Nucleic acids helical nature

Three classes of nucleic acid triple helices have been described for oligonucleotides containing only natural units. They differ according to the base sequences and the relative orientation of the phosphate-deoxyribose backbone of the third strand. All the three classes involve Hoogsteen or reverse Hoogsteen-like hydrogen bonding interaction between the triple helix form-... 

Especially attractive was the possibility to connect nucleosides, as has been realized, for instance, with the hexathymidine 141 and with the elongated and alternating strands 142 and 143. These compounds represent artificial oligonucleosides, which may interact with natural polynucleotides or nucleic acids. On treatment with Cu(i), 142 and 143 gave the double-helical complexes 144 and 145, respectively, inside-out analogues of double-stranded nucleic acids, which may be termed deoxy-... 

James D. Watson (1928-) and Francis H. C. Crick (1916- ) publish two landmark papers in the journal Nature. The papers are entitled Molecular structure of nucleic acids a structure for deoxyribose nucleic acid and Genetic implications of the structure of deoxyribonucleic acid. Watson and Crick propose a double helical model for DNA and call attention to the genetic implications of their model. Their model is based, in part, on the x-ray crystallographic work of Rosalind Franklin (1920-1958) and the biochemical work of Erwin Chargaff (1905- ). Their model explains how the genetic material is transmitted. 

Nucleic acid structures also involve assembly events determined by differential solubilities of different molecular constituents. The stacking of purine and pyrimidine bases in the helical structures of DNA relies on hydrophobic effects, whereas the positioning of phosphate groups in contact with solvent reflects their hydrophilic nature. Secondary structures of RNA likewise are influenced by differential solubilities of polar and nonpolar constituents. 

Moreover, natural nucleic acids give rise to two well-separated oxidation peaks in differential pulse voltammograms, which can be used to probe individual adenine-thymine (AT) and guanine-cytosine (GC) pairs in double helical DNA during its conformational changes [38]. Differences in signals obtained at carbon electrodes were observed according to whether, or not, the DNA was denatured [39]. 

This helicate formation mechanism can be extended to interactions with other materials. In the example shown in Fig. 4.2, hgands carrying nucleobases are used. The helicate forms a helical structure similar to the double helix of DNA, where the nucleobases in the helicate are on the outside of the helix. This helixate can form complexes with actual nucleic acid through complementary base pairing. The artificial supramolecular complex can read the programs of naturally-occurring molecules. 

Screw structures or helices (helix Greek = winding, convolution, spiral) are encountered in various variations in nature and technique. Propeller-shaped, helical structures play an important role in architecture, physics, astronomy and biology. Screw-shaped macromolecular skeletons of nucleic acids, proteins and polysaccharides are important structural elements in biochemistry. Their helix turns often are stabilized through hydrogen bonds, metal cations, disulfide linkages and hydrophobic interactions. 

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