STEREO-REPRODUCIBILITY OF THE PHOSPHORAMIDITE METHOD IN THE SYNTHESISOF OLIGONUCLEOTIDE PHOSPHOROTHIOATES

Stereo-reproducibility of phosphoroamidite-based phosphorothioate olig onucleotide synthesis is examined through synthesis of singly thioate- substituted oligodeoxynucleotide model compounds. Baseline RP-HPLC res olution of resulting Rp and Sp diastereomers allowed accurate determin ation of any enantiomeric excess at each phosphorothioate linkage. Our results show that phosphorothioate linkage formation is not a stereo- random process. AH investigated stereoisomeric phosphorothioate dieste r linkages were formed with a small, reproducible excess of the R isom er (2-6% per linkage). Regardless of the synthesized sequence, all Rp/ Sp diastereoisomer ratios were between 50:50 and 60:40, indicating tha t this synthesis process is under inherent stereochemical control. (C) 1995 Academic Press, Inc.