Abstract: The present invention comprises an improved method of synthesizing oligonucleotides. The method comprises employing dinucleotides (or “dimer blocks”) as the basic synthetic unit building block. The method results in extremely high purity oligonucleotides in which the N−1 content is very low, generally less than 1-2% of the full length, N, oligonucleotide. We have found that synthesis using dinucleotide phosphorothioates results in oligonucleotides having very little phosphodiester content. Furthermore, we have found that the amount of dimer required in each coupling step can be less than about 6 and is preferably about 2 equivalents. Synthesis of oligonucleotides according to the dimer block approach described herein can also be conducted without the capping step that has heretofore been deemed necessary after each coupling.

Abstract: The present invention comprises an improved method of synthesizing oligonucleotides. The method comprises employing dinucleotides (or "dimer blocks") as the basic synthetic unit building block. The method results in extremely high purity oligonucleotides in which the N-1 content is very low, generally less than 1-2% of the full length, N, oligonucleotide. We have found that synthesis using dinucleotide phosphorothioates results in oligonucleotides having very little phosphodiester content. Furthermore, we have found that the amount of dimer required in each coupling step can be less than about 6 and is preferably about 2 equivalents. Synthesis of oligonucleotides according to the dimer block approach described herein can also be conducted without the capping step that has heretofore been deemed necessary after each coupling.