Nucleic left-handed

Increasing the asymmetric unit from a mono- to a dinucleotide obviously increases the conformational flexibility of the nucleic acid structure. When the two nucleotides are only minor variants of one another, such as the differences between them are one of degree rather than kind, the resulting structure would still be of the same kind as that of the parent polymononucleotide helix. On the other hand, if the two nucleotides appear in quite different conformational domains, this could eventually lead to unusual secondary structures. For exampls, we have recently obtained from the synthetic DNA poly d(GC) poly d(GC) diffraction patterns which can be interpreted in terms of a left-handed helix... 

Finally, the recent discovery of the unusual left-handed polydinucleotide helices for the alternating purine-pyrimidine polymers opens up a whole new field of nucleic acid secondary structures. The relevance and importance of these new structures in the visualization of overwound, as well as underwound, and supercoiled DNA molecules in biological.systems need hardly be emphasized. 

Figure 10.27 Organizations of genetic material in the nucleus. Upper frame represents several nucleosomes (chromatosomes) arrayed in a beaded string fashion (100 A fibers) Circles represent histone octamers. HI histones are not shown. Lower frame represents a 100 A fiber wound in a solenoid fashion. There are six nucleosomes per turn in a solenoid (300 A fiber). DNA is wound around the histone octamer in a left-handed superhelix. In the solenoid, the winding may be left- or right-handed. (Reproduced by permission from Mainwaring WIP, Parish JH, Pickering JD, Mann NH. Nucleic Acid Biochemistry and Molecular Biology. Oxford Blackwell Scientific, 1982, pp. 268, 269.)...

Both right- and left-handed double helices occur in the nucleic acids [522, 653, 662]. In the double helical DNA structure proposed by Watson and Crick in 1953,... 

Thamann, T. J., Lord, R. C., Wang, A. H-J. and Rich, A. (1981) The high salt form of poly(dG-dC). poly(d dC) is left-handed Z-DNA Raman spectra of crystals and solutions. Nucleic Acids B s. 9, 5443- 5457. 

Thomas, T.J. and Thomas, T. (1989) Direct evidence for the presence of left-handed conformauon in a supramolecular assembly of polynucleotides. Nucleic Acids Res 17, 3795-3810... 

The natural standard amino acids are left-handed, whereas carbohydrates and nucleic acids are right-handed. This property has consequences the chiral groups arrange in particular patterns in space relative to each other. Two differently arranged molecules that carry the same substituents are enantiomers. They differ in their ability to rotate plane-polarized light by equal amounts but in opposite directions. A mixture of equal parts of an optically active isomer and its enantiomer is a racemate and does not have a net rotation of plane-polarized light. 

The vacuum-UV CD spectrum of nucleic acids appears to be a reliable diagnostic of the sense of the helix, with right-handed helices (A- and B-forms) giving a positive couplet centered near 180 nm and left-handed helices (Z-form) giving a negative couplet centered near 190 nm.By... 

On the microscopic scale, chirality is an important property in life science. All amino acids in proteins are left-handed. On the other hand, all the sugars in the nucleic acids are right-handed. This is in contrast to reagent glass organic chemistry, where usually racemates are produced. However, nature synthesizes completely pure chiral compounds. How chiral substances arrived in living materials is still an open question, it is addressed as chirality problem. Several environmental conditions during synthesis have been considered as the source of chirality, such as [43]... 

Naturally there are, and not only when it pertains to the historical framework, still many open questions. For example on which level is the handedness or chirality of biological macromolecules determined We know that aU proteins, as long as they are produced through the information-guided synthesis apparatus of the cell, exclusively use left-handed amino adds and therefore bmld left turned structures. In the case of the nucleic... 

Fig. 6. Nucleic acid polymorphs Fibrous polynucleotides unwinding right- to left-handed helices. Olson-and Z-families omitted. Modified from ref. 24.

One un-natural nucleic acid structure is of a cyclohexene nucleic acid (23) that adopts a left-handed helix belonging to the (mirrored) A-type helix with the cyclohexene moieties adopting a C3 -endo conformation. " ... 

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