Determination by MAD-DM of the structure of the DNA duplex d[ACGTACG(5-BrU)](2) at 1.46 angstrom and 100 K

A four-wavelength MAD experiment on a new brominated octanucleotide is repo rted here. d[ACGTACG(5-BrU)], C77H81BTN30O32P7, M-r, (DNA) = 2235, tetragon al, P4(3)2(1)2 (No. 96), a = 43.597, c = 26.268 Angstrom, V = 49927.5 Angst rom(3), Z = 8, T = 100 K, R = 10.91% for 4312 reflections between 15.0 and 1.46 Angstrom resolution. The self-complementary brominated octanucleotide d[ACGTACG(S-BrU)](2) has been crystallized and data measured to 1.45 Angstr om at both 293 K and a second crystal hash frozen at 100 K. The latter data collection was carried out to the same resolution at the four wavelengths 0.9344, 0.9216, 0.9208 and 0.9003 Angstrom, around the Br K edge at 0.92 An gstrom and the structure determined from a map derived from a MAD data anal ysis using pseudo-MIR methodology, as implemented in the program MLPHARE. T his is one of the first successful MAD phasing experiments carried out at S incrotrone Elettra in Trieste, Italy. The structure was refined using the d ata measured at 0.9003 Angstrom, anisotropic temperature factors and the re strained least-squares refinement implemented in the program SHELX96, and t he helical parameters are compared with those previously determined for the isomorphous d(ACGTACGT)(2) analogue. The asymmetric unit consists of a sin gle strand of octamer with 96 water molecules. No countercations were locat ed. The A-DNA helix geometry obtained has been analysed using the CURVES pr ogram.