Protein Molecular Weight
Molecular weight from an amino acid sequence, with composition and the A280 extinction coefficient.
21 residues · 2.384 kDa
A protein’s molecular weight is the sum of its residue masses plus one water molecule for the free ends. The insulin A chain, GIVEQCCTSICSLYQLENYCN, comes to 2383.71 Da — 21 residues at an average of about 110 Da each, which is the rule of thumb behind “roughly 110 Da per residue”.
Residue mass, not amino acid mass
Free alanine weighs 89.09 Da, but inside a chain it contributes only 71.08. The difference is a water molecule: forming each peptide bond releases one, so a residue is the amino acid minus H₂O. Summing residue masses and adding a single water back at the end accounts for the two free termini and gives the mass of the whole chain.
That is why a protein of n residues has n − 1 peptide bonds and loses n − 1 waters. Adding one water at the end, rather than none or n, is the step most hand calculations get wrong.
Average against monoisotopic
The masses here are average masses, weighted across the natural isotope abundances, and they are what SDS-PAGE, size-exclusion chromatography and everyday lab work use. Mass spectrometry with enough resolution instead reports monoisotopic mass, computed from the most abundant isotope of each element only. For a small peptide the two differ by around a dalton; for a large protein the gap grows to tens of daltons, so quoting the wrong one in a mass spec context is a real error.
one water for the free amino and carboxyl termini; average masses, not monoisotopic
- 1 Write the sequence in one-letter code. The insulin A chain is GIVEQCCTSICSLYQLENYCN — 21 residues.
- 2 Look up the residue mass of each letter. Glycine is 57.05, isoleucine 113.16, valine 99.13, and so on.
- 3 Add them all together. The twenty-one residues sum to 2365.69 Da.
- 4 Add one water. 2365.69 + 18.02 = 2383.71 Da, or about 2.38 kDa.
- 5 Sanity-check against 110 Da per residue. 21 × 110 ≈ 2310, close enough to confirm no residue was miscounted.
Residue masses at the extremes
In daltons. The spread is why composition matters and a residue count alone is only an estimate.
| Residue | One-letter | Mass |
|---|---|---|
| Glycine — the smallest | G | 57.05 Da |
| Alanine | A | 71.08 Da |
| Average across the twenty | — | ≈ 110 Da |
| Arginine | R | 156.19 Da |
| Tyrosine | Y | 163.18 Da |
| Tryptophan — the largest | W | 186.21 Da |
Why the extinction coefficient comes with it
Proteins absorb at 280 nm almost entirely because of their tryptophan and tyrosine residues, so the molar extinction coefficient can be predicted from composition: 5500 M⁻¹cm⁻¹ for each Trp and 1490 for each Tyr, following Pace’s coefficients. Dividing that by the molecular weight gives the absorbance of a 1 mg/mL solution, which is what turns a spectrophotometer reading into a concentration.
Two caveats. The figure here assumes all cysteines are reduced; disulfide bonds add about 125 M⁻¹cm⁻¹ per cystine and each one also removes two hydrogens from the mass. And a protein with no tryptophan or tyrosine has an extinction coefficient near zero, so A280 cannot measure it at all — a Bradford or BCA assay is needed instead. Post-translational modifications, signal peptide cleavage and bound cofactors all shift the real mass away from what the sequence alone predicts.