Phosphorothioate anti-sense oligonucleotides: the kinetics and mechanism of the sulfurisation of phosphites by phenylacetyl disulfide (PADS).

In the pharmaceutical industry the sulfurisation of nucleotide-phosphites to produce more biologically stable thiophosphates is often achieved using 'aged' solutions of phenylacetyl disulfide (PADS) which consist of a mixture of polysulfides that are more efficient sulfur transfer reagents. However, both 'fresh' and 'aged' solutions of PADS are capable of the sulfurisation of phosphites. The rates of both processes in acetonitrile are first order in sulfurising agent, phosphite and a pyridine base, although with 'aged' PADS the rate becomes independent of base at high concentrations.

Phosphorothioate anti-sense oligonucleotides: the kinetics and mechanism of the generation of the sulfurising agent from phenylacetyl disulfide (PADS).

The synthesis of phosphorothioate oligonucleotides is often accomplished in the pharmaceutical industry by the sulfurisation of the nucleotide-phosphite using phenylacetyl disulfide (PADS) which has an optimal combination of properties. This is best achieved by an initial 'ageing' of PADS for 48 h in acetonitrile with 3-picoline to generate polysulfides. The initial base-catalysed degradation of PADS occurs by an E1cB-type elimination to generate a ketene and acyldisulfide anion.