3'-modified oligonucleotides by reverse DNA synthesis.

Reverse DNA oligonucleotide synthesis (i.e. from 5'-->3') is a strategy that has yet to be exploited fully. While utilized previously for the construction of alternating 3'-3'- and 5'-5'-linked antisense oligonucleotides, the use of nucleoside 5'-phosphoramidites has not generally been used for the elaboration of (modified) oligonucleotides. Presently, the potential of reverse oligonucleotide synthesis for the facile synthesis of 3'-modified DNAs is illustrated using a phosphoramidite derived from tyrosine. The derived oligonucleotide was shown to have chromatographic and electrophoretic properties identical with the modified oligonucleotide resulting from the proteinase K digestion of the vaccinia topoisomerase I-DNA covalent complex. The results confirm the nature of the structure previously assigned to this product, and establish the facility with which proteinase K is able to complete the digestion of the polypeptide backbone of the DNA oligonucleotide-linked topoisomerase I.