New Approaches to Structure Determination from Uncrystallized Biomolecules

Jeffrey Donatelli, Lawrence Berkeley Lab

The development of X-ray free-electron lasers has enabled new experiments for studying uncrystallized biomolecules that were previously infeasible with traditional X-ray sources. One such emerging experimental technique is fluctuation X-ray scattering (FXS), where one collects a series of diffraction patterns, each from multiple particles in solution, using ultrashort X-ray pulses that allow snapshots to be taken below rotational diffusion times of the particles. The resulting images contain angularly varying information from which angular correlations can be computed, yielding several orders of magnitude more information than traditional solution scattering methods. In another technique known as single-particle diffraction (SPD), one collects several diffraction patterns, each from a single particle at a random orientation. In both experiments, determining molecular structure is challenging, since, in addition to solving the classical phase problem, one must also recover a 3D diffraction volume from a set of noisy 2D images taken from randomly oriented particles.

In this talk, we present the multi-tiered iterative phasing (M-TIP) algorithm for determining molecular structure from both FXS and SPD data, and show results from applying M-TIP to both simulated and experimental data.