SYNTHESIS AND PROPERTIES OF MIXED RIBOOLI GONUCLEOTIDE AND DEOXYRIBOOLIGONUCLEOTIDE DUPLEXES CONTAINING AN INTERNUCLEOTIDE TRISUBSTITUTED PYROPHOSPHATE BOND

Fragments of double-stranded RNAs and mixed double-stranded ribo/deoxy ribooligonucleotides containing an internucleotide trisubstituted pyro phosphate bond in a predetermined position of the sugar-phosphate back bone were synthesized. The modified internucleotide bond was created b etween synthetic oligonucleotides by chemical ligation with carbodiimi de. The chemical ligation conditions were optimized for a series of du plexes that differed in the content and mutual arrangement of ribo- an d deoxyribonucleotide blocks. The yield of oligonucleotides containing the trisubstituted internucleotide pyrophosphate bond was 90% in an a ll-deoxy duplex and 15-68% in ribooligonucleotide-containing duplexes. The modified duplexes thus obtained are relatively stable in an aqueo us solution at pH 5.5-7.5, whereas the modified internucleotide bond i s cleaved by amines to form phosphamide derivatives of the correspondi ng oligonucleotides. Automated solid-phase synthesis of oligonucleotid e 3'-ethylphosphates is described.