Dalton (Da)

The dalton, also written as the unified atomic mass unit, is one twelfth of the mass of an unbound carbon-12 atom in its ground state, roughly 1.66 times ten to the minus twenty-seventh kilograms. Its convenience is that it is numerically identical to grams per mole, so a peptide of 4114 daltons weighs 4114 grams per mole and no conversion is needed between the molecular and the bulk scale.

The scale gives an immediate sense of what a molecule is. Water is 18 daltons, an average residue about 110, native GLP-1 near 3300, semaglutide near 4100, insulin about 5800, albumin about 66 thousand and an immunoglobulin G about 150 thousand. Size drives disposition, since molecules well below the glomerular filtration threshold are cleared renally while albumin-sized species are largely retained, which is the whole basis of albumin-binding strategies. Dermatology has its own convention in the 500 dalton rule.

Mass is not the regulatory boundary, though it tracks it. The FDA separates a peptide from a protein biologic at 40 amino acids, a residue count rather than a dalton figure, and that line decides whether a product travels through a new drug application or a biologics license application, and whether a follow-on version can be a generic or must be a biosimilar.

Within analysis, the common slip is between monoisotopic and average mass. Monoisotopic mass uses the lightest isotope of each element, average mass weights isotopes by natural abundance, and the two diverge by roughly two daltons at four kilodaltons and more for larger molecules. A certificate reporting a mass without saying which convention it used, or comparing a measured average against a calculated monoisotopic value, can manufacture an apparent discrepancy or conceal a real one.