peptide-mass

A small, no-dependency Python calculator for the theoretical masses of linear peptides built from the 20 standard amino acids. It gives monoisotopic and average masses plus the [M+H]⁺ … [M+5H]⁵⁺ m/z values you'd expect in electrospray, so you can check the identity (LC-MS) line of a certificate of analysis without doing the arithmetic by hand.

It's the packaged form of the calculator in peptide-reference-data (dataset DOI 10.5281/zenodo.23284819), maintained by Peptide.Express.

Install

pip install peptide-mass

Use

peptide-mass GEPPPGKPADDAGLV
# monoisotopic 1418.7042 Da | average 1419.54 Da
#   [M+1H]1+  m/z 1419.7114 ...

peptide-mass CYIQNCPLG --c-amide --disulfides 1   # oxytocin
peptide-mass --csv peptides.csv                   # writes masses.csv

Options: --n-acetyl, --c-amide, --pyroglu (N-terminal Q or E), --disulfides N.

from peptide_mass import mass, mz
mono, avg = mass("CYIQNCPLG", c_amide=True, disulfides=1)
print(round(mz(mono, 2), 4))

Scope and limits

• Standard residues only. Non-standard residues and lipidated peptides aren't approximated.
• Values are for the neutral free peptide, not a TFA or acetate salt. Counter-ions and water change the weighed mass, not the MS parent mass.
• Average masses use IUPAC standard atomic weights; expect ±0.01–0.05 Da differences from other calculators.

License

MIT. For laboratory research use only. Not for human or veterinary use. Nothing in this package is dosing, clinical or medical information.