Building a Peptide Monoisotopic and Average Mass Calculator in Python

Description and use

A Python script that calculates monoisotopic and average peptide masses using authoritative atomic mass sources, validated against an open‑source package.

Problem

Synthetic peptide users need accurate mass predictions for mass spectrometry, but manual calculations are error‑prone.

Outcome

The script accurately reproduces peptide masses, demonstrating the difference between monoisotopic and average masses and providing a reliable tool for researchers.

Workflow

• Define monoisotopic, average, and proton masses from NIST and IUPAC sources.
• Store each amino acid residue as an elemental formula (C,H,N,O,S) minus H₂O.
• Sum residue formulas for the peptide, add one H₂O for termini.
• Calculate monoisotopic and average masses by multiplying element counts by their masses and summing.
• Divide by charge to obtain m/z.
• Compare results with an external library for validation.

Evidence

The math is simple: add up the residues, add one water, and divide by the charge.

Source

I used two primary sources and opened both to check the values: Monoisotopic masses: NIST, ... Average masses: IUPAC CIAAW... Proton mass: NIST CODATA...

Source

Rather than hard‑coding residue masses, I store each residue as an elemental formula.

Source