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L‑Edge X‑ray Absorption Spectroscopy of Dilute Systems Relevant to Metalloproteins Using an X‑ray Free-Electron Laser
L-edge spectroscopy of 3d transition
metals provides important
electronic structure information and has been used in many fields.
However, the use of this method for studying dilute aqueous systems,
such as metalloenzymes, has not been prevalent because of severe radiation
damage and the lack of suitable detection systems. Here we present
spectra from a dilute Mn aqueous solution using a high-transmission
zone-plate spectrometer at the Linac Coherent Light Source (LCLS).
The spectrometer has been optimized for discriminating the Mn L-edge
signal from the overwhelming O K-edge background that arises from
water and protein itself, and the ultrashort LCLS X-ray pulses can
outrun X-ray induced damage. We show that the deviations of the partial-fluorescence
yield-detected spectra from the true absorption can be well modeled
using the state-dependence of the fluorescence yield, and discuss
implications for the application of our concept to biological samples