Protein molecular weight formula
MW = Σ (amino acid residue masses) + 18.015 − 2.016 × disulfide bonds
Each residue is an amino acid minus one water, which leaves the peptide bond. The added water restores the free N-terminus and C-terminus.
Protein molecular weight examples
| Protein | Length | Formula | Molecular weight (g/mol) |
|---|---|---|---|
| Angiotensin II (DRVYIHPF) | 8 aa | C50H71N13O12 | 1,046.20 |
| Ubiquitin, human | 76 aa | C378H629N105O118S | 8,564.87 |
| Estimate for 300 residues | 300 aa | None | 33,000 |
Extinction coefficient and A280
Tryptophan and tyrosine absorb at 280 nm. Cystines add a small amount.
A280 of a 1 mg/mL solution = ε280 ÷ molecular weight. Use it to find concentration from a spectrophotometer reading.
Ubiquitin has one tyrosine and no tryptophan, so ε280 = 1490 M⁻¹ cm⁻¹ and A280 of 1 mg/mL = 0.174.
How to use this calculator
- Choose Sequence, or Length only for a quick estimate.
- Paste the protein sequence in one-letter codes.
- Enter the number of disulfide bonds in the advanced options.
- Calculate the molecular weight, formula, and extinction coefficient.
Formula and variables
MW = Σ residueᵢ + H2O − 2.016 × disulfide bonds
| Symbol | Meaning | Unit |
|---|---|---|
| residueᵢ | amino acid residue mass | g/mol |
| ε280 | molar extinction coefficient at 280 nm | M⁻¹ cm⁻¹ |
Worked example
Angiotensin II, DRVYIHPF, is C50H71N13O12. Its eight residues plus one water give 1046.19 g/mol.
Interpretation
The result is the average mass of the unmodified chain. Glycosylation, phosphorylation, and other modifications add mass.
Common mistakes
- Use one-letter codes. Three-letter codes such as Ala are not read.
- Replace ambiguous codes B, J, X, and Z with specific residues.
- Enter the number of disulfide bonds for proteins with cystines.
Frequently asked questions
How do you calculate protein molecular weight?
Add the residue mass of each amino acid, then add one water for the two chain ends. Subtract 2.016 g/mol per disulfide bond.
What is the average molecular weight of an amino acid?
A common estimate is 110 g/mol per residue. A 300-residue protein is about 33 kDa.
How is the extinction coefficient calculated?
ε280 = 5500 × W + 1490 × Y + 125 × disulfide bonds, in M⁻¹ cm⁻¹. These are the Pace et al. values for proteins in water.
Does the result include glycosylation or other modifications?
No. It gives the mass of the unmodified chain. Glycans, phosphates, and lipids add mass.
Can I enter a peptide with ambiguous residues?
No. Replace B, J, X, and Z with specific residues so the mass is exact. U is selenocysteine and O is pyrrolysine.
Can I use scientific notation?
Yes. Enter values such as 6.022e23 or 1.0e-6 in a numeric field.
How should I round the result?
Keep extra digits during the calculation. Round the final result to the precision required by your data.