Progress towards imaging biological filaments using X-ray free-electron lasers

X-ray free-electron lasers (XFELs) are opening new frontiers in structural biology. The extreme brilliance of these highly coherent X-ray sources allows for ever smaller crystals to be used while still being able to diffract enough photons to provide sufficient data for structure determination. Biomolecules arranged into filaments are an important class of targets that are expected to greatly benefit from the continuous improvements in XFEL capabilities. Here we first review some of the state-of the-art research in using XFELs for the imaging of biological filaments. Extrapolating current trends towards single particle imaging, we consider an intermediate case where diffraction patterns from single filaments can be measured and oriented to form a 3D dataset. Prospects for using iterative projection algorithms (IPAs) for ab initio phase retrieval with such data collected from single filaments are illustrated by the reconstruction of the electron density of a B-DNA structure from simulated, noisy XFEL data.