Perspectives on single molecule diffraction using the X-ray free electron laser

A free electron laser capable of operating at the short wavelengths and high energies characteristic of hard X-rays, could be a potentially powerful new tool for the determination of the structures of single biological macromolecules, subcellular organelles, or whole cells at high resolution. Current methods using relatively low-intensity, incoherent X-ray sources depend upon the high signal gain achieved by the coherent scattering of a large number of identical molecules from a crystalline sample. The prerequisite crystallization of biological macromolecules is a major bottleneck in this process and the discrete nature of the Fourier transforms of crystalline materials also complicates the process of the reconstruction of the original molecular structure from its corresponding diffraction pattern. The ability to record detailed diffraction patterns from single molecules would eliminate the need for crystalline samples. Furthermore, the continuous molecular transforms of single molecules can be sampled finely enough to enable the unrecorded phase information to be retrieved from the set of structural amplitudes without recourse to the additional, a priori structural information that is generally required for phase assignment using sets of diffraction amplitudes from crystalline samples. Currently, diffraction experiments with single biological macromolecules or other non-crystalline material are hampered by the significant radiation damage to the samples that occurs as a result of the very large radiation doses that are required. Model calculations indicate that this problem may be solved by using the ultra-short (< 100 fs) pulses of hard X-radiation that a new generation of X-ray free electron lasers should be capable of producing. In anticipation that the current, rapid development of this technology will make this potential a reality in the near future, the theoretical foundations of this new methodology are already being laid.