The X-ray crystal structure of d(TGCGCA)(2) has been determined at 120 K to
a resolution of 1.3 Angstrom. Hexamer duplexes, in the Z-DNA conformation,
pack in an arrangement similar to the 'pure spermine form' [Egli et al. (1
991). Biochemistry, 30, 11388-11402] but with significantly different cell
dimensions. The phosphate backbone exists in two equally populated discrete
conformations at one nucleotide step, around phosphate 11. The structure c
ontains two ordered cobalt hexammine molecules which have roles in stabiliz
ation of both the Z-DNA conformation of the duplex and in crystal packing.
A comparison of d(TGCGCA)(2) with other Z-DNA hexamer structures available
in the Nucleic Acid Database illustrates the elusive nature of crystal pack
ing. A review of the interactions with the metal cations Na+, Mg2+ and Co3 reveals a relatively small proportion of phosphate binding and that close
contacts between metal ions are common. A prediction of the water structure
is compared with the observed pattern in the reported structure.