Biomolecular Ultracentrifuge Facility
Beckman XL-I and XL-A instruments

The Analytical UltraCentrifuge (AUC) is the perfect union between a preparative, high-speed centrifuge and a dual-beam spectrophotometer. This union allows for detection of the movement of bio-molecules in real time. How rapidly a molecule moves in an aqueous solution depends on its size, shape and buoyancy. We can determine a 'Sedimentation Velocity' (S) for a particular particle (protein or DNA molecule, or a larger nucleo-protein complex) by subjecting it to an extremely high centrifugal force (at up to 60,000 RPM). This measurement gives very detailed information about the size and conformation of the particle or complex. Importantly, such a measurement allows for a determination of the homogeneity (purity) of the material, thus this technique is often a prelude to a crystallographic, 3-D structure determination, which requires highly pure materials. This technique will reveal if the protein self-associates, forming dimers, trimers or larger oligomers.

Alternatively, we can subject the particle or complex to a lesser centrifugal force and allow its natural buoyancy to counteract the centrifugal force. When this equilibrium is reached, the distance the particle or complex has traveled allows its molecular size. Such 'Sedimentation Equilibrium' experiments also allow for the determination of the self-association scheme of a protein, which often is an important factor in the biological function of the protein.

Practically, the sample solution is loaded into the sample cell, and is inserted into a titanium rotor capable of very high-speed rotation. The rotor-cell assembly is placed in the AUC with the optical detector.

In the Fall of 2005, our facility will become only the second facility in the world featuring an AUC retro-fitted with an AVIV fluorescence optical system. This system will increase the detection sensitivity by orders of magnitude and allow for detection of single, labeled molecules from within complex, impure mixtures.