A NEW SIMPLE METHOD OF REPRESENTING RELATIVE ORIENTATION OF BASES IN DNA HELICAL STRUCTURES

It is shown that the strandwise correlation of the torsion angles in N ucleic acid structures can throw light on the aspects like helical dis tortion, in double helical and other associated forms. The single crys tal X-ray coordinates for 7 A-DNA, 29 B-DNA, 8 Z-DNA and 4 tRNA struct ures are used to calculate the glycosyl angle chi and they are plotted in a two dimensional plot. A new torsion angle pi is defined as O4'-C 1'-N1(9)-X, where X is a fictitious atom on the normal at N1(9). The c hi 1 vs. chi 2 plots (where 1 and 2 refer to I and II-strand of DNA re spectively) show the characteristic clustering features of A, B and Z DNAs. They are also compactly reflected in the modified graphical repr esentation like (chi 1 + chi 2) vs. \chi 1 - chi 2\ plot. chi 1 vs. ch i 2 plot of the Hoogsteen type of base pairing obtained from the tRNA structures is off diagonal. The distortions due to complexing of the s tructures under each category, in particular, B-DNAs is readily observ ed. The observed points of A/B hybrid form clusters between A and B un complexed forms. The new torsion angle pi shows similar behavior as th at of chi.