Molecular weight is a molecule’s mass in daltons (Da), where one dalton is one twelfth the mass of a carbon-12 atom and is numerically equal to one gram per mole. Research peptides typically fall between about 300 Da for a tripeptide such as GHK-Cu and roughly 4,000 to 5,000 Da for the larger incretin analogs; semaglutide is about 4,114 Da.
Why molecular weight matters in peptide research
Two uses dominate. It converts mass to amount of substance, so a molarity calculation is impossible without it: moles equal grams divided by molecular weight, and a 5 mg vial of a 4,114 Da peptide contains about 1.22 µmol. It also anchors identity confirmation, because mass spectrometry on a certificate of analysis is judged by how closely the observed mass matches the calculated one.
Two subtleties trip people up. Average mass, weighted across natural isotopes, differs from monoisotopic mass, computed from the most abundant isotope of each element; below about 2,000 Da they can differ by a dalton or more, and a COA should say which it reports. And a TFA salt or PEG conjugate weighs more than the free peptide, so labelled milligrams are not all peptide. The molecular weight and moles guide works through examples.