Synthesis and hybridization properties of l-oligodeoxynucleotide analogues fixed in a low anti glycosyl conformation

Exploring the role of chirality in nucleic acid recognition

The study of the base-pairing properties of nucleic acids with sugar moieties in the backbone belonging to the L-series (β-L-DNA, β-L-RNA, and their analogs) are reviewed. The major structural factors underlying the formation of stable heterochiral complexes obtained by incorporation of modified nucleotides into natural duplexes, or by hybridization between homochiral strands of opposite sense of chirality are highlighted. In addition, the perspective use of L-nucleic acids as candidates for various therapeutic applications, or as tools for both synthetic biology and etiology-oriented investigations on the structure and stereochemistry of natural nucleic acids is discussed.

Synthesis and hybridization studies of 2'-amino-alpha-L-LNA and tetracyclic "locked LNA"

A convergent route to a new class of locked nucleic acids, i.e., 2'-amino-alpha-L-LNA, has been developed. The optimized synthetic route to the corresponding phosphoramidite building block of thymine proceeds in 4% overall yield over 15 steps from the starting diol. Crucial synthetic steps include (a) introduction of a C2-azido group prior to nucleobase coupling, (b) Vorbrüggen glycosylation primarily affording the desired alpha-anomer, (c) separation of alpha-L-ribo- and beta-L-ribo-configured bicyclic nucleosides, and (d) selection of a suitable protecting group to avoid intramolecular Michael addition of the C2'-amino group onto the C6-position. Incorporation of a 2'-amino-alpha-L-LNA monomer into oligodeoxyribonucleotides results in modest changes in thermal stability with complementary DNA, whereas significant increases in thermal stability are observed with RNA complements along with excellent Watson-Crick discrimination. These results, along with the flexibility of the synthetic strategy allowing chemoselective N2'-functionalization at a late stage, render 2'-amino-alpha-L-LNA a promising building block for nucleic acid based nanobiotechnology and therapeutics. A slight modification in strategy facilitated the synthesis of the corresponding phosphoramidite building blocks of Michael adducts, which due to their tetracyclic skeletons exhibit a conformationally restricted furanose ring and glycosidic torsion angle (anti-range). Incorporation of such a "locked LNA" monomer into oligodeoxyribonucleotides results in large decreases in thermal affinity toward DNA/RNA complements.