Results tagged “MX,” from The SIBYLS Beamline

A recent IDAT publication from James, Chris, and Ken investigating the source of the point spread function in detectors using CCDs coupled fiber optic tapers.

The point-spread function (PSF) of a fiber-optic taper-coupled CCD area detector was measured over five decades of intensity using a 20 µm X-ray beam and 2000-fold averaging. The “tails” of the PSF clearly revealed that it is neither Gaussian nor Lorentzian, but instead resembles the solid angle subtended by a pixel at a point source of light held a small distance (27 µm) above the pixel plane. This converges to an inverse cube law far from the beam impact point. Further analysis revealed that the tails are dominated by the fiber-optic taper, with negligible contribution from the phosphor, suggesting that the PSF of all fiber-coupled CCD-type detectors is best described as a Moffat function.

the authors go on to suggest that:

…we expect that by fitting an expression for the spot-PSF convolution as described here directly to pixel values will result in more accurate spot intensity integrals than those currently being obtained using conventional profile-fitting methods (which assume that the intensity of a pixel is due exclusively to X-ray photons falling directly upon it). A “fitting approach” would eliminate systematic errors in background estimation arising from the tails and also suppress the influence of shot noise from X-ray photons falling on pixels outside the “true” spot area.

Holton, J. M., Nielsen, C. and Frankel, K. A.”The point-spread function of fiber-coupled area detectors” Journal of Synchrotron Radiation, 19, 6, (Nov, 2012)


DNA double strand break repair via nonhomologous end joining is a critical regulatory function that maintains genomic integrity. One of the major factors involved in this process is the XLF-XRCC4 protein complex. Although mutation of either XLF or XRCC4 leads to defects in break repair, the function of the XLF-XRCC4 complex has remained enigmatic. In their Paper of the Week, Hammel et al. used structure-based methods to elucidate the mechanism by which XLF-XRCC4 promotes double strand break repair. The authors solved the crystal structure of the XLF-XRCC4 complex using the N-terminal head domains of each protein and identified two key structural features that stabilize the complex: a key-lock interaction that links the two proteins and a set of hydrogen-bonding interactions that supplement the key-lock bond. Furthermore, the authors found that the C-terminal domain of XLF was crucial for promoting the formation and extension of filaments of the XLF-XRCC4 complex, allowing for interaction with DNA in a concentration-dependent manner. The crystal structure also identified a putative DNA-binding region, located at the XLF-XRCC4 interface, which was confirmed through addition of DNA oligomers. Subsequent addition of the break repair complex nucleator Ku and DNA ligase IV allowed the authors to develop a model for nonhomologous end joining in which Ku initially binds the damaged DNA site and recruits the XLF-XRCC4 complex, which is necessary for proper alignment of damaged DNA for repair by DNA ligase IV. Importantly, the elucidation of the structure of the XLF-XRCC4 scaffold provides potential targets for anticancer therapeutics.

Hammel M, Rey M, Yu Y, Mani RS, Classen S, Liu M, Pique ME, Fang S, Mahaney BL, Weinfeld M, Schriemer DC, Lees-Miller SP, Tainer JA. “XRCC4 protein interactions with XRCC4-like factor (XLF) create an extended grooved scaffold for DNA ligation and double strand break repair.” J Biol Chem. 2011 Sep 16;286(37):32638-50. Epub 2011 Jul 20.

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