Fast SAXS studies of sensitive biological samples
Small-angle X-ray scattering (SAXS) measurements of low-concentrated and radiation-sensitive biological samples using line collimation ensure short measurement time and allow to analyze the low-resolution 3D structure with excellent data quality.
SAXS of biological samples
SAXS provides valuable structural information of biological samples such as proteins, RNA, DNA and their complexes in solution under near-native conditions. However, obtaining high-quality SAXS data of biological samples is complicated due to low sample concentrations resulting in a low signal-to-noise of the scattering curves and low sample stability.
Line collimation offers several advantages to overcome these challenges:
- an increased signal-to-noise ratio compared to point collimation,
- excellent sample stability (no sample deterioration by radiation damage),
- high resolution (i.e. smaller accessible q-vectors) along with high flux ensuring short measurement times.
For data analysis and structural studies the line-smeared SAXS data are evaluated using standard programs.
Experimental and Results
In this study scattering curves of the protein lysozyme (5 mg/ml) obtained on laboratory SAXS ma-chines using line collimation (SAXSpace system with sealed tube) or point collimation (system with rotating anode) were compared. Exposure times and qmin for both experiments were the same. The 1D scattering curve and the pair-distance distribution function p(r) of the data measured with line-collimation perfectly matches the point-collimation data after desmearing (Figure 1). The IFT algorithm implemented in the GIFT1 program was used.
Fig. 1 Radial density distribution of lysozyme: comparison of theoretical and experimental data.
A low-resolution model was then reconstructed from the desmeared data using the program DAM-MIF2. As shown in Figure 2, the crystal structure (PDB 132l) clearly matches the low-resolution envelope.
Fig. 2 Low-resolution 3D envelope models of lysozyme and transportin-1
Further SAXS data was recorded for the 100 kDa human nuclear import receptor transportin-1 at a protein concentration of 8 mg/ml. As observed for lysozyme, the low-resolution model obtained with DAMMIF from scattering data measured in line collimation is in perfect agreement with the crystal structure (PDB 4FDD). This demonstrates that obtaining SAXS data using line-collimation is excellently suited to study biological samples in solution.
1 O. Glatter, J. Phys.: Condens. Matter 18 (2006) 2403
2 D. Franke, D.I. Svergun, J. Appl. Cryst. 42 (2009) 342-346