CRYSTAL-STRUCTURE OF D(GCGCGCG) WITH 5'-OVERHANG G-RESIDUES

The crystal structure of the DNA heptamer d(GCGCGCG) has been solved a t 1.65 Angstrom resolution by the molecular replacement method and ref ined to an R-value of 0.184 for 3598 reflections. The heptamer forms a Z-DNA d(CGCGCG)(2) with 5'-overhang G residues instead of an A-DNA d( GCGCGC)(2) with 3'-overhang G residues. The overhang G residues from p arallel strands of two adjacent duplexes form a trans reverse Hoogstee n G.G basepair that stacks on the six Z-DNA basepairs to produce a pse udocontinuous helix. The reverse Hoogsteen G.G basepair is unusual in that the displacement of one G base relative to the other allows them to participate in a bifurcated (G1)N2 ... N7(G8) and an enhanced (Ga)C 8-H ... O6(G1) hydrogen bond, in addition to the two usual hydrogen bo nds. The 5'-overhang G residues are anti and C2'-endo while the 3'-ter minal G residues are syn and G2'-endo. The conformations of both G res idues are different from the syn/C3'-endo for the guanosine in a stand ard Z-DNA. The two cobalt hexammine ions bind to the phosphate groups in both GpC and CpG steps in Z(I) and Z(II) conformations. The water s tructure motif is similar to the other Z-DNA structures.