Glossary
Molecular Weight (Da)
A peptide’s molecular weight in daltons links mass to moles and underpins MS identity checks. Also covers average versus monoisotopic mass on a certificate.
Molecular weight is the mass of a molecule expressed in daltons (Da); a dalton is defined as one twelfth of a carbon-12 atom’s mass and numerically matches a gram for every mole. Catalogue peptides span roughly 300 Da (small tripeptides such as GHK-Cu) to around 4,000 to 5,000 Da (the larger incretin analogs); semaglutide weighs approximately 4,114 Da.
Two key uses
It serves two main purposes. First, it translates mass into amount of substance, so no molarity calculation is possible without it: divide grams by the molecular weight to get moles, so 5 mg of a peptide weighing 4,114 Da is roughly 1.22 µmol. Second, it is the reference point for confirming identity, since the MS result on a COA is judged by how well the measured mass agrees with the calculated one.
Two finer points often cause confusion. Average mass, weighted over the natural isotopes, is not the same as monoisotopic mass, calculated from each element’s most common isotope; below about 2,000 Da they can be a dalton or more apart, so the certificate should say which one it reports. Salt forms (TFA) and PEG conjugates also weigh more than the bare peptide, so part of each labelled milligram is not peptide. Worked examples are in our guide to molecular weight and moles.
Related terms
See also mcg vs mg and the molarity calculator