What is Protein Concentration Measurement?

Protein concentration measurement is fundamental in biochemistry and molecular biology. Accurate quantification is essential for enzyme activity assays, SDS-PAGE loading, Western blotting, protein purification monitoring, and standardizing experimental conditions. The Beer-Lambert law provides the theoretical foundation for spectrophotometric protein quantification.

The Beer-Lambert law states that absorbance is directly proportional to both the concentration of the absorbing species and the path length of light through the sample: A = ε × c × l. The extinction coefficient (ε) is unique to each protein at a given wavelength and depends on its amino acid composition, particularly aromatic residues (Trp, Tyr, Phe).

Common methods include: A280 (direct UV absorbance, requires pure protein), Bradford assay (binding of Coomassie dye, 595nm), and BCA assay (copper reduction, 562nm). A280 is fastest but requires knowledge of extinction coefficient. Bradford and BCA use standard curves with known protein concentrations (typically BSA).

How to Measure Protein Concentration

Beer-Lambert Law

A = ε × c × l

• A = Absorbance (no units, typically 0-3)

• ε = Molar extinction coefficient ((mg/mL)⁻¹ cm⁻¹ or M⁻¹ cm⁻¹)

• c = Concentration (mg/mL or M)

• l = Path length (cm, usually 1 cm cuvette)

c = (A / (ε × l)) × Dilution Factor

Common Methods

A280 (UV Absorbance)

• Wavelength: 280 nm (aromatic amino acids)

• Pros: Fast, non-destructive, small sample volume

• Cons: Requires pure protein, needs extinction coefficient

• Typical ε: 0.5-2.0 (mg/mL)⁻¹ cm⁻¹

Bradford Assay

• Wavelength: 595 nm (Coomassie Brilliant Blue G-250)

• Pros: Fast, sensitive (1-20 µg/mL), compatible with reducers

• Cons: Affected by detergents, requires standard curve

• Detection range: 1-1400 µg/mL

BCA Assay

• Wavelength: 562 nm (copper reduction by protein)

• Pros: Very sensitive, tolerates detergents

• Cons: Incompatible with reducing agents, requires heating

• Detection range: 20-2000 µg/mL

Worked Example

Calculate BSA protein concentration:

Given:

• Absorbance at 280nm: 0.543

• Extinction coefficient (BSA): 1.0 (mg/mL)⁻¹ cm⁻¹

• Path length: 1 cm (standard cuvette)

• Dilution factor: 10 (sample was diluted 1:10)

Step 1:

Apply Beer-Lambert law:

c = A / (ε × l)
c = 0.543 / (1.0 × 1)
c = 0.543 mg/mL

This is the concentration in the measured (diluted) sample

Step 2:

Account for dilution:

c_original = c_measured × Dilution Factor
c_original = 0.543 × 10
c_original = 5.43 mg/mL

Result:

5.43 mg/mL

The original protein sample concentration is 5.43 mg/mL or 5,430 µg/mL

Frequently Asked Questions

How do I find extinction coefficient?

For purified proteins, calculate from sequence using ProtParam (ExPASy) which counts Trp, Tyr, and Cys residues. Typical values: BSA = 0.7, IgG = 1.4, lysozyme = 2.6 (all in (mg/mL)⁻¹ cm⁻¹ at 280nm). For unknown proteins, use Bradford or BCA with BSA standard.

Why dilute samples?

Spectrophotometers are accurate in absorbance range 0.1-1.0. Outside this range, measurements become nonlinear. If A > 1.0, dilute the sample. The dilution factor corrects back to original concentration. Most assays have optimal absorbance ranges for best accuracy.

What interferes with measurements?

A280: nucleic acids (absorb at 280nm), turbidity, reducing agents. Bradford: detergents (SDS, Triton), high salt, basic pH. BCA: reducing agents (DTT, β-ME), chelators (EDTA), lipids. Always include appropriate blanks with your buffer.

Which method should I use?

A280 for pure proteins (fastest, no reagents). Bradford for crude lysates (compatible with reducers). BCA for samples with detergents or when higher sensitivity needed. For unknown proteins, use Bradford or BCA with BSA standard curve.

What is a standard curve?

For Bradford/BCA, prepare known BSA concentrations (0-2 mg/mL), measure absorbance, plot A vs concentration, fit a line/curve. Unknown protein absorbance is compared to this curve to determine concentration. Curves are method and plate-reader specific.

Why use BSA as standard?

Bovine serum albumin (BSA) is cheap, pure, stable, and well-characterized. It's the universal protein standard. However, different proteins bind Bradford/BCA reagents differently, so BSA curves give approximate concentrations. For precision, use a purified version of your target protein as standard.

Can I use different path lengths?

Yes. Microvolume (1mm path) uses only 1-2 µL sample. Standard cuvettes are 1 cm path. Formula still applies: shorter path = lower absorbance, so scale accordingly. Most spectrophotometers have cuvette adapters or built-in microvolume pedestal (e.g., NanoDrop).

How accurate are these methods?

A280: ±5% with known ε and pure protein. Bradford: ±10-15% due to protein-to-protein variation in dye binding. BCA: ±5-10%, more consistent across proteins. Always measure in triplicate and use freshly prepared standards for best accuracy.

A280 Protein Concentration Calculator

Beer-Lambert Law Calculator