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Z Shakked

Publications and source records attributed to Z Shakked.

29 records · Page 2Linked to original sources
PubMed1986

Ordered water structure in an A-DNA octamer at 1.7 A resolution.

The crystal structure of the deoxyoctamer d(G-G-Br U-A-BrU-A-C-C) was refined to a resolution of 1.7 A using combined diffractometer and synchrotron data. The analysis was carried out independently in two laboratories using different procedures. Although the final results are identical the comparison of the two approaches highlights potential problems in the refinement of oligonucleotides when only limited data are available. As part of the analysis the positions of 84 solvent molecules in the asymmetric unit were established. The DNA molecule is highly solvated, particularly the phosphate-sugar back-bone and the functional groups of the bases. The major groove contains, in the central BrU-A-BrU-A region, a ribbon of water molecules forming closed pentagons with shared edges. These water molecules are linked to the base O and N atoms and to the solvent chains connecting the O-1 phosphate oxygen atoms on each strand. The minor groove is also extensively hydrated with a continuous network in the central region and other networks at each end. The pattern of hydration is briefly compared with that observed in the structure of a B-dodecamer.

Amino Acid Sequence↗
PubMed1984

Base-stacking interactions in double-helical DNA structures: experiment versus theory.

Atom-atom potential energy calculations have been undertaken for deriving stacking energies in double-helical structures. A comparison between the energy patterns of A- and B-type double-helical fragments determined by single-crystal X-ray diffraction methods versus idealized uniform models based on fibre diffraction data shows that the van der Waals stacking energy is largely sensitive to local changes in the relative orientation of adjacent base pairs. The sequence-dependent conformational variability observed in the high-resolution structures appears to be a consequence of the equipartitioning of the stacking energy along the double helix. The large energy variations expected for a uniform structure are dampened considerably in the observed structures by means of local changes in conformational features such as helix rotation and roll angles between base pairs.

DNA↗
PubMed1983

Sequence-dependent conformation of an A-DNA double helix. The crystal structure of the octamer d(G-G-T-A-T-A-C-C).

The crystal structures of the synthetic self-complementary octamer d(G-G-T-A-T-A-C-C) and its 5-bromouracil-containing analogue have been refined to R values of 20% and 14% at resolutions of 1.8 and 2.25 A, respectively. The molecules adopt and A-DNA type double-helical conformation, which is minimally affected by crystal forces. A detailed analysis of the structure shows a considerable influence of the nucleotide sequence on the base-pair stacking patterns. In particular, the electrostatic stacking interactions between adjacent guanine and thymine bases produce symmetric bending of the double helix and a major-groove widening. The sugar-phosphate backbone appears to be only slightly affected by the base sequence. The local variations in the base-pair orientation are brought about by correlated adjustments in the backbone torsion angles and the glycosidic orientation. Sequence-dependent conformational variations of the type observed here may contribute to the specificity of certain protein-DNA interactions.

Base Composition↗
PubMed1983

Proton nuclear Overhauser effect study of the structure of a deoxyoligonucleotide duplex in aqueous solution.

Nuclear Overhauser enhancements (NOEs) have been observed between some of the nonexchangeable protons of the self-complementary octanucleoside heptaphosphate d(G-G-T-A-T-A-C-C) in conditions under which duplex formation takes place. Comparison of inter- and intraresidue NOEs has made possible the assignment of all the aromatic and 1'- and 2'-ribosyl signals. In particular, and with the exception of the 5' terminal, NOEs of comparable magnitude are observed between H-8 of purines or H-6 of pyrimidines and pairs of anomeric protons. There are the H-1's of the nucleotide containing the relevant aromatic proton and those of the adjacent residue in the 5' direction. Differentially evolving NOEs are also generated between aromatic protons and the C-2' methylene protons on both the same residue and on the neighboring 5' residue. The relative magnitudes of these NOEs are discussed in terms of overall helical geometry and are consistent with a generally accepted B-type DNA model (R. Chandrasekaran and S. Arnott, personal communications).

Base Sequence↗
PubMed1981

Crystalline A-dna: the X-ray analysis of the fragment d(G-G-T-A-T-A-C-C).

An A-DNA type double helical conformation was observed in the single crystal X-ray structure of the octamer d(G-G-T-A-T-A-C-C), 1, and its 5-bromouracil-containing analogue, 2. The structure of the isomorphous crystals (space group P61) was solved by a search technique based on packing criteria and R-factor calculations, with use of only low order data. At the present stage of refinement the R factors are 31% for 1 and 28% for 2 at a resolution of 2.25 A (0.225 nm). The molecules interact through their minor grooves by hydrogen bonding and base to sugar van der Waals contacts. The stable A conformation observed in the crystal may have some structural relevance to promoter regions where the T-A-T-A sequence is frequently found.

Base Sequence↗
PubMed1980

Interactions of tris buffer with nucleotides: the crystal structure of tris(hydroxymethyl)methylammonium adenosine 5'-diphosphate dihydrate.

The crystal and molecular structures of the Tris salt of adenosine 5'-diphosphate were determined from X-ray diffraction data. The crystals are monoclinic, space P21, and Z = 2 with a=9.198 (2) A, b=6.894 (1) A, c=18.440 (4) A, and beta = 92.55 (2) degrees. Intensity data were collected on an automated diffractometer. The structure was solved by the heavy-atom technique and refined by least squares to R = 0.047. The ADP molecule adopts a folded conformation. The conformation about the glycosidic bond is anti. The conformation of the ribose ring is close to a perfect C(2')-endo-C-(3')-exo puckering. The conformation about C(4')-C(5') is gauche-gauche, similar to other nucleotide structures. The pyrophosphate chain displays a nearly eclipsed geometry when viewed down the P-P vector, unlike the staggered conformation observed in crystal structures of other pyrophosphates. The less favorable eclipsed conformation probably results from the observed association of Tris molecules with the polar diphosphate chain through electrostatic interactions and hydrogen bonds. Such interactions may play an important role in Tris-buffered aqueous solutions of nucleotides and metal ions.

Adenosine Diphosphate↗