Visualisation of a 2'-5 ' parallel stranded double helix at

X-ray crystallographlc studies on 3'-5 ' ollgomers have provided a great deal of information on the stereochemistry and conformational flexibility of nucleic acids and polynucleotides. In contrast, there is very little Information available on 2'-5' polynucleotides. We have now obtained the crystal structure of Cytidylyl-2',5'-Adenoslne (C2'p5'A) at atomic resolution to establish the conformational differences between these two classes of polymers. The dlnucleoside phosphate crystallises in the monocllnlc space group C2, with a = 33.912(4)A, b = 16.824(4)A, c = 12.898(2)A and 0 = 112.35(1) with two molecules in the asymmetric unit. Spectacularly, the two independent C2'p5'A molecules in the asymmetric unit form right handed miniature parallel stranded double helices with their respective crystallographic two fold (b axis) symmetry mates. Remarkably, the two mini duplexes are almost indistinguishable. The cytosines and adenines form self-pairs with three and two hydrogen bonds respectively. The conformation of the C and A residues about the glycosyl bond is anti same as in the 3'-5' analog but contrasts the anti and syn geometry of C and A residues in A2'p5'C. The furanose ring conformation is C3'endo, C2'endo mixed puckering as in the C3'p5'A-proflavine complex. A comparison of the backbone torsion angles with other 2'-5 ' dinucleoside structures reveals that the major deviations occur in the torsion angles about the C3'-C2 ' and C4'-C3 ' bonds. A right-handed 2'-5 ' parallel stranded double helix having eight base pairs per turn and 45 ° turn angle between them has been constructed using this dinucleoside phosphate as repeat unit. A discussion on 2'-5 ' parallel stranded double helix and its relevance to biological systems is presented