A novel Z-structure for poly d(GC).poly d(GC).

Introduction Recently a structure was proposed for poly d(GC). poly d(GC) (1) at high salt. The structure is a six-fold helix (n = 6) with height per residue, h = 7.25 1 similar to the crystal structure (2) of d(CG)5 (n = 6, h ~7.40 8). Asymmetric unit in both the cases is a dinucleotide, The structure of poly d(GC).poly d(GC) was reported to be very similar to that of d(CG)5. However, the proposed polymer structure (l), on inspection differs from the structure of d(W)5 in two major aspects: (i) progression of the sugarphosphate backbone and separation between two neighbouring chains and (ii) staclcing arrangement of the oases. In this communication, we present a novel left handed Z-DNA model for poly d(GC).poly d(GC). We also show that the present model is very similar to the structure of d(CG)g in most aspects including the two mentioned above.