TABLE OF CONTENTS The interactions and oligomeric states of macromolecules are often the source of their biological function. These can be transient or permanent self-interactions, or interactions with other molecules protein protein, protein nucleic acid and protein lipid. Sometimes the existence of these interactions becomes apparent when an X-ray crystal structure is solved, and then analytical ultracentrifugation (AUC) can prove a vital accessory technique to show that this observed oligomerisation occurs in solution, i.e., in a state unconstrained by crystal contacts. On this point we should however note that (i) protein crystals are frequently 50 70% solvent, so the argument that the crystalline state presents the molecule in an unnatural environment where its conformation and interactions can be expected to be artefactual may be specious and (ii) protein protein interactions in a biologically significant oligomer can be expected to be stronger than the crystal contacts and therefore to be preserved accurately in the crystalline state. Nevertheless, by showing, for example, that a dimer is present in the crystal structure and in solution, a major insight into the mechanism of the protein as a complex is obtained, and AUC is much the best method of doing this. AUC wins over other hydrodynamic techniques, such as dynamic light scattering and analytical gel filtration, because analysis of the data obtained is not hobbled, as in these techniques it can be, by the assumption that the macromolecule of interest is globular, or by interaction with a solid matrix. Oligomerisation can be measured either using...
