The properties and capabilities are described of an analytical ultracentrifuge, Beckman Optima XL-I, with both interference and absorption optics, that will be used by a group of Purdue research scientists to investigate the interactions between macromolecules in biologically important systems. The acquisition of this instrument is part of a plan to assemble an array of instruments in a center for biophysical studies of macromolecules. This array of instruments currently includes a surface plasmon resonance instrument, an isothermal calorimeter, and a ten-year old Beckman Model XL-A analytical ultracentrifuge. Future development of this center targets the addition of state-of-the-art fluorescence, FTIR and Raman spectrometers, plus a stopped flow rapid reaction instrument. This proposal for the XL-I analytical ultracentrifuge is submitted by a group of six investigators involved in studies of protein-protein, protein-nucleic acid, protein-ligand and protein- membrane interactions. These investigators have expertise in high resolution crystallographic structural studies as well as in biochemistry, biophysics and molecular genetics, and are interested in complementing their structural results with the biophysical measurements that can illuminate the details of assembly of large complexes, changes in interaction states with ligand substrates or effectors or with modifications that mimic cellular signaling states. Investigations of stability of large complexes and search for conditions of monodisperse preparations of membrane proteins are also proposed, as an aid to the crystallization of these difficult systems. Initial studies with the Beckman XL-A have demonstrated the usefulness of analytical ultracentrifugation in a wide variety of systems including: the binding affinity of membrane proteins for their soluble counterparts, assembly of animal viruses, signal transduction protein interactions, interactions between toxins and their cellular receptors as well as the assembly of large protein complexes for transmembrane transport. The XL-I will extend the range of studies into those involving weaker interactions as well as to studies where the optical characteristics of the samples are inappropriate for the absorption optics of the XL-A and will compliment and extend the information obtained by both structural and biophysical methods at Purdue.
