Guanosine structure

FIGURE 12.14 Comparison of the deoxy-guanosine conformation in B- and Z-DNA. In B-DNA, the Cl -N-9 glycosyl bond is always in the anti position (lefi). In contrast, in the left-handed Z-DNA structure, this bond rotates (as shown) to adopt the syn conformation. 

The 1-methoxy-compound " is not formed, but rather 7-methyl guanosine (32b), with a betaine structured - "... 

Recently, Switzer and co-workers have further extended the multi-stranded motifs for nucleic acids with the formation of a quintet assembly with oligonucleotides containing 2,deoxy-iso-guanosine (74). To support the quintet, metal ions larger than those appropriate for quartet stabilization were required, and Cs+ ions were found to best meet this requirement. From modeling studies, a structure in which a central Cs+ interacts with ten 02 iG atoms at a distance of 3.5 A was proposed. 

We succeeded in isolating the same adduct reported by Maliepaard et al. in trace quantities, but a 13C-NMR spectrum of the crude reaction mixture revealed the presence of hundreds of additional products (Fig. 7.4). The chemical shift of the guanosine 2-amino adduct (35.4 ppm) is not obvious in this spectrum. The bands of chemical shifts centered at 65 and 55 ppm correspond to structures with oxygen attachment to the C-10 center, but the band of products centered at 28 ppm represented unknown structures. 

Nucleic acids which contain only adenosine, guanosine and uridine are able to form A-U Watson-Crick pairs and G-U wobble pairs. They should be able to build up complex secondary and tertiary structures. 

Kadokura, M., Wada, T., Urashima, C., and Sekine, M. (1997). Efficient synthesis of g-methyl-capped guanosine. V-mphosphate as a SLtcrminal unique structure of U6 RNA via a new triphosphate bond formation involving activation of methyl phosphor-imidazolidate using ZnCl2 as a catalyst in DMF under anhydrous conditions. Tetrahedron Lett. 38, 8359-8362. 

Very interesting information relevant to the stereochemical results of alkylation of DNA comes from studies of nucleosides alkylated by activated PAHs. Eight such structures have been reported (115-118). Three are products of the interaction of chloromethyl PAHs with N6 of adenosine XXX-XXXII, two with deoxyadenosine (XXXIII, XXXIV), two are para-substituted benzyl derivatives of guanosine, alkylated at 0-6 (XXXV, XXXVI) (117) and one is an acetyl aminofluorene derivative of guanosine, alkylated at C8 (XXXVII) (118). 

Outstanding in this field has been the contribution of Todd and his co-workers. The significance of their work has been, first, that it has led to successful syntheses of adenosine, guanosine, cytidine, and uridine. Equally important, however, has been its contribution toward the final confirmation of the structures of the nucleosides. This may be illustrated in the following way. By the route I to V, which had been developed for the synthesis of pentosylpurines (purine 1-deoxypentosides), it was possible to obtain 9-/3-n-mannopyranosyladenine (V), in which the position of the carbohydrate residue was known with certainty.19 This nucleoside, on oxi-... 

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