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The crystal structure of the telomeric sequence d(UBrAGG) interacting with an anthraquinone derivative has been solved by MAD. In all previously studied complexes of intercalating drugs, the drug is usually sandwiched between two DNA base pairs. Instead, the present structure looks like a crystal of stacked anthraquinone molecules in which isolated base pairs are intercalated. Unusual base pairs are present in the structure, such as G·G and A·UBr reverse Watson-Crick base pairs. © 2010 International Union of Crystallography All rights reserved., We acknowledge support from the European Project MPRN-CT-2000-00009 and from the Ministerio de Ciencia e Innovación, project No. BFU 2009-10380.

The coiled-coil structure formed by the complex of the DNA duplex d(ATATATATAT)2with pentamidine is presented. The duplex was found to have a mixed structure containing Watson–Crick and Hoogsteen base pairs. The drug stabilizes the coiled coil through the formation of cross-links between neighbouring duplexes. The central part of the drug is found in the minor groove as expected, whereas the charged terminal amidine groups protrude and interact with phosphates from neighbouring molecules. The formation of cross-links may be related to the biological effects of pentamidine, which is used as an antiprotozoal agent in trypanosomiasis, leishmaniasis and pneumonias associated with AIDS. The DNA sequence that was used is highly abundant in most eukaryotic genomes. However, very few data are available on DNA sequences which only contain A·T base pairs.

The structure of the oligonucleotide duplex d(AAATATTT)(2) has been obtained in two crystal forms. In both cases the duplexes show Watson-Crick base pairs and are organized as a helical arrangement of stacked oligonucleotides. The helices contain either 11 duplexes in seven turns (form I) or 14 duplexes in nine turns (form II). As a result, the crystals have rather large unit cells. Such an organization of DNA duplexes has not been previously described in oligonucleotide crystal structures. The columns of stacked duplexes are organized in a pseudohexagonal arrangement. They do not show any direct lateral interactions; instead, these are indirectly mediated by shared and disordered hydrated ions. Such interactions are similar to those found in DNA fibres. The structural parameters of the individual base steps coincide with those found in other structures which contain CG terminal base pairs.

A program has been developed to determine the diffraction pattern given by partially ordered fibres formed by macromolecules with helical symmetry. It is particularly useful for visualizing the splitting of layer lines typical of coiled coils. The program produces as output the diffraction diagram calculated for helices that are oriented along their axis but are randomly oriented in other directions. The results can be numerically analyzed and also visualized on-screen. The program has been applied to the diffraction patterns given by DNA and protein coiled coils.

The deoxyoligonucleotide d(CGCAATTGCG) has previously been crystallized in four different space groups. The crystals diffract to moderate resolution (2.3-2.9 A). Here, a fifth crystal form that diffracts to higher resolution (1.6 A) is presented which was obtained thanks to the use of Co2+ and cryogenic temperatures. The availability of five different crystal structures allows a thorough analysis of the conformational variability of this DNA sequence. It is concluded that the central hexamer sequence CAATTG has a practically constant conformation under all conditions, whilst the terminal base pairs at both ends vary considerably as a result of differing interactions in the crystals. The new crystal structure presented here is stabilized by guanine-Co2+-guanine interactions and the formation of C1+ -G8.C3 triplexes between neighbouring duplexes. As a result of the higher resolution of the crystal structure, a more regular structure was obtained and a clear definition of the spine of hydration was observed which was not visible in the four previous structures.

The structure of the hydrochloride salt of amino-11-undecanoic acid cyclic dimer (C 22 H 42 N 2 O 2 .HCl) corresponds to two monomeric units of nylon 11. The intensity data were collected at 203 K. Two molecules are linked by two short hydrogen bonds with O…O distances of 2.406(6) and 2.464 (6) A between half-protonated amide-O atoms. The methylene groups are all-trans except for two terminal groups, which display a (-)synclinal form (gauche). The ring is closed through bent amide groups, which are reminiscent of β-turns in proteins. The all-trans methylene chains along both sides of the ring are not parallel, but are organized at an angle of about 10.9 (6)°.

The title compound, C8H12N2O5, does not show conformational symmetry in the crystal. It is stabilized by intermolecular hydrogen bonds producing a three-dimensional network of hydrogen bonds. The two mol­ecules in the asymmetric unit differ in the conformation of the six-membered ring and the way in which each participates in the hydrogen-bonding network.