Reporter molecules as probes of DNA conformation: structure of a crystalline complex containing 2-methyl-4-nitroaniline ethylene dimethylammonium hydrobromide--5-iodocytidylyl (3'-5')guanosine

2-Methyl-4-nitroaniline ethylene dimethylammonium hydrobromide forms a crystalline complex with the self-complementary dinucleoside monophosphate, 5- iodocytidylyl (3'-5')guanosine. The crystals are tetragonal, with a = b = 32.192 A and c = 23.964 A, space group P4(3)2(1)2. The structure has been solved to atomic resolution by Patterson and Fourier methods, and refined by full matrix least squares. 5- Iodocytidylyl (3'-5')guanosine molecules are held together in pairs through Watson-Crick base-pairing, forming an antiparallel duplex structure. Nitroaniline molecules stack above and below guanine-cytosine pairs in this duplex structure. In addition, a third nitroaniline molecule stacks on one of the other two nitroaniline molecules. The asymmetric unit contains two 5- iodocytidylyl (3'-5')guanosine molecules, three nitroaniline molecules, one bromide ion and thirty-one water molecules, a total of 160 atoms. Details of the structure are described.