A revolution in biological imaging: Surpassing x-ray crystallography
Stefan Hau-Riege, Lawrence Livermore National Laboratory

Abstract:
The discovery of x-ray light has provided scientists with an extraordinary tool to probe the atomic structure of matter. The intensity distribution of scattered x-rays when illuminating an object provides information about the atomic positions in the sample. Since the first x-ray sources were rather weak, scientists imaged multiple copies of the same object simultaneously to boost the signal. By arranging these objects periodically in a crystal, the signal adds up “coherently” under certain conditions, meaning that the scattered intensity signal scales as N2 instead of N, where N is the number of objects -- x-ray crystallography was born. X-ray crystallography has revolutionized physics, chemistry, and biology [1], and it is still the workhorse of structural biology today. But it has its limitations: It can take years to produce acceptable biological crystals, if that is possible at all, and the confinement of a crystal can artificially alter the structure of a protein.

In an effort to return to single-particle imaging, it was realized that simply increasing the x-ray light output does not help since the x rays increasingly damage the sample. Solem and Baldwin achieved a breakthrough in biological imaging by delivering large x-ray radiation doses before radiation-induced structural damage occurs, allowing snapshot-imaging of biological processes [2]. Neutze et al. pointed out that the upcoming x-ray free electron lasers (XFELs) potentially provide sufficiently short pulse of high intensity to enable this diffract-and-destroy scheme in single biomolecules [3]. Now that XFELs are becoming available, we see this dream becoming reality. In this presentation, we will discuss the status, promise, challenges, limitations, and application of this revolutionary new imaging technique.

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