Molecular Weight¶
Definition¶
Molecular weight (MW) is the sum of the atomic weights of all atoms in a peptide molecule, expressed in Daltons (Da). It is a fundamental identifier used to confirm peptide identity by mass spectrometry.
Overview¶
The molecular weight of a peptide is calculated from its amino acid sequence plus any chemical modifications. It is the primary parameter measured by mass spectrometry to confirm identity.
Technical Explanation¶
Average vs. Monoisotopic Mass:
| Type | Definition | Use |
|---|---|---|
| Average Mass | Weighted average of all natural isotopes | General reference |
| Monoisotopic Mass | Mass using the most abundant isotope of each element | MS peak assignment |
Calculation: MW = Sum of residue masses + mass of terminal groups + mass of modifications
Example: For a peptide with sequence A-B-C: - Sum individual amino acid residue masses (each residue mass = amino acid MW − 18 Da for water loss) - Add H₂O at ends (18 Da) - Add modification masses (if any)
Impact of Counter-Ion: The molecular weight reported is typically the peptide free base or peptide salt, depending on convention. Always verify which form is being reported.
Importance in Peptide Documentation¶
Theoretical and experimental (MS-observed) molecular weights are compared on the COA to confirm identity. A match within ±0.5 Da confirms correct peptide synthesis and modification.
Related Terms¶
Frequently Asked Questions¶
Q: What is the molecular weight range of research peptides? A: Most research peptides range from ~200 Da (dipeptides) to ~5000 Da (50-mers), though longer sequences are possible.
Q: Why might observed and theoretical MW not match exactly? A: Differences can indicate incorrect sequence, incomplete or excessive modifications, counter-ion presence, or adduct formation (sodium, potassium) during MS analysis.