular weight (in kDa) to convert molar concentration into mass concentration.
Formula
c (M) = A₂₈₀ ÷ (ε × l)
Where ε is the molar extinction coefficient (M⁻¹·cm⁻¹) and l is path length (cm). To convert molarity to mass concentration:
Conc (mg/mL) = c (M) × MW (kDa) × 1000
Because 1 g/L equals 1 mg/mL, the same numeric value applies to µg/µL.
Full Description
The Beer–Lambert law relates absorbance to concentration for chromogenic molecules. Aromatic amino acids (tryptophan, tyrosine) enable proteins to absorb strongly at 280 nm, making the A₂₈₀ assay a popular, label-free quantification technique.
Ensure your buffer lacks components absorbing at 280 nm (e.g., imidazole) or blank accordingly. For complex mixtures or impure samples, complementary assays such as Bradford or BCA may provide more reliable results.
Frequently Asked Questions
How do I find the extinction coefficient?
Use sequence analysis tools or look up literature values for your protein. Many vendors publish ε for recombinant proteins.
Can I use this calculator for antibody quantification?
Yes. Antibodies typically have ε ≈ 210,000 M⁻¹·cm⁻¹ and MW ≈ 150 kDa. Enter these values along with measured A₂₈₀.
What if my protein absorbs at different wavelengths?
Modify ε and absorbance wavelength accordingly. The Beer–Lambert equation still applies as long as ε corresponds to the chosen wavelength.
Do reducing agents affect measurements?
Reducing agents like DTT or β-mercaptoethanol can absorb at 280 nm. Include them in the blank to offset background absorbance.