Sum the residues, add one water
A protein's mass is the sum of its amino-acid residue masses plus a single water molecule. Each peptide bond releases water as it forms, and residue masses are already the dehydrated values, so you only add water back once for the two free chain ends.
Written the other way from whole-amino-acid masses, it is the sum of the free amino acids minus (n−1) waters for the (n−1) peptide bonds. Same number, since a residue mass is an amino acid with one water taken off.
Quick estimate: length times 110
The average residue weighs about 110 Da, so you can eyeball molar mass straight from the number of amino acids.
- 100 residues → about 11 kDa
- 300 residues → about 33 kDa
- 500 residues → about 55 kDa
Use the exact residue sum when you need it to match a mass spectrum; the 110 Da shortcut is only for a ballpark.
Average residue masses (daltons)
| Amino acid | Code | Residue mass |
|---|---|---|
| Glycine | G | 57.05 |
| Alanine | A | 71.08 |
| Serine | S | 87.08 |
| Leucine / Isoleucine | L / I | 113.16 |
| Lysine | K | 128.17 |
| Phenylalanine | F | 147.18 |
| Tyrosine | Y | 163.18 |
| Tryptophan | W | 186.21 |
Glycine is the lightest residue and tryptophan the heaviest, roughly a three-fold spread, which is why composition, not just length, sets the exact mass. Add 18.02 Da once for the terminal water.
Common questions
How do you get molar mass from an amino-acid sequence?
Add the residue mass of every amino acid in the chain, then add one water molecule (18.02 Da) for the free ends. Residue masses already have water removed, so you do not subtract per bond. The result is the average molar mass in daltons.
Why is the rule of thumb about 110 Da per residue?
It is the average residue mass across the twenty standard amino acids, weighted for how often each appears in real proteins. So a quick estimate is chain length times 110 Da: a 300-residue protein is roughly 33 kDa.
What is the difference between average and monoisotopic mass?
Average mass uses the natural isotope mix of each element and suits chromatography and general work. Monoisotopic mass uses only the lightest isotope of each element and is what high-resolution mass spectrometry measures. They differ by a few tenths of a percent.
