Double-helix structure

F. H. C. Crick. J. B. Watson and M. H. F. Wilkins (with Rosalind Franklin) establish the double helix structure of nucleic acids (Nobel Pnze 1962). 

The central role played by DNA in cellular life guarantees a place of importance for the study of its chemical and physical properties. It did not take long after Watson and Crick described the now iconic double helix structure for a question to arise about the ability of DNA to transport electrical charge. It seemed apparent to the trained eye of the chemist or physicist that the array of neatly stacked aromatic bases might facilitate the movement of an electron (or hole) along the length of the polymer. It is now more than 40 years since the first experimental results were reported, and that question has been answered with certainty. 

The term peptide nucleic acids was chosen because of the peptide bond in the polymer (see Sect. 5.2). The bond between the polyamide strand and the organic bases involves an acetyl group. The formation of DNA-like double helix structures by PNAs was described by Pernilla Wittung et al. (1994). The question arises as to whether peptide nucleic acids can in fact be synthesized under prebiotic conditions. 

The systematic catenation of cyclotetrasilane rings gives rise to a double helix structure of two poly silane main chains. 

Fundamental knowledge on the structures and properties of the ladder polysilanes has accumulated in our research for the past 15 years. Some results were unpredictable, including the silicon double helix structure, the domino oxidation, the formation of persistent radical anions, the Diels-Alder reactions at the 1,4-positions of anthracene, etc. These results let us recognize that the construction of novel structures will open the new chemistry. 

Figure 8 The structure of amylopectin (a) organisation of the molecule (b) crystalline and amorphous regions (c) possible double helix structure...

Rees and coworkers158 showed that, at 15°, i-carrageenan forms a gel whose 13C-n.m.r. signals are so broad that they cannot be detected, in contrast to those given by the solution at 80° (see Fig. 28). At the lower temperature, segmental motion is restricted by frequent, interunit junction-zones in a double-helix structure, in contrast to the gel of a /8-D-(l— 3)-linked D-glucopyranan, where the intermolecular association is not so complete, and portions of the polymer are sufficiently mobile to provide broad signals.159... 

The twist angles between the terminal rungs of 141, 142 and 143 are 44.0, 63.0, and 80.3°, respectively. The double helix structures crystallize with equal amounts of left- and right-handed screw sense forms, as expected in the absence of any chiral field. Some ladder polysilanes crystallize with both helical forms in the same centrosymmetric unit cell for others, the two forms can crystallize separately. It will be interesting to carry out the crystallization in a chiral field. 

The double helix structure for DNA was first described in J. D. Watson and F. H. C. Crick, Nature 171 737-738 (1953). This is the single most famous publication in all of molecular biology. 

The role of DNA in storing and transferring genetic material is dependent on the properties of the four bases. These bases are complementary in that guanine is always associated with cytosine, and adenosine with thymine. Watson and Crick, some 40 years ago, showed that the stability of DNA is due to the double helix structure of the molecule that protects it from major perturbations. Information is ultimately transferred by separating these strands which then act as templates for the synthesis of new nucleic acid molecules. 

Figure 7. Mutually perpendicular views of the (a) agarose, (b) iota-carrageenan, and (c) kappa-carrageenan double-helix structures. The two chains are shown with open and full bonds, and the 06—02 hydrogen bonds by broken lines. (Reproduced with permission from ref. 28. Copyright 1989 Elsevier.)...

In 1952, Bloch and Woodward suggested a mechanism for the cycliza-tion of squalene to cholesterol. In 1962, Francis Crick and James Watson described the double helix structure of proteins. Hodgkin determined the structure of vitamin B12 and of penicillin through collaboration between Woodward and Eschenmoser, involving postdoctoral fellows. In 1877, Alexander Fleming discovered penicillin which was active against tuberculosis. 

The double helix structure of DNA has the hydrophobic bases pointing to the centre of the helix in an almost planar arrangement. These base-pairs are closely stacked perpendicular to the long axis of the chain, and are attracted to each other by Van der Waals forces. The hydrophilic phosphates are negatively charged at the pH of the cell and point to the outside. 

When wRNA strand is synthesized, the DNA-RNA double helix separates. The wRNA migrates to the cell cytoplasm while DNA returns to its normal double helix structure. Similarly tRNA and rRNA are synthesized. 

An unusual photochemical reaction of 2-pyridones, 2-aminopyridinium salts and pyran-2-ones is photodimerization to give the so-called butterfly dimers. These transformations are outlined in equations (13) and (14). Photodimerization by [2+2] cyclization is also a common and important reaction with these compounds. It has been the subject of particular study in pyrimidines, especially thymine, as irradiation of nucleic acids at ca. 260 nm effects photodimerization (e.g. equation 15) this in turn changes the regular hydrogen bonding pattern between bases on two chains and hence part of the double helix structure is disrupted. The dimerization is reversed if the DNA binds to an enzyme and this enzyme-DNA complex is irradiated at 300-500 nm. Many other examples of [2+2] photodimerization are known and it has recently been shown that 1,4-dithiin behaves similarly (equation 16) (82TL2651). 

As with proteins, the nucleic acid polymers can denature, and they have secondary structure. In DNA, two nucleic acid polymer chains are twisted together with their bases facing inward to form a double helix. In doing so, the bases shield their hydrophobic components from the solvent, and they form hydrogen bonds in one of only two specific patterns, called base pairs. Adenine hydrogen bonds only with thymine (or uracil in RNA), and guanine pairs only with cytosine. Essentially every base is part of a base pair in DNA, but only some of the bases in RNA are paired. The double-helix structure... 

Fio. 5.A8. The double-helix structure of DNA. D, deoxyribose P, phosphate ester bridge A, adenine C, cytosine G, guanine and T, thymine... 

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