Molecular Weight (Da)

Molecular weight is the mass of a molecule expressed in daltons (Da); one dalton equals a twelfth of the mass of a carbon-12 atom and corresponds numerically to one gram per mole. Research peptides generally range from around 300 Da for a tripeptide like GHK-Cu up to about 4,000 to 5,000 Da for the bigger 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: moles are grams divided by molecular weight, meaning a 5 mg vial of a 4,114 Da peptide holds roughly 1.22 µmol. Second, it is the reference point for confirming identity, because the mass spectrometry result on a certificate of analysis is assessed 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, which uses the most abundant isotope of every element; under about 2,000 Da the two may differ by one dalton or more, and a COA ought to state which is given. And a TFA salt or a PEG conjugate is heavier than the free peptide, so not every labelled milligram is peptide. The molecular weight and moles guide works through examples.

Related terms

micrograms vs milligrams · molarity calculator