Kilodalton

Kilodalton is a unit of mass used in biochemistry and molecular biology to express the molecular weight of macromolecules such as proteins and nucleic acids. It is equal to 1000 daltons, with the common symbol kDa. The dalton (Da) is defined as one twelfth the mass of a carbon-12 atom and is approximately 1.6605 × 10^-27 kilograms (about 1.6605 × 10^-24 grams). Consequently, 1 kDa is about 1.6605 × 10^-24 kilograms or 1.6605 × 10^-21 grams, and in chemical terms corresponds to 1000 grams per mole.

In practice, biomolecule masses are typically reported in kilodaltons. This unit is particularly convenient for proteins, enzymes, nucleic acids, and large complexes. For reference, a small protein may be in the range of 5–20 kDa, a typical enzyme 40–100 kDa, and the bacterial ribosome is on the order of a few megadaltons (roughly 2–3 MDa, i.e., several thousand kDa).

The kilodalton remains standard in contexts where molecular mass is relevant to stoichiometry and separation techniques such as gel filtration and mass spectrometry. For related mass relations, 1 base pair (bp) has a mass of about 660 Da, so 1 kilobase pair (kbp) is roughly 660 kDa. The term is commonly abbreviated as kDa or kD, with kDa being the preferred designation in scientific writing.