Molecular Weight (MW)

Molecular weight is the mass of one molecule of a peptide, expressed in daltons or in the equivalent unit grams per mole. It is calculated, not weighed: add the residue masses of every amino acid in the sequence, then add one water molecule for the free termini, because each peptide bond formed expels a water. Two versions circulate. The average mass uses the natural isotopic abundance of each element and is the number quoted on labels; the monoisotopic mass uses only the lightest isotope of each element and is the number a high-resolution mass spectrometer reports.

Real values give a feel for the scale. An average residue contributes roughly 110 daltons, so a thirty-residue chain lands near 3.3 kDa before any modification. Oxytocin, nine residues with a disulfide, is about 1007 daltons. Semaglutide, a 31-residue backbone carrying a C18 diacid linker, comes to roughly 4114. Human insulin, two chains and three disulfides, is about 5808. Each of these is a fixed, checkable number, which is why mass spectrometry can confirm identity to within a fraction of a dalton.

The number does real work. It converts a stated milligram amount into moles, which is the only honest way to compare two analogs of different sizes on a molar basis. It also predicts behaviour: molecules well under the glomerular filtration threshold are cleared renally, and passive penetration through intact skin falls away sharply above a few hundred daltons.

Where it goes wrong is the vial. A lyophilised peptide is a salt, usually with trifluoroacetate or acetate counterions, and it holds residual water, so the powder mass is not the peptide mass. A certificate reporting a found mass matching the free-base molecular weight confirms the molecule, not how much of it is in the container.