Effect of chirality of ribose on nucleic acid structure and function

Organisms utilize the only D-enantiomer of ribose as a sugar unit of nuclei c acids. This homochirality of nucleic acids is considered to be essential for their higher-order structures and functions. Although there had been a few reports on the synthesis of L-deoxynucleosides, effects of the substitu tion of L-ribose for the D-enantiomer on the structure and properties of nu cleic acids have hardly been investigated due to difficulties in large-scal e synthesis of L-deoxynucleosides. We have developed an approach for the ef ficient, short step synthesis of L-deoxynucleosides, and have investigated the structure and properties of oligonucleotides containing them. The double-helical conformations of an L-hexadeoxynucleotide, L-d(CGCGCG) w ere clearly shown to be an exact minor image of those of the corresponding natural one by CD (circular dichroism) and X-ray crystallographic analysis. This L-hexadeoxynucleotide was applied to the study of the specific double -stranded DNA recognition mechanism of bleomycin. It was found that the con ventional DNA-binding domain of bleomycin binds to the L-hexadeoxynucleotid e to essentially the same extent as natural one, although bleomycin can not cleave it. This result suggests that the DNA-binding domain is not respons ible for the specific DNA recognition. Thus, L-DNA would be useful for dist inguishing between enantio-specific and nonspecific interactions in DNA-dru g interaction studies. The structures of heterochiral oligonucleotides, which contain an unnatural L-nucleotide residue in the sequence of natural type of DNA chains, were i nvestigated by UV and H-1-NMR experiments. The results demonstrated that th e L-nucleotide residue in the heterochiral oligonucleotide forms stable Wat son-Crick base-pairing with the complementary natural residue, while the ov erall duplex stability is slightly decreased. The unusual conformational fe atures of the L-nucleotide residue in the heterochiral DNA may be useful fo r the design of a novel antisense molecule resistant to nucleases in vivo.