Ultracentrifugering

Ultracentrifugering, also known as ultracentrifugation, is a laboratory technique that uses very high centrifugal forces to separate biomolecules, viruses, and other particles based on size, density, and shape. Ultracentrifuges generate relative centrifugal forces (RCF) typically from about 100,000 g to well over 400,000 g, achievable with fixed-angle or swinging-bucket rotors. Separation can occur by sedimentation velocity, where particles move through a dense medium at different rates, or by sedimentation equilibrium (isopycnic centrifugation), where particles band at a position corresponding to their buoyant density in a density gradient such as sucrose or cesium chloride.

Common approaches include differential ultracentrifugation, where successive spins pellet larger components and leave smaller ones in the supernatant; rate-zonal ultracentrifugation, which separates particles by size in a gradient; and density-gradient ultracentrifugation, which separates by buoyant density and can yield highly purified fractions.

Equipment consists of an ultracentrifuge with rotors designed for high-speed operation. Rotors are either fixed-angle, which hold tubes at a constant angle relative to the axis, or swinging-bucket, which allow horizontal alignment during centrifugation. Proper balancing, cooling, and rotor compatibility with sample tubes and gradients are essential due to the extreme forces involved.

Applications span biochemistry and molecular biology, including purification of cellular organelles (nuclei, mitochondria, lysosomes), ribosomes, viruses, protein complexes, lipoproteins, and nucleic acids, as well as analytical measurements of sedimentation coefficients expressed in Svedberg units (S).

The technique has a long history in physical chemistry and biochemistry and is named after Theodor Svedberg, whose work on high-speed centrifugation contributed to its development.