Resonant Soft X-Ray Scattering Provides Protein Structure with Chemical Specificity

We introduce resonant soft X-ray scattering (RSoXS) as an approach to study the structure of proteins and other biological molecules in solution. Scattering contrast calculations suggest that RSoXS has comparable or even higher sensitivity than hard X-ray scattering because of contrast generated at the absorption edges of constituent elements, such as carbon and oxygen. Here, we demonstrate that working near the carbon edge reveals the envelope function of bovine serum albumin, using scattering volumes of 10⁻⁵ μL that are multiple orders of magnitude lower than traditional scattering experiments. Furthermore, tuning the X-ray energy within the carbon absorption edge provides different signatures of the size and shape of the protein by revealing the density of different types of bonding motifs within the protein. The combination of chemical specificity, smaller sample size, and enhanced X-ray contrast will propel RSoXS as a complementary tool to existing techniques for the study of biomolecular structure. Ye et al. demonstrate resonant soft X-ray scattering as a tool to examine the shape and size of proteins in solution. Working at X-ray absorption edges provides an opportunity to selectively highlight structure associated with specific chemistries, thereby establishing a promising technique for the study of complex biological assemblies.