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Internet Electronic Journal of Molecular Design - IEJMD, ISSN 1538-6414, CODEN IEJMAT
| ABSTRACT - Internet Electron. J. Mol. Des. May 2005, Volume 4, Number 5, 342-354 |
Microcrystal Structure Determination Using Synchrotron Radiation and a CCD Detector
Michael J. Bucknum, Doletha M. E. Szebenyi, Emil Lobkovsky, Shirish A. Patil, Robert S. Klein, and Eduardo A. Castro
Internet Electron. J. Mol. Des. 2005, 4, 342-354
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Abstract:
The structure determination of the nucleoside analog metRITone,
2-methylthio-6-β-D-ribofuranosylimidazo[2,1-f][1,2,4]triazin-4(3H)-one,
is presented. This molecule forms needle-shaped crystals with
extremely small external dimensions (≈ 5 μm × 10 μm
× 100 μm). It
crystallizes in space group P212121
with a = 21.368(5) Å, b = 4.790(5) Å and c = 12.876(5) Å.
Single crystal diffraction data
collected on a four circle diffractometer, using a sealed tube
generator with a Cu anode and a scintillation detector, are compared
to data collected with synchrotron radiation and a CCD (charge
coupled device) detector. Least squares refinement of the two
structures of metRITone was carried out with the program SHELXL-93.
Isotropic refinement of 21 non-hydrogen atoms was carried forth
to an initial reliability index (R1) of about 9% for the diffractometer
data and 6% for the synchrotron-CCD data. None of the hydrogen
atoms could be conclusively located in the difference Fourier maps
generated in the structure solution step. For the synchrotron-CCD
data, anisotropic least squares refinement of non-hydrogen atoms,
with addition of "riding" hydrogens, was carried out using
SHELXL-93; the final crystallographic reliability factor (R1) was
0.0315 (wR2 = 0.0731), at a resolution of 0.95 Å. The number of
reflections in the diffractometer dataset was insufficient for
anisotropic refinement. The metRITone structure determined using
the synchrotron-CCD dataset is described below; it is consistent with
structures of related compounds. Comparison of the quality of the
structure solution and refinement, in terms of the combined figure of
merit (CFOM) and the signal-to-noise ratio as a function of
diffraction resolution, has been made between the two datasets; the
synchrotron-CCD dataset appears superior to the diffractometer
dataset by these measures, in addition to being more complete.
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