Anti conformation of nucleosides

This conformation would place the chromophore residue into a stacked situation within the adjacent DNA bases while replacing the counterbase of the complementary strand. To obtain more structural information, NOESY experiments were performed on the pyrene-modified nucleosides. The spectra of the modified pyrimidines Py-dU and Py-C clearly showed a significant NOESY cross peak between H-6 of the dU part or the C part, respectively and H-2 of the corresponding 2 -deoxyribose moiety. The NOESY cross peak was comparably as strong as the cross peaks between H-2 and H-l or H-3, respectively. These NMR results can only be explained with the preferred anti conformation of these nucleosides. 

The conformation of nucleosides and nucleotides has been reviewed.The conformation of 2-alkylthio-2, 3 -0-isopropylidene-adenosine derivatives has been studied by i.r. and c.d. measurements a strong hydrogen bond exists between N-i and 0-5 in carbon tetrachloride, indicating a syn conformation for the base c.d. curves, which are markedly dependent on the sulphur substituent, suggest a syn conformation in chloroform and an anti arrangement in water. 

Glycosldlc bond conformations of nucleosides and nucleotides have been Investigated by V( C,Hl ) measurements (Lemleux, 1973 Schwelzer and Krelshman, 1973 Davies, 1976). It was shown that observation of vicinal coupling constants between H(l ) and both C(2) and C(6) of pyrimidine rings Is necessary to determine either syn- or anti-type conformations unless HI ) > (Davies, 1976). It Is expected... 

Fig. 2-8. Proposed ranges of cn for syn and anti conformation of a pyrimidine nucleoside.

Using phosphotriester methods, dinucleoside (3 - 50-monophosphates containing 6-methyl-2,-deoxyuridine at the 3 - or 5 -end have been prepared.44 N.m.r. spectroscopy indicates that this nucleoside possesses the syn conformation in these compounds, and, on treatment with snake venom phosphodiesterase, d(m6UpT) is degraded, while d(Apm6U) is not, indicating that this enzyme, a 3 -exonuclease, requires the anti conformation to be present in the substrate. Two modified nucleo-side-5 -monophosphates, (20) and (21), which are resistant to 5 -nucleotidase, have been isolated from tRNA snake venom hydrolysates.45 A synthesis of (20) has been reported.46... 

Figure 5-11 Views of a nucleoside in syn and anti conformations.66 (C) Courtesy of Dr. Muttaiya Sundaralingam.

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