Adrienne Adams

Structural Characterisation of Bisintercalation in Higher-order DNA at a Junction-like Quadruplex

We report the single-crystal X-ray structure for the complex of the bisacridine bis-(9-aminooctyl(2-(dimethylaminoethyl)acridine-4-carboxamide)) with the oligonucleotide d(CGTACG)(2) to a resolution of 2.4A. Solution studies with closed circular DNA show this compound to be a bisintercalating threading agent, but so far we have no crystallographic or NMR structural data conforming to the model of contiguous intercalation within the same duplex. Here, with the hexameric duplex d(CGTACG), the DNA is observed to undergo a terminal cytosine base exchange to yield an unusual guanine quadruplex intercalation site through which the bisacridine threads its octamethylene linker to fuse two DNA duplexes. The 4-carboxamide side-chains form anchoring hydrogen-bonding interactions with guanine O6 atoms on each side of the quadruplex. This higher-order DNA structure provides insight into an unexpected property of bisintercalating threading agents, and suggests the idea of targeting such compounds specifically at four-way DNA junctions.

Differences between central and peripheral rat α-adrenoceptors revealed using binuclear ligands

We have used two homologous series of binuclear ligands, diacridines and diquinolines, and the radioligand receptor assay to compare the topology of alpha 1- and alpha 2-adrenoceptors in rat cerebral cortex and kidney membranes. While the chain length-dependence of affinity of the diacridines, as well as that of the diquinolines, for the alpha 1-adrenoceptors of these central and peripheral tissues are similar, we find marked differences in affinity profiles for interaction with central and peripheral alpha 2-adrenoceptors. In the context of our previously proposed model for the binding of diacridines and diquinolines to alpha-adrenoceptors the results suggest that the surface features of central and peripheral alpha 2-adrenoceptors differ in the area surrounding the noradrenaline binding site. This difference may prove to be of therapeutic relevance.

Structures of two minor-groove-binding quinolinium quaternary salts complexed with d(CGCGAATTCGCG)2 at 1.6 and 1.8 Å resolution

The structures of the complexes formed between two anticancer minor-groove-binding quinolinium quaternary salts and the dodecamer d(CGCGAATTCGCG)(2) have been refined to resolutions of 1.6 and 1.8 Angstrom. The complexes crystallized in space group P2(1)2(1)2(1) and the structures are isomorphous with previously solved dodecamer structures. The ligands both bind in the central AT region of the minor groove and although the crystallization conditions and structures of the ligands are very similar, they bind in opposing orientations. The structures are compared with two previously published structures of quinolinium quaternary salts, refined at 2.5 Angstrom resolution, complexed with d(CGCGAATTCGCG)(2) and an e(6)G-modified DNA.

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

A four-wavelength MAD experiment on a new brominated octanucleotide is reported here. d[ACGTACG(5-BrU)], C77H81BrN30O32P7, Mr (DNA) = 2235, tetragonal, P43212 (No. 96), a = 43.597, c = 26.268 A, V = 49927.5 A3, Z = 8, T = 100 K, R = 10.91% for 4312 reflections between 15.0 and 1.46 A resolution. The self-complementary brominated octanucleotide d[ACGTACG(5-BrU)]2 has been crystallized and data measured to 1.45 A at both 293 K and a second crystal flash 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 A, around the Br K edge at 0.92 A and the structure determined from a map derived from a MAD data analysis using pseudo-MIR methodology, as implemented in the program MLPHARE. This is one of the first successful MAD phasing experiments carried out at Sincrotrone Elettra in Trieste, Italy. The structure was refined using the data measured at 0.9003 A, anisotropic temperature factors and the restrained least-squares refinement implemented in the program SHELX96, and the helical parameters are compared with those previously determined for the isomorphous d(ACGTACGT)2 analogue. The asymmetric unit consists of a single strand of octamer with 96 water molecules. No countercations were located. The A-DNA helix geometry obtained has been analysed using the CURVES program.

Structure of the DNA octanucleotide d(ACGTACGT)2.

d(ACGTACGT), C(78)H(84)N(30)O(32)P(7).20H(2)O, M(r) (DNA) = 2170, tetragonal, P4(3)2(1)2 (No 96), a = 42.845 (1), b = 42.845(1), c = 24.804 (1) A, V = 45532.5 (2) A(3), z = 8,lambda(MoKalpha) = 0.71069 A, micro (MoKalpha) = 0.10 mm(-1), T = 295 K, R = 0.18 for 1994 unique reflections between 5.0 and 1.9 A resolution. The self-complementary octanucleotide d(ACGTACGT)(2) has been crystallized and its structure determined to a resolution of 1.9 A. The asymmetric unit consists of a single strand of octamer with 20 water molecules. It is only the second example of an octanucleotide having terminal A.T base pairs whose structure has been determined by X-ray crystallography. The sequence adopts the modified A-type conformation found for all octanucleotide duplexes studied to date with the helix bent by approximately 15 degrees and an average tilt angle of 0 degrees. Unusually the data collection was carried out using a 3 kW molybdenum sealed-tube source. The conformational details are discussed in comparison with other closely related sequences.