Tool
Peptide molecular weight calculator
| Length | 15 residues |
|---|---|
| Molecular formula | C62H98N16O22 |
| Average molecular weight | 1,419.56 g/mol |
| Monoisotopic mass | 1,418.7042 Da |
| [M+H]+ | m/z 1,419.7114 |
| [M+2H]2+ | m/z 710.3594 |
| [M+3H]3+ | m/z 473.9087 |
| [M+4H]4+ | m/z 355.6833 |
Composition: A×2 · D×2 · E×1 · G×3 · L×1 · K×1 · P×4 · V×1
How the calculation works
Each of the twenty standard amino acids contributes a fixed elemental composition once it is part of a chain. The calculator sums those residue compositions, adds one water for the free termini, applies any terminal groups and disulfide bridges, and writes the result as a molecular formula in Hill order.
From the formula it computes two different masses. The average molecular weight uses standard atomic weights and is the figure for weighing and molar arithmetic. The monoisotopic mass uses the most abundant isotope of each element and is what mass spectrometry resolves. The [M+nH]ⁿ⁺ values add n protons and divide by n, which is where an electrospray spectrum shows the molecule.
Checked against published formulas
The same arithmetic reproduces the formula and molecular weight published for every compound in the reference library that has a standard sequence — for example BPC-157 (GEPPPGKPADDAGLV, C₆₂H₉₈N₁₆O₂₂), sermorelin (C-terminal amide) and thymosin alpha-1 (N-terminal acetyl). A compound whose record includes a non-standard group, such as the hexenoyl N-terminus of tesamorelin, needs that group's formula and is outside what a one-letter sequence can describe.
What a calculated mass does and does not establish
A calculated mass is the expected value. Identity is established when a laboratory measures the material and the observed mass matches it; purity is a separate measurement, usually by HPLC. Both belong on a batch's certificate of analysis, and the weight of peptide in a vial is the net peptide content, not the molecular weight multiplied by anything.
Common questions
What is the difference between average molecular weight and monoisotopic mass?
Average molecular weight uses each element's standard atomic weight, which averages over its natural isotopes; it is the figure for weighing and molar calculations. Monoisotopic mass uses only the most abundant isotope of each element (carbon-12, hydrogen-1, nitrogen-14, oxygen-16, sulfur-32); it is the figure a mass spectrometer resolves as the first peak of the isotope pattern.
Why does the calculator add water to the residues?
Each peptide bond forms with the loss of one water, so a residue inside a chain is its amino acid minus H₂O. A linear peptide is therefore the sum of its residues plus one water, which restores the free amine at the N-terminus and the free acid at the C-terminus.
How do C-terminal amidation and N-terminal acetylation change the mass?
A C-terminal amide replaces the terminal OH with NH₂, lowering the monoisotopic mass by about 0.984 Da. An N-terminal acetyl group adds C₂H₂O, about 42.011 Da. Many synthetic peptides carry one or both, so the published formula of a compound is the check on which applies.
What does each disulfide bridge do to the mass?
Forming a disulfide bond between two cysteines removes two hydrogen atoms, about 2.016 Da per bridge. The calculator subtracts that for the number of bridges entered and requires two cysteines for each.
Why do the [M+2H]²⁺ and [M+3H]³⁺ values matter?
Electrospray ionisation usually puts several protons on a peptide, and the spectrometer measures mass-to-charge ratio, not mass. A 3,000 Da peptide is typically observed as its 2+, 3+ or 4+ ion, so the expected m/z values are what identity is confirmed against.
Can it handle modified or non-standard residues?
No. It accepts the twenty standard one-letter codes and the two common terminal groups. D-amino acids have the same mass as their L-forms, but residues such as 2-aminoisobutyric acid, lipidated side chains or metal complexes need their own formula, and the compound's published record is the source for those.
For dilution arithmetic, see the solution dilution calculator. Every compound's sequence, formula and expected ions are on its compound reference page.
For laboratory research use only. Not for human or veterinary consumption.