> 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-formula.md).

# Molecular Formula

## Definition

A molecular formula is a representation of the number and type of atoms in a peptide molecule, written using standard chemical element symbols (e.g., C₈₃H₁₃₂N₂₄O₂₅S₂ for a typical peptide).

## Overview

The molecular formula provides the complete elemental composition of a peptide. It is derived from the amino acid sequence, terminal groups, and any modifications. The formula is used to calculate exact molecular weight and to verify correct synthesis.

## Technical Explanation

**Derivation:** The molecular formula is built by summing the element counts from:

* Each amino acid residue (known from sequence)
* Terminal groups (free NH₂ and COOH, or modified ends)
* Modifications (acetylation adds C₂H₂O)
* Counter-ion (if reporting salt form)

**Example:** For the dipeptide Gly-Gly (H-Gly-Gly-OH):

* Glycine residue: C₂H₃NO (×2)
* Terminal: +H₂O
* Total: C₄H₈N₂O₃

## Importance in Peptide Documentation

The molecular formula is used to:

* Calculate theoretical molecular weight
* Determine elemental composition for combustion analysis
* Support identity confirmation
* Calculate peptide content for assay determination

## Related Terms

* [Molecular Weight](/docs/glossary/chemical-terms/molecular-weight.md)
* [Identifier (CAS)](/docs/glossary/chemical-terms/cas-number.md)
* [Identity](/docs/glossary/analytical-terms/identity.md)

## Frequently Asked Questions

**Q: Does the molecular formula include the counter-ion?** A: It depends. Some documentation reports the formula for the peptide free base, others include the counter-ion (e.g., TFA salt). The COA should specify which form is reported.

**Q: Why does elemental analysis not match the formula exactly?** A: Small deviations are normal due to residual water, trace impurities, and the hygroscopic nature of peptides. Larger deviations may indicate quality issues.


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