Molecular weight (MW), more precisely the relative molecular mass, is the sum of the atomic weights of all atoms in a molecule, reported in daltons (Da) or grams per mole (g mol⁻¹). For a peptide it is calculated as the sum of the residue masses of its constituent amino acids minus one molecule of water (18.015 Da) per peptide bond, with additional adjustments for N- and C-terminal modifications and any bound counterions.
Two forms are commonly cited: the monoisotopic mass(used in high-resolution mass spectrometry) and the average mass (used in most laboratory concentration calculations). Reported MW should always specify which form and whether counterions or hydration water are included.
MW converts a mass of lyophilised powder into a molar amount and underpins every reconstitution and dilution calculation.