Abstract
A synthetic protocol for (34) S-labeled phosphorothioate oligonucleotides (PS ONs) was developed to facilitate MS-based assay analysis. This was enabled by a highly efficient, two-step, one-pot synthesis of (34) S-labeled phenylacetyl disulfide ((34) S-PADS), starting from (34) S-enriched elemental sulfur ((34) S(8) ). (34) S-PADS was subsequently used for stable isotope labeling (SIL) of oligonucleotides containing a phosphorothioate backbone. The (34) S-SIL PS ONs are shown to retain the same melting temperature, antisense activity, and secondary structure as those of the corresponding unlabeled (32) S PS ONs.