Abstract
In order to understand why DNA and RNA have the 3''-5'' and not the 2''-5'' link and to delineate the stereochemistry of the 2''-5'' phosphodiester links, a very accurate X-ray diffraction analysis was carried out on crystallized A2''p5''C, an analog of A2''p5''A. Contrary in numerous reports in the literature that concluded that the tendency for 2''-5'' nucleotides to stack intramolecularly is stronger than for 3''-5'' counterparts, there was hardly any intramolecular base stacking for this molecule but an intramolecular stacking of the ribose oxygen-4'' of cytidine on top of the adenine ring was found. Although A2''p5''C shows the standard conformational features usually found for 3''-5'' nucleotides, the overall stereochemistry of 2''-5'' nucleotides is quite different because the 2'' link orients the backbone inwards to the bases unlike the 3'' and 5'' links that orient it away from the bases. With the conformational features found for A2''p5''C, it is possible to build a very comapct right-handed single-stranded helix but not a double helix. Such a preference for single-stranded helices may be the reason for the absence of 2''-5'' bonds in DNA and RNA even though the 2''-5'' bonds are formed more readily than 3''-5'' bonds.

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