Synthetic oligonucleotides as RNA mimetics:: 2′-modified RNAs and N3′→P5′ phosphoramidates

Significant interest in synthetic DNA and RNA oligonucleotides and their analogues has marked the past two decades of research in chemistry and biochemistry. This attention was largely determined by the great potential of these compounds for various therapeutic applications such as antisense, antigene and ribozyme-based agents. Modified oligonucleotides have also become powerful molecular biological and biochemical research tools that allow fast and efficient regulation of gene expression and gene functions in vitro and in vivo. These applications in turn are based on the ability of the oligonucleotides to form highly sequence-specific complexes with nucleic acid targets of interest. This review summarizes recent advances in the design, synthesis, biochemical and structural properties of various RNA analogues. These comprise 3'-modified oligonucleotide N3' --> P5' phosphoramidates, analogues with modifications at the 2'-position of nucleoside sugar rings, or combinations of the two. Among the properties of the RNA minetics reviewed here are the thermal stability of their duplexes and triplexes, hydrolytic resistance to cellular nucleases and biological activity in in vitro and in vivo systems. In addition, key structural aspects of the complexes formed by the RNA analogues, including interaction with water molecules and ions, are analyzed and presented.