Determination of multiwavelength anomalous diffraction coefficients at high x-ray intensity

TL;DR

This paper develops a method to accurately determine multiwavelength anomalous diffraction coefficients under high x-ray intensity, enabling improved phase determination in femtosecond crystallography with XFELs.

Contribution

It revisits the generalized Karle-Hendrickson equation to account for atomic fluctuations and proposes an experimental procedure for MAD coefficient determination at high intensities.

Findings

Experimental procedure for MAD coefficient measurement proposed

Accounted for atomic fluctuations induced by intense x-ray pulses

Validated method with crystalline samples of known structures

Abstract

The high-intensity version of multiwavelength anomalous diffraction (MAD) has a potential for solving the phase problem in femtosecond crystallography with x-ray free-electron lasers (XFELs). For MAD phasing, it is required to calculate or measure the MAD coefficients involved in the key equation, which depend on XFEL pulse parameters. In the present work, we revisit the generalized Karle-Hendrickson equation to clarify the importance of configurational fluctuations of heavy atoms induced by intense x-ray pulses, and investigate the high-intensity cases of transmission and fluorescence measurements of samples containing heavy atoms. Based on transmission/fluorescence and diffraction experiments with crystalline samples of known structures, we propose an experimental procedure to determine all MAD coefficients at high x-ray intensity, which can be used in \emph{ab initio} phasing for unknown structures.