The crystal structure of deoxycytidine 5'-phosphate monohydrate (5'-dCMP), C9H14N3O7PH2O, has been determined by X-ray diffraction techniques. Two entirely independent investigations were carried out.In one case, 1149 intensity data were collected by multiple-film equiinclination Weissenberg technique using CuKa radiation. In the other case, 1248 three-dimensional intensity data were collected on a four-circle diffractometer, also using Cu Ka radiation. In both cases the structure was refined by full-matrix least-squares techniques. The final R values for the two investigations were 0.070 (from film data) and 0.035 (from diffractometer data); in the latter case, when secondary extinction corrections were applied the R value was reduced to 0.023. The unit cell is orthorhombic, space group P212121, with average a = 6.786, b = 11.345, and c = 16.769 Á. The nucleotide exists as a zwitterion with N(l) protonated by one of the phosphate protons. A comparison of the geometries of the N(l)-protonated and neutral cytosine derivatives shows that there are marked differences in the bond angles and bond distances involving N(l), C(2), and C(4). A novel glycosyl torsion angle, -6.0°, and sugar conformation C(3')ex"-C(2 ')ex0, where both C(3') and C(2') are on the opposite side of C(5 0, are observed in 5 '-dCMP. The conformation about the C(4')-C(5') bond is gauche-gauche, the only conformation observed so far for the known 5 '-nucleotides. A correlation between the furanoside ring conformations and the glycosyl torsion angles is presented. All available hydrogens participate in the hydrogen bonds. The molecules related by the screw axis parallel to the a axis are linked together by pairs of hydrogen bonds, resulting in an infinitely extended spiral around the screw axis. The crystal structure can be considered essentially a close packing of these infinite spirals linked by hydrogen bonds to each other and the water of crystallization.
We describe the origins, purposes, and findings of a national study to determine whether a large-scale program of blind proficiency testing in U.S. DNA laboratories is feasible and/or practical. Proficiency testing in clinical laboratories is reviewed, particularly as mandated by the Clinical Laboratory Improvement Acts and its role in the regulation of those laboratories. Proficiency testing in forensic urine drug testing labs is also briefly reviewed. Studies involving comparisons between open and blind proficiency testing are discussed. The clinical laboratory proficiency testing and regulation landscape provides the background for the DNA Act of 1994, and the congressional mandate to investigate blind proficiency testing in forensic DNA laboratories. Four models of blind proficiency testing are defined and discussed, along with the advantages and disadvantages of each and estimates of the costs of a large-scale program. The purposes of proficiency testing in a quality-assurance context are likewise discussed and related to the models and the arguments generally proffered for and against blind vs. open proficiency testing.
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