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.