> For the complete documentation index, see [llms.txt](https://docs.rplpeptides.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.rplpeptides.com/docs/glossary/chemical-terms/molecular-weight.md).

# 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

* [Mass Spectrometry](/docs/glossary/analytical-terms/mass-spectrometry.md)
* [Identity](/docs/glossary/analytical-terms/identity.md)
* [Counter-Ion](/docs/glossary/chemical-terms/counter-ion.md)
* [Molecular Formula](/docs/glossary/chemical-terms/molecular-formula.md)

## 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.


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