Protein Extinction Coefficient (ε₂₈₀) Calculator

Estimate protein molar extinction coefficient at 280 nm from Trp, Tyr, and Cys content.

Number of Tryptophan (Trp, W) residues

ε₂₈₀ = 5,500 M⁻¹cm⁻¹ per Trp

Number of Tyrosine (Tyr, Y) residues

ε₂₈₀ = 1,490 M⁻¹cm⁻¹ per Tyr

Number of disulfide bonds (Cys-Cys)

ε₂₈₀ = 125 M⁻¹cm⁻¹ per SS bond

Protein MW (Da) (Da)

For calculating extinction coefficient in L/g·cm

Please check your inputs and try again.

Worked Examples

Example 1: Typical enzyme: 3 Trp, 8 Tyr, 1 SS

ε = 3×5500 + 8×1490 + 1×125

Result: 29,545 M⁻¹cm⁻¹

Use in Beer-Lambert to find concentration

Example 2: Antibody: 8 Trp, 16 Tyr, 16 SS

ε = 44000+23840+2000

Result: ≈213,000 M⁻¹cm⁻¹

IgG typical: ~210,000 M⁻¹cm⁻¹

Frequently Asked Questions

How to use ε₂₈₀ to measure protein concentration?

Measure A₂₈₀ of your sample, then c = A₂₈₀/ε₂₈₀ (Beer-Lambert). If using mass concentration: c(mg/mL) = A₂₈₀ × MW/ε₂₈₀. Use 1 cm cuvette.

Why Trp and Tyr absorb at 280 nm?

The aromatic rings in Trp and Tyr absorb UV light at ~280 nm via π→π* electronic transitions. Trp has a much larger cross-section (5,500 M⁻¹cm⁻¹) than Tyr (1,490) due to its larger indole ring system.