We have determined the single crystal x-ray structure of the synthetic DNA hexamer d(pCpGpCpGpCpG) in two different crystal forms. The hexamer pCGCGCG has the Z-DNA conformation and in both cases the asymmetric unit contains more than one Z-DNA duplex. Crystals belong to the space group C222(1) with a = 69.73, b = 52.63, and c = 26.21 A, and to the space group P2(1) with a = 49.87, b = 41.26, c = 21.91 A, and gamma = 97.12 degrees. Both crystals show new crystal packing modes. The molecules also show striking new features when compared with previously determined Z-DNA structures: 1) the bases in one duplex have a large inclination with respect to the helical axis, which alters the overall shape of the molecule. 2) Some cytosine nitrogens interact by hydrogen bonding with phosphates in neighbor molecules. Similar base-phosphate interactions had been previously detected in some B-DNA crystals. 3) Basepair stacking between the ends of neighbor molecules is variable and no helical continuity is maintained between contiguous hexamer duplexes.
The structure of C5H5FeR(+·) ions was studied by tandem mass spectromerry that included the neutralization-reionization (NR) method. Halogen-containing species (R = F, Cl, Br, I) showed fragmentation that was consistent with a structure that has the cyclopentadienyl ring and R as separate ligands at the metal atom (structure A). This structure also was identified for C5H5FeO(+) and CpFeOH(+·) ions, but these species also easily isomerized to metal-cyclopentadiene structure, RC5H5Fe(+·) (B), followed by hydrogen rearrangement(s) and CO loss. B was the dominant structure of C5H5FeH(+·) and C5H5FeC6H 5 (+·) ions. All ions that have structure A showed recovery signals in their NR mass spectra that indicated that their stable neutral counterparts were generated. The NR mass spectra also provided complementary information about the structure of ions before neutralization and after reionization.
Fragmentation of the acetyl-and propionyl-ferrocenes under electron impact together with the rupture of C-CO bonds, characteristic of ketones, also includes elimination of water and migration of the OH group to the central metal atom, which is due to keto-enol tautomerization of the acyl group in molecular or fragment ions.
+ ions (R = H, Me), which are abundant in the mass spectra of many types of coordination compounds, were studied by the MNDO method. In all cases the most stable isomers correspond to the zinc ion coordinated with the nitrogen(s) of the nitrile ligand. For IZn(NCR),J +, the N-Zn-N angles are -108'.
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