The first experimental data from single-particle scattering experiments from free electron lasers (FELs) are now becoming available. The first such experiments are being performed on relatively large objects such as viruses, which produce relatively
low-resolution, low-noise diffraction patterns in so-called diffract-and-destroy experiments. We describe a very simple test on the angular correlations of measured diffraction data to determine if the scattering is from an icosahedral particle. If this is confirmed, the efficient algorithm proposed can then combine diffraction data from multiple shots of particles in random unknown orientations to generate a full 3D image of the icosahedral particle. We demonstrate this with a simulation for the satellite tobacco necrosis virus (STNV), the atomic coordinates of whose asymmetric unit is given in Protein Data Bank entry 2BUK.
We demonstrate that a common-line method can assemble a 3D oversampled diffracted intensity distribution suitable for high-resolution structure solution from a set of measured 2D diffraction patterns, as proposed in experiments with an X-ray free ele
ctron laser (XFEL) (Neutze {it et al.}, 2000). Even for a flat Ewald sphere, we show how the ambiguities due to Friedels Law may be overcome. The method breaks down for photon counts below about 10 per detector pixel, almost 3 orders of magnitude higher than expected for scattering by a 500 kDa protein with an XFEL beam focused to a 0.1 micron diameter spot. Even if 10**3 orientationally similar diffraction patterns could be identified and added to reach the requisite photon count per pixel, the need for about 10**6 orientational classes for high-resolution structure determination suggests that about ~ 10**9 diffraction patterns must be recorded. Assuming pulse and read-out rates of 100 Hz, such measurements would require ~ 10**7 seconds, i.e. several months of continuous beam time.
