Nucleic acid 3-anomers

Chromatographic evidence has been obtained for the formation of 6-thioguanosine di- and triphosphates in Ehrlich ascites cells [197] and their formation is supported by the demonstrated incorporation of thioguanine into nucleic acids as thioguanylic acid [198]. The incorporation of both the a- and 3-anomers of 2 -deoxythioguanosine (XXXVI) [199] into DNA without... 

Mixtures of the anomers of glycofuranosides have often been used as starting materials for various syntheses, instead of the individual anomers. Thus, methyl a, /3-D-ribofuranoside is often encountered in the literature of the nucleic acids. Such mixtures can be converted into the esters, or into the glycofuranosyl halides, and then into the individual glycofuranosides. Some of these applications will be discussed later (see pp. 121). 

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).

Note that it is the /3 anomer in each case that is present in the nucleic acid (Chap. 2)... 

The polysaccharides are, after the proteins and the nucleic acids, the third important group of biological polymers. From the point of view of the quantum-mechanical calculations of their conformational problems, mention may be made in the first place of the work of Neely 13> who has studied by the Extended Hiickel Theory the relative conformational stabilities of a few of the different possible chair and boat conformations of D-glucose, II. Within the conformations studied, the chair one, known under the designation Cl, appears so far the most stable. Experimentally, it is apparently the most stable of all. Moreover, the (3-anomer comes out as 9 kcal/mole more stable than the a-one. Although this difference is far too large with respect to experiment, it is in the proper direction. 

Many unusual nucleotides have been found as minor components of nucleic acids, especially in the soluble or transfer ribonucleic acids. Most of these minor components contain methylated aglycons in their structure. A review of these nucleotides has been presented by Dekker, and general techniques for their isolation as nucleosides have been reported by Hall. In addition, 5,6-dihydrouridylic acid (34) has been isolated by enzymic hydrolysis of certain transfer ribonucleic acids from yeast, and 4-thiouridylic acid (35) was obtained from the alkaline hydrolyzate of transfer ribonucleic acid from Escherichia coli. A nucleotide whose ultraviolet absorption spectrum was very similar to that of 2-thiouridine has been reported to be present in transfer ribonucleic acid. Although the a anomer (36) of cytidylic acid has been detected (and identified) in a yeast ribonucleic acid hydrolyzate, it is not certain whether this -cytidylic acid is a minor component of ribonucleic acid or an artifact produced during the alkaline hydrolysis. Among the minor nucleotide components of transfer ribonucleic acid, pseudouridylic acid (37)89-98 jg unique, in that the D-ribosyl moiety is linked to the aglycon... 

Fig. 3-27 The sugar in DNA is 2-deoxy-D-ribose (2-deoxy-P-D-ribofuranose ieft) in RNA it is o-ribose (P-D-ribofuranose right), it is the -anomer in each case that is present in the nucleic acid.

It has also been demonstrated that the distribntion of nucleoside anomers produced in acidic aqueous solutions is kinetically controlled on short time scales, but will reach an equilibrium distribution that minimizes steiic interactions between the base and sugar, which for aldopentoses results in the predominance of the a- and P-pyranosyl forms (25,27-29). Therefore, it would not have been the selective production of p-fiiranosyl nucleosides that influenced their incorporation into piebiotic nucleic adds. 

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