Guanosine bonds

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. 

Schwalb NK, Temps F (2007) Ultrafast electronic relaxation in guanosine is promoted by hydrogen bonding with cytidine. J Am Chem Soc 129 9272... 

The translation model was refined and developed further in a later article (Lehmann and Kuhn, 1984). It is assumed that in the early stages of evolution, guanosine and cytidine were the most important building blocks for RNA syntheses, as they allow strong base pairing (via three hydrogen bonds). Quantitative considerations lead to an upper limit of about 50 nucleotides for a reasonable chain length. 

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. 

A more complex structure is that of leinamycin 45 (Scheme 15), a material with potent cytotoxic and antitumor properties, isolated from a Streptomyces sp. A 1,2 dithiolane-3-one ring is spiro fused to a complex macrolactam96 (and references therein). Leinamycin has the remarkable ability to cleave DNA. In brief, leinamycin reacts with a thiol and, after a profound rearrangement, forms an episulfonium ion. This ion alkylates the N7 position of guanosine residues in double stranded DNA an unstable adduct is depurinated by hydrolysis of the glycosidic bond between the alkylated base and a deoxyribose residue. Some structurally less complex l,2-dithiolane-3-one 1-oxides have a similar DNA cleaving ability.97... 

Another ingenious modification of the HNN-COSY scheme involves the replacement of the homonuclear 15N-COSY transfer by a 15N-TOCSY transfer [37]. As the homonuclear TOCSY transfer is twice as fast as the COSY transfer, a significant sensitivity increase is achieved. The application is, however, limited to cases where the 15N donor and acceptor resonances are at similar frequencies, so that the power of the 15N-TOCSY radio frequency pulses needed is not too strong. A very interesting application of this scheme was presented for the sensitive detection of very small (0.14 Hz, Tab. 9.1) h4/NiNi couplings in N1-H1 06=C6-N1 H-bonds of guanosine tetrads. 

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