megadalton

Megadalton is a unit of mass used primarily in biochemistry and molecular biology to express the mass of very large macromolecules and assemblies. It equals one million daltons (Da). A dalton, also known as an atomic mass unit (amu), is defined as 1/12 the mass of a carbon-12 atom. One dalton is about 1.66053906660×10^-27 kilograms, so one megadalton is approximately 1.66053906660×10^-21 kilograms, or about 1.66053906660×10^-18 grams. The symbol MDa is commonly used; MDal is another occasionally seen variant.

In practice, masses of proteins and complexes are often reported in kilodaltons (kDa) for smaller molecules and in megadaltons (MDa) for large assemblies. For example, a bacterial ribosome has a mass of roughly 2.5–3 MDa, and many viral capsids and multimeric protein complexes fall within the 0.1–10 MDa range. The megadalton scale helps convey the size and stoichiometry of large assemblies that exceed the typical protein-molecule range.

Analytical methods suitable for megadalton species include mass spectrometry techniques capable of handling large ions, such as native or charge-detection MS, as well as analytical ultracentrifugation, and various light-scattering and size-exclusion approaches. Understanding megadalton-scale masses is important for characterizing assembly states, structural organization, and the physical properties of macromolecular machines.