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High-intensity phasing with x-ray free-electron lasers
Sang-Kil Son, Lorenzo Galli, Henry N. Chapman, and Robin SantraEuropean XFEL and DESY Photon Science Users' Meeting
(DESY, Hamburg, Germany, January 28-30, 2015) [poster]
X-ray free-electron lasers (XFELs) show promise for revealing molecular structure using serial femtosecond crystallography (SFX), but the associated phase problem remains largely unsolved. Many of the ab initio methods that are used for phasing diffraction data collected with synchrotron radiation employ anomalous scattering from heavy atoms, for example, multiwavelength anomalous diffraction (MAD). Because of the extremely high intensity of XFELs, samples experience severe and unavoidable electronic radiation damage, especially to heavy atoms. The scattering factors of heavy atoms are dramatically changed due to ionization during an intense x-ray pulse, which hinders direct implementation of those phasing techniques with XFELs. A generalized version of MAD at high x-ray intensity has been proposed previously, suggesting that element-specific and fluence- dependent electronic damage could be used to determine phases. Here, we show two recent results towards a new high-intensity phasing (HIP). We demonstrate that simulated SFX data of Cathepsin B can be phased by the different ionization degree of S atoms between two datasets at low and high x-ray fluences, similar to the technique of radiation induced phasing. We present an experimental evidence of the different ionization degree of Gd atoms between two datasets obtained from a Gd derivative of lysozyme microcrystals, which is used to identify the positions of the Gd atoms. New opportunities and challenges of high-intensity phasing methods with XFELs will be discussed.
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