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

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.