The effect of K‐ fluorescence reabsorption of selenium on the performance of an imaging detector for protein crystallography

Abstract

AbstractProtein crystallography is an important technique in the determination of three dimensional atomic structures of proteins. In order to reconstruct the atomic structure of protein from experimental diffraction data, both the magnitude and phase of the atomic scattering factor should be known. The phase can be calculated using the Multiple Wavelength Anomalous Dispersion (MAD) method. Selenium (Se) is commonly used as an anomalous scatterer in the MAD method since the K ‐edge of Se (12.6 keV) is readily accessible on most synchrotron X‐ray sources. Recently, we proposed a novel direct X‐ray conversion imager for protein crystallography which employs amorphous Se for the X‐ray to charge conversion and an amorphous silicon flat panel thin film transistor array as the charge image readout method. For the protein crystallography X‐ray energy range (6‐20 keV), X‐ray interaction in Se is due to the photoelectric effect. Therefore there is a possibility of generation and reabsorption of K ‐fluorescence at or above the K ‐edge. The reabsorption of a fraction of the generated K ‐fluorescence is a random process which leads to fluctuations in conversion gain and hence addition of image noise. In this paper, the physics of K ‐fluorescence is explained and the probability of K ‐fluorescence reabsorption of Se for the useful X‐ray energy of protein crystallography is calculated. Finally, the possible consequences from K ‐fluorescence reabsorption of Se on the performance of a protein crystallography imager where Se is used both as an anomalous scatterer and a photoconductor is elucidated. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)