SOLUTION X-RAY SCATTERING STUDIES ON PROTEIN INTERACTION

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Solution x-ray scattering data from biological macromolecules often contain both the form factor, due to the three dimensional structure of the molecule, and the structure factor, which reflects intermolecular interactions. We will characterize and minimize the latter effect by screening a number of different solution conditions, for each protein under study, prior to studying the protein structures. Many proteins are less prone to aggregate under certain conditions, such as in the presence of salt ions or inert chemicals such as glycerol. Radiation induced aggregation can often be suppressed in these conditions as well. We have been studying lysozyme intermolecular interactions in a variety of conditions, aimed at establishing generalized methods to predict solution conditions suitable for solution x-ray scattering studies. We use the sum of potential terms including a hard sphere term, a long-range repulsive electrostatic term, and a short range attractive potential, which itself contains Van der Waals, hydrophobic, dipole-dipole interactions as well as multipole interactions in simulations. As empirically predicted, we have verified that glycerol weakens the attractive interactions among lysozyme molecules. In-depth analysis will determine whether this change is caused by an altered dielectric constant of the bulk solvent, which would influence electrostatic potential, or by modified short-range hydrophobic forces by glycerol molecules acting on the surface of the macromolecules. We also study effects of various salts, such as Cl- and F-, which are expected to have identical effects on electrostatic potentials due to the same net charge while they are likely to have different effects on short range forces due to different ionic radii. Our results so far show that the lysozyme pair-potential is more attractive in the presence of NaCl than NaF. Quantitative interpretation of this result is being developed, but NaF may be a better alternative to keep proteins less aggregating in solution x-ray studies. We will a