Abstract
Small-angle X-ray scattering is widely utilized to study biological macromol-ecules in solution. For samples containing specific (e.g. metal) atoms, additional information can be obtained using anomalous scattering. Here, measuring samples at different energies close to the absorption edges of relevant elements provides specific structural details. However, anomalous small-angle X-ray scattering (ASAXS) applications to dilute macromolecular solutions are challenging owing to the overall low anomalous scattering effect. Here, pilot ASAXS experiments from dilute solutions of ferritin and cobalt-loaded apoferritin are reported. These samples were investigated near the resonance X-ray K edges of Fe and Co, respectively, at the EMBL P12 bioSAXS beamline at PETRA III, DESY. Thanks to the high brilliance of the P12 beamline, ASAXS experiments are feasible on dilute protein solutions, allowing one to extract the Fe- or Co-specific anomalous dispersion terms from the ASAXS data. The data were subsequently used to determine the spatial distribution of either iron or cobalt atoms incorporated into the ferritin/apoferritin protein cages.
Highlights
Soft matter and biological systems are actively utilized in applied science and biophysical processes (Henderson et al, 2005; Holm et al, 2001)
For the determination of the p(r) function the program GNOM from the ATSAS suite was used (Manalastas-Cantos et al, 2021; Franke et al, 2017; Svergun, 1992). p(r) is an indirect Fourier transformation yielding the distribution of distances between volume elements weighted by the excess density distribution and can be calculated as described by Svergun & Koch (2003): 3. Results and discussion Fig. 1 shows smallangle X-ray scattering (SAXS) data measured from the Bovine serum albumin (BSA) samples at different X-ray energies for the Anomalous small-angle X-ray scattering (ASAXS) measurements across the Co and Fe absorption edges
The presented ASAXS analysis demonstrates the incorporation of cobalt into the inner core of apoferritin, the cobalt moiety forming a structure similar to that of a hollow sphere
Summary
Soft matter and biological systems are actively utilized in applied science and biophysical processes (Henderson et al, 2005; Holm et al, 2001). One of the key aspects of investigating such systems relates to obtaining information about the spatial distribution of counter-ions around the biopolymers in solution (Ballauff & Jusufi, 2006). Answering such scientific questions is not trivial and needs advanced analysis methods. Anomalous small-angle X-ray scattering (ASAXS) is one method that allows one to probe the structure of biopolymers and obtain element-specific information. In an ASAXS experiment, one utilizes the anomalous dispersion of X-rays by specific elements associated with these biopolymers to assess their (time-preserved) spatial distributions.
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