Protein Concentration from A280
BCA/Bradford assay or A280: C = A/(ε×l). mg/mL. Extinction coefficient from sequence or database.
Why This Biology Metric Matters
Why: Accurate protein concentration is essential for enzyme assays, Western blots, and structural biology.
How: A280: C (mg/mL) = A280/(ε×l). ε from ProtParam or known values. BCA/Bradford: standard curve from known protein.
- ●A280 assumes Trp/Tyr content. BSA ε ≈ 43,824 M⁻¹cm⁻¹.
- ●BCA and Bradford more sensitive; A280 for pure proteins.
- ●Buffer absorbance at 280 nm can interfere; blank carefully.
Sample Scenarios
BSA Standard Measurement
Typical BSA concentration measurement at 280nm
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IgG Purification Check
Checking IgG concentration after protein purification
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Lysozyme Activity Assay
Measuring lysozyme concentration for enzyme activity
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Custom Protein Analysis
Custom protein with known extinction coefficient
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Tryptophan-Rich Protein
Protein with high tryptophan content (high ε)
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Low Concentration Sample
Diluted sample requiring careful measurement
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Enter Measurement Parameters
🔬 Protein Selection
📊 Measurement Parameters
💧 Optional: Sample Volume
For educational use only. Always confirm dosages and care with a licensed veterinarian.
🧬 Biology Facts
C = A280/(ε×l). Beer-Lambert for proteins. ε in M⁻¹cm⁻¹.
— A280
BCA and Bradford: colorimetric, standard curve. More sensitive.
— Assays
BSA ε ≈ 43,824; IgG ≈ 210,000. ProtParam for custom.
— Extinction
mg/mL common. Convert to molar: MW from sequence.
— Units
What is Protein Concentration Measurement?
Protein concentration measurement is a fundamental technique in biochemistry and molecular biology. The UV absorbance method at 280nm is one of the most common, rapid, and non-destructive methods for determining protein concentration. It relies on the fact that proteins contain aromatic amino acids (tryptophan, tyrosine, and phenylalanine) that absorb ultraviolet light at 280nm.
UV Absorbance Method
Measures absorbance at 280nm where aromatic amino acids absorb light. Fast, non-destructive, and requires minimal sample.
Beer-Lambert Law
The fundamental principle: absorbance is proportional to concentration, pathlength, and extinction coefficient.
Extinction Coefficient
Unique to each protein, depends on the number and type of aromatic amino acids. Higher ε = more sensitive detection.
How the Beer-Lambert Law Works
The Beer-Lambert Law describes the relationship between light absorption and the properties of the material through which light passes:
Where:
• A = Absorbance (unitless, typically 0-2)
• ε = Molar extinction coefficient (M⁻¹ cm⁻¹)
• b = Pathlength (cm, usually 1.0 cm for standard cuvettes)
• C = Molar concentration (M)
To calculate protein concentration in mg/mL, we rearrange and multiply by molecular weight and dilution factor:
Where:
• C = Mass concentration (mg/mL)
• MW = Molecular weight (g/mol)
• n = Dilution factor (if sample was diluted)
Key Considerations
- Keep absorbance 0.1–1.0 for accurate measurements
- Always use buffer blank to subtract background
- Nucleic acids absorb at 280nm—check A260/A280 ratio (~0.6 for pure protein)
- Verify cuvette pathlength (usually 1.0 cm)
When to Use UV Absorbance vs Other Assays
✅ UV Absorbance (280nm) - Best For:
- • Pure protein samples (no nucleic acids)
- • Known protein with known extinction coefficient
- • Quick, non-destructive measurements
- • Monitoring protein purification
- • When sample volume is limited
- • Real-time concentration monitoring
⚠️ Consider Other Methods When:
- • Sample contains nucleic acids (use Bradford, BCA, or Lowry)
- • Protein has very low/no aromatic amino acids
- • Unknown protein without known ε
- • Need absolute accuracy (use amino acid analysis)
- • Sample has interfering compounds
- • Very low concentrations (<0.1 mg/mL)
Common Protein Extinction Coefficients
Extinction coefficients vary significantly based on aromatic amino acid content. Tryptophan contributes most (~5500 M⁻¹ cm⁻¹), followed by tyrosine (~1490 M⁻¹ cm⁻¹), and phenylalanine (~200 M⁻¹ cm⁻¹).
| Protein | Extinction Coefficient (ε) | Molecular Weight | Notes |
|---|---|---|---|
| BSA (Bovine Serum Albumin) | 43,824 M⁻¹ cm⁻¹ | 66,463 g/mol | Common standard protein, widely used in biochemical assays |
| IgG (Immunoglobulin G) | 210,000 M⁻¹ cm⁻¹ | 150,000 g/mol | Antibody protein, commonly purified from serum |
| Lysozyme | 37,901 M⁻¹ cm⁻¹ | 14,000 g/mol | Small antimicrobial enzyme, high extinction coefficient |
| Insulin | 6,335 M⁻¹ cm⁻¹ | 5,734 g/mol | Hormone protein, low extinction coefficient |
| Streptavidin | 176,000 M⁻¹ cm⁻¹ | 55,000 g/mol | Biotin-binding protein, very high extinction coefficient |
Troubleshooting Common Issues
❌ Contamination Issues
Problem: Nucleic acids (DNA/RNA) also absorb at 280nm, causing overestimation.
Solution: Use A260/A280 ratio. Pure protein should have ratio ~0.6. If ratio >0.8, consider Bradford or BCA assay.
⚠️ Interference from Buffers
Problem: Some buffers (Tris, imidazole, DTT) absorb at 280nm.
Solution: Use buffer blank or measure at different wavelength (205nm for peptide bonds).
📏 Pathlength Accuracy
Problem: Using wrong pathlength (e.g., 0.5 cm cuvette but entering 1.0 cm).
Solution: Always verify cuvette pathlength. Many modern cuvettes are 1.0 cm, but some are 0.5 cm or 1.0 mm.
✅ Optimal Absorbance Range
Best Practice: Keep absorbance between 0.1-1.0 for accurate measurements.
Too Low (<0.1): High noise, inaccurate. Dilute less or use longer pathlength.
Too High (>2.0): Non-linear response. Dilute sample further.
Frequently Asked Questions
Q: Why measure at 280nm specifically?
A: Proteins contain three aromatic amino acids that absorb UV light: tryptophan (peak ~280nm), tyrosine (peak ~275nm), and phenylalanine (peak ~258nm). The 280nm wavelength is chosen because it's where tryptophan and tyrosine absorb most strongly, providing the best sensitivity for most proteins. Tryptophan has the highest extinction coefficient (~5500 M⁻¹ cm⁻¹ per residue), making it the primary contributor to protein absorbance.
Q: How do I find the extinction coefficient for my protein?
A: Extinction coefficients can be found in several ways: (1) Literature values - many common proteins have published ε values, (2) Calculate from sequence - tools like ProtParam (ExPASy) calculate ε from amino acid composition, (3) Experimental determination - measure absorbance of known concentration, (4) Use Bradford/BCA assay first to establish concentration, then calculate ε. For proteins with unknown ε, the Bradford or BCA assay is recommended instead of UV absorbance.
Q: What if my absorbance is too high or too low?
A: Optimal absorbance range is 0.1-1.0. If absorbance >1.0: Dilute your sample (e.g., 1:10 or 1:100) and multiply the calculated concentration by the dilution factor. If absorbance <0.1: Use a longer pathlength cuvette (e.g., 1 cm instead of 0.5 cm), or concentrate your sample. Very low absorbance values have high noise and poor accuracy. Always use a buffer blank to subtract background absorbance.
Q: Can I use this method if my sample contains DNA or RNA?
A: Nucleic acids also absorb strongly at 260nm and weakly at 280nm, which can interfere with protein measurements. Check the A260/A280 ratio: Pure protein should be ~0.6, while nucleic acid contamination gives ratios >0.8. If contaminated, use alternative methods like Bradford assay, BCA assay, or Lowry assay. Alternatively, measure at 205nm (peptide bond absorption) which is less affected by nucleic acids.
Q: How accurate is the UV absorbance method?
A: Accuracy depends on several factors: (1) Known extinction coefficient (±5-10% if from literature), (2) Absorbance measurement precision (±2-5% with good spectrophotometer), (3) Pathlength accuracy (±1% for standard cuvettes), (4) Sample purity (nucleic acid contamination can cause 10-50% overestimation). Overall, expect ±10-15% accuracy for pure proteins with known ε. For highest accuracy, use amino acid analysis or quantitative mass spectrometry.
Q: What's the difference between mg/mL and molar concentration?
A: mg/mL (mass concentration) tells you how many milligrams of protein per milliliter of solution - useful for practical applications like loading gels or preparing solutions. Molar concentration (M or mM) tells you how many moles of protein molecules per liter - important for stoichiometric calculations, enzyme kinetics, and binding studies. To convert: Molar = (mg/mL) / (MW in g/mol) × 1000. The calculator provides both units for convenience.
Q: Can I measure protein concentration in crude cell lysates?
A: UV absorbance at 280nm is generally NOT recommended for crude lysates due to interference from nucleic acids, small molecules, and other cellular components. Use colorimetric assays instead: Bradford assay (fast, sensitive, but affected by detergents), BCA assay (more compatible with detergents, very sensitive), or Lowry assay (classic method, more time-consuming). After purification, UV absorbance becomes more reliable.
Q: How do I account for buffer absorbance?
A: Always use a buffer blank! Measure your buffer alone at 280nm and subtract this value from your sample absorbance. Some buffers absorb significantly: Tris-HCl (minimal), HEPES (minimal), imidazole (absorbs at 280nm - use 290nm instead), DTT/TCEP (absorb at 280nm - use reducing agent-free buffer for blank). For best results, use the same buffer composition for both sample and blank.
Comparison with Other Protein Assay Methods
| Method | Principle | Sensitivity | Advantages | Limitations |
|---|---|---|---|---|
| UV Absorbance (280nm) | Aromatic amino acid absorption | 0.1-1 mg/mL | Fast, non-destructive, no reagents needed | Requires known ε, affected by nucleic acids |
| Bradford Assay | Coomassie Blue dye binding | 0.01-1.4 mg/mL | Very sensitive, fast, simple protocol | Affected by detergents, variable between proteins |
| BCA Assay | Biuret reaction + BCA enhancement | 0.0005-2 mg/mL | Most sensitive, compatible with detergents | More expensive, longer incubation time |
| Lowry Assay | Biuret + Folin-Ciocalteu reaction | 0.01-1 mg/mL | Classic method, well-established | Time-consuming, many interfering substances |
| Amino Acid Analysis | Complete hydrolysis + HPLC | Any concentration | Most accurate, absolute quantification | Expensive, time-consuming, requires expertise |
Aromatic Amino Acids and Their Contribution
The UV absorbance of proteins at 280nm comes from three aromatic amino acids. Understanding their individual contributions helps explain why extinction coefficients vary so much between proteins:
Tryptophan (Trp, W)
Extinction coefficient: ~5,500 M⁻¹ cm⁻¹ per residue
Peak wavelength: 280nm
Contribution: Highest - primary contributor to most protein absorbance
Most proteins contain 1-2% tryptophan. Proteins rich in Trp (like antibodies) have very high ε values.
Tyrosine (Tyr, Y)
Extinction coefficient: ~1,490 M⁻¹ cm⁻¹ per residue
Peak wavelength: 275nm
Contribution: Moderate - significant contributor
More common than tryptophan (~3-4% of residues). Can be ionized at high pH, affecting absorbance.
Phenylalanine (Phe, F)
Extinction coefficient: ~200 M⁻¹ cm⁻¹ per residue
Peak wavelength: 258nm
Contribution: Low - minimal contribution at 280nm
Common amino acid (~4% of residues) but contributes little to 280nm absorbance due to low ε and different peak.
Calculating Extinction Coefficient: For a protein with known sequence, ε can be calculated as: ε = (nTrp × 5,500) + (nTyr × 1,490) + (nPhe × 200) M⁻¹ cm⁻¹, where n is the number of each residue. Disulfide bonds can also contribute slightly (~200 M⁻¹ cm⁻¹ per bond).
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