Summary

Analytical ultracentrifugation (AUC) uses high rotational speeds to separate and study macromolecules in solution. You can measure their sedimentation coefficients with velocity and equilibrium experiments. Detection methods, such as fluorescence optics, can detect very small amounts down to nanomolar-picomolar levels. Automation and data tools help you understand molecular weight, sample shape and how particles bind easier and faster. This method works for proteins, nucleic acids, drugs and biopharmaceuticals. Key challenges include maintaining sample purity and interpreting complex results.

Analytical ultracentrifugation (AUC) is a powerful technique for analyzing the physical properties of macromolecules in a solution. Unlike preparative ultracentrifugation, which is primarily focused on isolating macromolecules, analytical ultracentrifuges work by spinning a sample at high speeds while monitoring the movement and distribution of its contents in real-time. AUC is invaluable for biophysics and molecular biology, revealing the size, shape, interactions, and conformational changes of proteins, nucleic acids and nanoparticles. Thus, researchers can evaluate sample homogeneity, binding interactions and aggregation within a sample without labeling or immobilization.

Swedish chemist Theodor Svedberg founded the principles of ultracentrifugation, earning him the Nobel Prize in Chemistry in 1926.

How Analytical Ultracentrifugation (AUC) Works?

Centrifugation and sedimentation are fundamental principles of AUC. When a sample is subjected to high rotational speeds, the particles within experience a force that pushes them away from the axis of rotation, ultimately settling against the tube's barriers. This process is known as sedimentation.

Conventional centrifuges rely mostly on gravitational force for sedimentation. Due to their small sizes, macromolecules experience negligible gravitational forces. Instead, their movement is mediated primarily by the much greater centrifugal force, a function of the particle mass, angular velocity (rotational speed) and distance from the axis. Centrifugal force is balanced out by buoyant and frictional forces, determining the particle's net sedimentation behavior.

The simplest unit of measure in AUC is the sedimentation coefficient, the rate at which a particle moves in a centrifugal field, which increases with the particle's rotational speed, mass and density. The effect of friction and buoyant forces on a particle is depicted by the frictional coefficient and partial specific volume (i.e., the volume of one gram of particle or solute), respectively.

Types of AUC Experiments and Sedimentation Techniques

Sedimentation Velocity AUC (SV-AUC)

AUC sedimentation velocity experiments aim to measure the sedimentation rate of macromolecules when a centrifugal force is applied. In SV experiments, the solute begins to sediment towards the bottom because of centrifugal forces. On the other hand, frictional and buoyant forces promote the diffusion of solutes. This interplay eventually gives rise to a concentration boundary moving in the direction of sedimentation. Thus, researchers can elicit the dynamic concentration profiles of solutes to study their hydrodynamic properties, molecular mass, shape and size distributions. SV experiments are performed to inspect the solute homogeneity of samples.

Sedimentation Equilibrium AUC (SE-AUC)

In contrast to the sedimentation velocity experiments, sedimentation equilibrium (SE-AUC) works with lower rotor speeds. The sample is spun until the centrifugal, frictional and buoyant forces all reach an equilibrium. Thus, a steady-state concentration gradient of the particle is generated. By repeating the experiment in different solute concentrations, researchers can gain insight into kinetic properties, such as the equilibrium dissociation constants of protein complexes. However, sedimentation equilibrium experiments require longer run times than sedimentation velocity.

Multi-signal and fluorescence-detected AUC

Through advancements in AUC detection methods, researchers can study the heterogeneity of their samples with greater sensitivity and precision. Multiple optical detection systems, such as inference and absorbance optics, can be simultaneously integrated into an analytical ultracentrifuge to track different solutes in the samples. More recently, fluorescence optics became vital to analytical ultracentrifuges, improving their dynamic range and increasing sensitivity to detect solutes found in nanomolar-picomolar concentrations. Integration of fluorescent labeling can help researchers achieve more specific and sensitive analysis with smaller sample volumes required.

AUC Experiment Types and Detection Techniques

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Data Analysis and Interpretation

Cutting-edge analysis tools are necessary to convert raw signals into quantitative data for interpretation. Therefore, analytical centrifuge experiments must be complemented with software packages.

The forces generated during AUC experiments help create concentration gradients and sedimentation profiles for particles, illustrating the concentration's time-course profile at different radial distances. Data analysis tools can leverage sedimentation profiles to extract key biophysical parameters, such as molecular weight, equilibrium binding constants, shape, elongation and conformational flexibility.

Despite the innovation in data analysis, AUC data analysis presents challenges. Deconvolution of sample content is challenging for highly heterogeneous mixtures, making interpreting sedimentation profiles difficult. Aggregation artifacts can further skew analysis and cause miscalculations of molecular weights or binding constants. Furthermore, analytical centrifuge resolution may not be sufficient to distinguish between species with similar sedimentation profiles.

Applications of Analytical Ultracentrifugation

The molecular characterization capabilities of analytical ultracentrifugation are applicable in structural biology, biochemistry and biomedicine.

Protein and nucleic acid characterization

AUC experiments help researchers determine the molecular weights of proteins and nucleic acids while uncovering their monomeric or oligomeric states. Changes in sedimentation profiles can be used to infer conformational changes, such as protein folding.

Biopharmaceutical and drug discovery research

Analytical ultracentrifugation (AUC) characterizes biopharmaceuticals like recombinant proteins and antibodies. It helps characterize protein-protein interactions and protein-ligand interactions to confirm the bioactivity of therapeutics. Furthermore, AUC can aid in quality control and regulatory compliance by monitoring protein integrity, stability and aggregation.

Viral and nanoparticle analysis

Developing viral vectors and nanoparticles as drug carriers requires characterization by analytical centrifugation. Sedimentation profiles help determine particle size and integrity, quantify cargo packaging efficiency, and detect impurities in viral vectors, such as host DNA or replication-competent particles.

Matrix-free separation advantages

Thanks to its matrix-free and solution-based approach, analytical centrifugation provides advantages over separation analysis methods, including chromatography and surface plasmon resonance. Macromolecules can be analyzed without immobilization, which allows monitoring of conformational changes and binding kinetics in native biological conditions.

Challenges and Limitations of AUC

Despite its contribution to macromolecule characterization, analytical ultracentrifugation poses some limitations.

• Stringent sample purity requirements - additional purification steps are often needed to remove impurities and minor aggregates that can distort sedimentation profiles
• Solvent compatibility constraints - the solvent must be compatible with optical detection systems, limiting buffer choices
• Complex and dense data output - AUC produces rich datasets that can be overwhelming to interpret without specialized expertise
• Difficulty resolving overlapping species - species with similar sedimentation coefficients can be challenging to distinguish between
• Sensitivity limitations of optical systems - traditional optics may fail to detect low-abundance species or small molecules with very slow sedimentation behavior
• Extra preparation steps for fluorescence detection - while fluorescent optics improve sensitivity, they require sample labeling, adding additional time and preparation
• Temperature and handling variability - manual workflows can introduce variability in sample handling and temperature control

These challenges are partially addressed with AUC automation and AI-driven analysis. Automated workflows improve consistency in sample handling to reduce variability while regulating the temperature. AI-assisted algorithms in software packages can accelerate workflows and enhance accuracy by guiding model selection, parameter estimation, species classification and deconvoluting overlapping species.

References

1. Svedberg T. The Ultra-Centrifuge and the Study of High-Molecular Compounds. 1937.
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4. Bou-Assaf GM, Budyak IL, Brenowitz M, Day ES, Hayes D, Hill J, et al. Best practices for aggregate quantitation of antibody therapeutics by sedimentation velocity analytical ultracentrifugation. J Pharm Sci 2022;111(7):2121-2133.
5. Henrickson A, Gorbet GE, Savelyev A, Kim M, Hargreaves J, Schultz SK, et al. Multi-wavelength analytical ultracentrifugation of biopolymer mixtures and interactions. Anal Biochem 2022;652:114728.
6. Richter K, Wurm C, Strasser K, Bauer J, Bakou M, VerHeul R, et al. Purity and DNA content of AAV capsids assessed by analytical ultracentrifugation and orthogonal biophysical techniques. Eur J Pharm Biopharm 2023;189:68-83.
7. Demeler B. Methods for the design and analysis of analytical ultracentrifugation experiments. Curr Protoc 2024;4(2):e974.