Free lab tool
Peptide molecular weight calculator
Paste a peptide sequence in one-letter code to get its molecular formula, average mass in g/mol and monoisotopic mass in Da. Add an N-terminal acetyl, C-terminal amide or disulfide bonds to match the material you hold.
Spaces, numbers and dashes are ignored. Example: BPC-157.
- Average mass
- 1419.54 g/mol
- Monoisotopic mass
- 1418.7042 Da
- Molecular formula
- C₆₂H₉₈N₁₆O₂₂
- Length
- 15 residues
Amino acid composition
- P Proline ×4
- G Glycine ×3
- A Alanine ×2
- D Aspartic acid ×2
- E Glutamic acid ×1
- K Lysine ×1
- L Leucine ×1
- V Valine ×1
How the calculation works
A peptide is a chain of amino acid residues. Every peptide bond releases one molecule of water, so each residue contributes its amino acid formula minus H2O. The finished chain keeps one water's worth of atoms at its two ends: a free amine at the N-terminus and a free carboxylic acid at the C-terminus.
The calculator sums the residue formulas, adds that terminal water, applies the modifications you select, and converts the formula to mass with IUPAC standard atomic weights (average) and the masses of the most abundant isotopes (monoisotopic).
Worked example: BPC-157
BPC-157 is the 15-residue sequence GEPPPGKPADDAGLV with a free N-terminus and a free C-terminal acid. The calculator returns C62H98N16O22, an average mass of 1419.54 g/mol and a monoisotopic mass of 1418.704 Da, in line with the published formula for the compound.
The monoisotopic figure is what a high-resolution mass spectrum shows for the [M] species; the singly protonated ion [M+H]+ appears about 1.007 Da higher. The average figure is the one to use when you weigh material or work out a molar concentration.
Questions
Frequently asked
Average mass uses standard atomic weights, which reflect the natural mix of isotopes; it is the value to use for weighing and molarity. Monoisotopic mass uses only the most abundant isotope of each element (12C, 1H, 14N, 16O, 32S) and is the value matched against the main peak in a high-resolution mass spectrum.
Each amino acid residue in the chain contributes its formula minus one water, because a water molecule is lost at every peptide bond. The calculator adds the residue formulas, adds one water for the free N- and C-termini, applies any terminal modifications, then multiplies each element by its atomic mass.
A C-terminal amide replaces -OH with -NH2, lowering the mass by about 0.98 Da. An N-terminal acetyl group adds C2H2O, about 42.04 Da. Each disulfide bond removes two hydrogens, about 2.02 Da.
Catalog values are often for the salt form (acetate or trifluoroacetate) or include non-standard residues and side chains, such as Aib or a fatty-acid chain, that a one-letter calculator cannot represent. Check the formula on the Certificate of Analysis for the lot you hold.
No. It accepts the 20 standard one-letter codes plus the common terminal modifications and disulfide bonds. For peptides with non-standard residues or lipid side chains, use the published formula and weight shown on the compound's reference page.
Calculations run in your browser and are provided for laboratory planning. Check every result against your own records and the lot's Certificate of Analysis. All Neupeptides products are for laboratory research use only.