Organisms utilize the only D-enantiomer of ribose as a sugar unit of nucleic 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 substitution of L-ribose for the D-enantiomer on the structure and properties of nucleic acids have hardly been investigated due to difficulties in large-scale synthesis of L-deoxynucleosides. We have developed an approach for the efficient, short step synthesis of L-deoxynucleosides, and have investigated the structure and properties of oligonu-cleotides containing them. The double-helical conformations of an L-hexadeoxynucleotide, L-d(CGCGCG) were clearly shown to be an exact mirror 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 conventional DNA-binding domain of bleomycin binds to the L-hexadeoxynucleotide to essentially the same extent as natural one, although bleomycin can not cleave it. This result suggests that the DNA-binding domain is not responsible for the specific DNA recognition. Thus, L-DNA would be useful for distinguishing between enantio-specific and nonspecific interactions in DNA-drug interaction studies. The structures of heterochiral oligonucleotides, which contain an unnatural L-nucleotide residue in the sequence of natural type of DNA chains, were investigated by UV and ¹H-NMR experiments. The results demonstrated that the L-nucleotide residue in the heterochiral oligonucleotide forms stable Watson-Crick base-pairing with the complementary natural residue, while the overall duplex stability is slightly decreased. The unusual conformational features of the L-nucleotide residue in the heterochiral DNA may be useful for the design of a novel antisense molecule resistant to nucleases in vivo.