Stability of oligodeoxynucleoside phosphorodithioates and phosphorothioates in aqueous ammonia

Oligonucleotide analogues like oligodeoxynucleoside phospho-rothioates and phosphorodithioates (Fig. 1) are of interest for antisense applications (1). Efficient methods exist for the preparation of both analogues, but the products often contain less sulfur than intended, as shown by the presence of variable amounts of phosphate linkages in oligonucleoside phosphoro-thioates, and phosphorothioate linkages in oligonucleoside phos-phorodithioates. Incomplete oxidation by the sulfurization reagents used in phosphoramidite or thiophosphoramidite methods could explain the appearance of these impurities, but cannot explain why products prepared by triester methods are also often contaminated. We have recently described a dithiophosphortriester method that gives oligodeoxynucleoside phosphorodithioates free of phosphorothioate impurities (2). However, when the oligonu-cleoside phosphorodithioates were kept in prolonged contact wit