It has been shown by us that oligonucleotides containing cyclonucleosides with a high anti glycosidic conformation take left-handed, single and double helical structures (S. Uesugi, J. Yano, E. Yano and M. Ikehara, J. Am. Chem. Soc. 99,2313 (1977) and references therein). In order to see whether DNA can adopt the high anti left-handed double helical structure or not, a self-complementary hexanucleotide containing 6,2'-O-cyclocytidine (C 0). 8,2'-O-cycloguanosine (G 0), deoxycytidine and deoxyguanosine, C 0 G 0 dCdGC 0 G 0, was synthesized. Corresponding hexanucleotide containing only cyclonucleosides, C 0 G 0 C 0 G 0 C 0 G 0, was also synthesized. Their conformation was examined by UV, CD and 1H NMR spectroscopy. C 0 G 0 C 0 G 0 C 0 G 0 forms an unusually stable, left-handed duplex. Imino proton NMR spectra and the results of nuclear Overhauser effect experiments strongly suggest that C 0 G 0 dCdGC0 G 0 take a left-handed double helical structure where the deoxynucleoside residues are involved in hydrogen bonding and take a high anti glycosidic conformation. Thus it is revealed that DNA could form a high anti, left-handed double helix which is different from that of Z-DNA under some constrained conditions.

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The structure of d(TTAAAAGAAAAGGG):d(CCCTTTTCTTTTAA) has been characterized by NMR. The minor grooves of the two dA-tracts are suggested to be rather narrow, and the portion linking the two dA tracts exhibits a slightly deviated structure from a standard B DNA, in order to maintain the narrowness of the minor groove. The structure of the dG-tract is also slightly deviated. Additionally, specific broadening of resonances is observed for the residues at or near the junction between the dA-tract and the dG-tract, suggesting local structural polymorphology.