In Situ Structural Changes of Biological Macromolecules with BioSAXS

Biological small angle X-ray scattering (BioSAXS) is a nowadays well-established technique to study low resolution structural changes within biological macromolecules in their native environment. In addition, SAXS is a powerful tool for structural characterization and the quantitative analysis of flexible system as a highly complementary method to the high resolution methods of X-ray crystallography and NMR. In general, BioSAXS allows studying three dimensional low resolution structures in different chemical environments under defined conditions (e.g., temperature, pH, dielectric properties, etc.) using not only ab initio methods but also rigid body modeling. Furthermore, the shape (form factor) and interaction potential (structure factor) of the active quaternary protein structure can be assessed using the pair distance distribution function (PDDF) with the use of the generalized indirect Fourier transformation (GIFT). Especially in situ measurements of biological macromolecules have gained a lot of attention in the last decades due to the related structural changes and correlated bioactivity (i.e., native and denatured state).As model substance, Lysozyme was analyzed in terms of its quaternary structural changes upon in situ heat treatment and changes in the dielectric constant of the chemical environment. The validation of the form and structure factor components of the analyzed system allows determining the influence of the changed environmental conditions on the protein structure. The changed chemical environment causes an elongation of the previously globular protein structure. In aqueous environment, Lysozyme is stabilized via electrostatic stabilization. In ethanolic solution, the stabilization mechanism is changed, resulting in a change of structure (e.g., elongation).