Nucleic acids furanose

Figure 3 The underlying tree of a furanose ring in nucleic acids. Atoms are numbered 1,. . . , 5 corresponding to the natural tree ordering. All bond lengths are fixed. Arrows illustrate five internal coordinates that determine the ring conformation.

Nucleic acids are linear polymers of nucleotides linked 3 to 5 by phosphodi-ester bridges (Figure 11.17). They are formed as 5 -nucleoside monophosphates are successively added to the 3 -OH group of the preceding nucleotide, a process that gives the polymer a directional sense. Polymers of ribonucleotides are named ribonucleic acid, or RNA. Deoxyribonucleotide polymers are called deoxyribonucleic acid, or DNA. Because C-1 and C-4 in deoxyribonucleotides are involved in furanose ring formation and because there is no 2 -OH, only... 

FIGURE 11.18 Furanoses are represented by lines phosphodiesters are represented by diagonal slashes in this shorthand notation for nucleic acid structures. 

Nucleic acids have two kinds of pentoses. The recurring deoxyribonucleotide units of DNA contain 2 -deoxy-D-ribose, and the ribonucleotide units of RNA contain D-ribose. In nucleotides, both types of pentoses are in their j3-furanose (closed five-membered ring) form. As Figure 8-3 shows, the pentose ring is not planar but occurs in one of a variety of conformations generally described as puckered. ... 

Figure 20-1 Structure and configuration of the D-aldoses from C3 to C6l showing the configurational relationship to D-glyceraldehyde. Open-chain and cyclic forms are shown. The oxacyclohexane (pyranose) form is more stable than the oxacyclopentane (furanose) form for the free sugar. The oxacyclopentane structure is shown for ribose because this is the form in which it occurs in many important substances, such as the nucleic acids. Only the a anomers are shown (see Section 20-2B).

Until recently the lactol ring structure of 2-desoxy-D-ribose in nucleic acid had been proved conclusively only for the thymidine nucleoside component and in this case it was furanose in form.26 Subsequently Brown and Lythgoe,27 by application of the periodate oxidation procedure to the 2 -desoxy ribosides of guanine, hypoxanthine, cytosine and thymine, afforded proof of the presence of a furanose sugar in each compound. 

Fig. 12. Projection of the two extreme conformers of the furanose ring in nucleic acids. S occurs in B-DNA, N occurs in RNA...

Nucleosides and Nucleotides occur in a limited number of preferred conformations. Except for the planar purine or pyrimidine base, the nucleotide component of the nucleic acids contains only single bonds (see Fig. 17.1). Rotation about bonds is restrained by the requirements of the furanose ring closure, and for the acyclic bonds, rotation is restricted by electronic factors and is limited by intramolecular interactions. Therefore only a few conformations have to be considered if nucleosides and nucleotides are described in their three-dimensional shapes. 

The substitution of pentose by hexose sugars has received much attention because oligonucleotides containing hexose sugars have many useful properties. The first hybridisation system between pyranose and furanose nucleic acids has been reported. a-Pyranose oligonucleotides (a-homo-DNA) forms stable duplexes with RNA in a parallel orientation. NMR studies show that the a-homo-DNA bases (in a-homo-DNA RNA duplexes) are equatorially arranged, whilst in the RNA strand they are pseudoaxial. The helical structure does not conform to either A- or B-forms. 

In 1935, Levenc and Tipson established the furanose structure of thymidine and suggested that the other nucleosides of desoxyribose-nucleic acid are likewise furanose. On this assumption the phosphoryl groups must be situated at positions (3) and (5), and they therefore suggested " the following formula for the tetranucleotide of desoxyribose-nucleic acid. 

Nucleic acids have a phosphodiester backbone that includes a ribose or deoxyri-bose group that is bound to a base (G, A, C, U, or T). The furanose sugar can adopt different conformations, and the link of the sugar to the base may have various torsion angles. 

Occurrence. D-Ribose (9) and 2-deoxy-D-eryf/iro-pentose ( 2-deoxy-D-ribose ) are the carbohydrate constituents of nucleic acids, which are found in all cells. D-Ribose is also a constituent of several coenzymes. In these natural products, the sugar occurs in the (3-furanose form. 

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