kDa

Kilodalton (kDa) is a unit of molecular mass used to express the mass of macromolecules such as proteins and nucleic acids. It is defined as 1,000 daltons, and the dalton (Da) is exactly 1/12 the mass of a carbon-12 atom. A dalton is equivalent to one gram per mole, so 1 kDa equals 1,000 g/mol.

The kilodalton scale is convenient for biology because the masses of biological macromolecules span from a few kilodaltons to several hundred kilodaltons. For example, lysozyme is about 14.3 kDa, albumin is around 66 kDa, and the hemoglobin tetramer has a molecular mass near 64 kDa (each subunit about 16 kDa). Large protein complexes and multi-subunit assemblies can reach well into the hundreds of kilodaltons or higher.

MW can be determined by various methods. Mass spectrometry (such as MALDI-TOF or electrospray) provides high-accuracy molecular weights. Sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE) estimates molecular weight from migration relative to protein standards. Gel filtration (size-exclusion chromatography) and analytical ultracentrifugation give information about size and oligomeric state, often expressed in kDa.

For nucleic acids, the average mass per base pair is about 660 Da; thus a DNA fragment of 1,000 base pairs is roughly 660 kDa. While kDa is commonly used in biology to describe molar mass, it does not measure physical dimensions, and it is distinct from units of length.

See also: dalton, molecular weight.