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, 147 (1984). The crystal structure of 5-(propyn-I-yl)-I-(P-D-arabinofuranosyl)uracil, an analog of the active antiherpes nucleoside I-(P-D-arabinofuranosyl)thymine, was determined by X-ray diffraction. The compound crystallizes in the space group P2,2,2, with a = 4.925(1), b = 14.326(2), c = 17.454 A. Reflections were measured on a diffractometer and the structure was solved by direct methods. Least-squares refinement converged at R = 0.032 for 1159 observed reflections. The sugar ring exhibits an 'E or a C(3')endo conformation with a pseudorotation angle P = 28.3" and puckering amplitude T,,, = 3 1.7". The orientation of the -CH20H side chain is g'. The base is in an at~ti conformation with respect to the sugar ring, with a glycosidic torsion angle x = 33.7". Changes in the C(5)-C(6) and C (6) [Traduit par le journal]

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5-Chloro-7-nitro-2,3-dihydroxyquinoxaline crystallizes in the triclinic system, space group P1, with a = 9.280 (2), b = 12.870 (3), c = 8.503 (2)/k, a= 90.95 (2),fl= 97.08 (2),),= 118.48 (2) °. 3085 independent reflexions were measured on a single-crystal diffractometer. The structure was solved by direct methods and refined by full-matrix least squares to a final R of 0.046. The compound exists as crystals in the keto form only (the average C-O bond distance is 1.215 A). The carbon ring is aromatic with an average C-C bond distance of 1.387 A; the heterocyclic ring is not aromatic. The molecules are linked by strong N-H...O hydrogen bonds.

Multidomain proteins provide special problems in the application of the molecular replacement method of structure determination. The structure of the Fab fragment from the autoimmune poly(dT)-specific antibody HED10 has been determined using molecular replacement. An analysis of the effects of varying the model and the parameters used in the rotation function indicates that dividing the molecule into individual relatively rigid domains simplifies interpretation of the results, and that the optimal parameters depend on the molecule under study.

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