Ultracentrifugation is a high-speed centrifugation method that separates particles in a solution based on their density, size and mass by using extremely high centrifugal forces. Unlike standard centrifugation, ultracentrifugation operates at speeds over 100,000 rpm, allowing the separation of subcellular particles, macromolecules, viruses and nanoparticles. This technique is crucial in life sciences and biopharma workflows for analyzing, purifying and characterizing complex biological systems.

Principles of Ultracentrifugation

Similarly to a standard centrifuge, an ultracentrifuge follows the principle of sedimentation, suggesting that high-speed spinning causes particles in a solution to experience a force that repels them from the axis of rotation. This force, known as the centrifugal force, is proportional to the molecular weight of the particle, the radius of the rotor and the square of the angular velocity (i.e., how fast the particle rotates around the axis).

The liquid also exerts opposing forces on the particles, such as friction and buoyant forces. These forces have a significant impact on the sedimentation. Denser particles experience centrifugal force, overcoming buoyant force and sink quickly. Less-dense particles remain suspended or float, while equal-density particles do not sediment. Large, dense particles settle rapidly, while smaller, lighter ones remain suspended at various levels.

This principle was studied by the Swedish chemist Theodor Svedberg, who won the Nobel Prize for his work on ultracentrifugation. He also invented the sedimentation coefficient or Svalbard Unit to measure particle size based on sedimentation velocity.

Ultracentrifugation vs High-speed Centrifugation

It is important to note that while ultracentrifuges and high-speed centrifuges both use centrifugal forces instead of gravity-based sedimentation, they should not be confused. Although high-speed centrifuges can handle larger sample volumes, the extreme speed of ultracentrifugation makes it more efficient at separating small particles, such as viruses, than high-speed centrifugation.

Key Components of an Ultracentrifuge System

Rotors

The rotor is the central component that holds and spins the tubes, generating centrifugal force. Different rotor types are:

1. Swinging Bucket Rotors: Used for horizontal spinning of the tubes, ideal for forming well-defined bands in gradient separations
2. Vertical Rotors: Tubes are held in a vertical orientation parallel to the axis of rotation, which supports higher speed limits and enables rapid separation, especially in isopycnic centrifugation.
3. Fixed-angle rotors hold the tubes at 14-40 degrees to the vertical axis while spinning and are ideal for rapid pelleting.

Detection systems

Analytical ultracentrifuges comprise detection systems, such as interference and absorption optical systems, as well as control panels for real-time monitoring and analysis of particles as they sediment.

Ultracentrifuge and heating system

Temperature maintenance is vital when spinning samples at extreme speeds. Cooling mechanisms, such as an interior refrigeration system, maintain the environment within a desired temperature range to prevent the denaturation of temperature-sensitive biomolecules, such as enzymes.

High-speed centrifuge vacuum systems

Ultracentrifuge rotors are placed in a vacuum chamber to minimize air friction that may cause heat buildup. This allows the ultracentrifuge to reach and exceed 100,000 rpm without generating heat or causing mechanical wear. Vacuum systems enhance rotor longevity and ensure stable, reproducible separations.

Types of Ultracentrifugation

Preparative and analytical ultracentrifuges are the two main types, each serving a distinct purpose.

Analytical ultracentrifugation (AUC) analyzes the physical properties of particles in a liquid mixture. Analytical ultracentrifuges monitor particle velocity and position in real time via absorbance or interference optics. It has two analysis modes. Sedimentation velocity uses high-speed spinning to determine particle size, shape, sedimentation coefficients, sample heterogeneity and molecular interactions, such as protein-protein binding.

In contrast, the sedimentation equilibrium mode operates at a lower spinning speed to achieve an equilibrium state in which sedimentation and diffusion balance. Thus, the molar masses of the analytes and the equilibrium constants for analyte interactions can be calculated.

Preparative ultracentrifugation is solely used for isolating and purifying desired macromolecules like viruses, nucleic acids and proteins from mixtures, based on size and density. They don't provide real-time detection like AUCs but enable precise separation for analysis after centrifugation. Their capacity to handle large samples and compatibility with robust methods make them vital for vaccine and gene therapy development.

Ultracentrifugation Techniques and Methods

Preparative ultracentrifugation can be conducted in three different modes to separate particles.

Differential ultracentrifugation

Differential ultracentrifugation involves varying the rotational speed to settle particles over multiple rounds. First, the ultracentrifuge is spun slowly to sediment the large particles. After collecting the pellet, the supernatant is spun at a higher speed to settle smaller particles. This method is beneficial for organelle isolation.

Density gradient centrifugation

Density gradient centrifugation separates particles into distinct bands using a specialized medium, such as a sucrose or cesium chloride gradient. This medium generates a density gradient within the tube. Thus, different particles settle at distinct points where their density matches that of the medium. Density gradient centrifugation offers high-resolution purification, separating particles with similar sizes and densities. Therefore, it is a powerful tool for generating high-purity viral vectors by isolating them from contaminated ones.

Application of Ultracentrifugation in Life Sciences

Ultracentrifugation plays a vital role in life sciences by separating macromolecules, organelles, viruses and nanoparticles, which are difficult to isolate using traditional centrifugation.

Protein purification by ultracentrifugation

Ultracentrifugation is suitable for separating individual proteins and large protein complexes without the risk of denaturation. Rate-zonal centrifugation is often preferred for size-based separation of enzymes and protein biomarkers.

Virus purification using ultracentrifuges

Ultracentrifuges are significant components of viral vector manufacturing protocols to eliminate residual DNA, host cell proteins and replicating viruses from the vector product. Density gradient ultracentrifugation with iodixanol or cesium chloride helps achieve high purity and yield.

DNA/RNA fractionation with ultracentrifuges

DNA molecules in various forms, such as circular, linear or supercoiled, can be separated by isopycnic centrifugation. Ultracentrifugation is widely used for DNA or RNA isolation during plasmid DNA/RNA preparation.

Nanoparticle separation

Ultracentrifugation is integral in nanoparticle characterization. By applying high centrifugal forces, researchers can determine nanoparticle diameter and weight, which helps them optimize these particles for small-molecule drug discovery, delivery and biomaterial applications.

Advantages and Limitations of Ultracentrifugation

Ultracentrifugation allows the separation of very small biomolecules and nanoparticles that cannot be distinguished using standard methods. It provides high-resolution purification for complex samples, supporting both analytical and preparative processes. The technique also maintains the structural integrity of sensitive biomolecules, such as proteins and is ideal for scalable biopharmaceutical applications, including viral vector purification and advanced therapeutic development.

However, ultracentrifugation requires significant capital investment, specialized infrastructure and technical expertise for proper setup and operation. Compared to alternative methods such as chromatography, it typically offers lower throughput and may result in sample loss during fractionation. In addition, high-resolution separations often require longer run times, which can impact overall workflow efficiency.

References

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