CRYSTAL-STRUCTURE OF THE SELF-COMPLEMENTARY 5'-PURINE START DECAMER D(GCACGCGTGC) IN THE A-DNA CONFORMATION .2.

The crystal structure of the alternating 5'-purine start decamer d(GCG CGCGCGC) was found to be in the left-handed Z-DNA conformation. Inasmu ch as the A . T base pair is known to resist Z-DNA formation, we subst ituted A . T base pairs in the dyad-related positions of the decamer d uplex. The alternating self-complementary decamer d(GCACGCGTGC) crysta llizes in a different hexagonal space group, P6(1)22, with very differ ent unit cell dimensions a = b = 38.97 and c = 77.34 Angstrom compared with the all-G . C alternating decamer, The A . T-containing decamer has one strand in the asymmetric unit, and because it is isomorphous t o some other A-DNA decamers it was considered also to be right-handed, The structure was refined, starting with the atomic coordinates of th e A-DNA decamer d(GCGGGCCCGC), by use of 2491 unique reflections out t o 1.9-Angstrom resolution. The refinement converged to an R value of 1 8.6% for a total of 202 nucleotide atoms and 32 water molecules. This research further demonstrates that A . T base pairs not only resist th e formation of Z-DNA but can also assist the formation of A-DNA by swi tching the helix handedness when the oligomer starts with a 5'-purine: also, the length of the inner Z-DNA stretch (d(CG)(n)) is reduced fro m an octamer to a tetramer, It may be noted that these oligonucleotide properties are in crystals and not necessarily in solutions.