A Versatile and Convenient Synthesis of 34S‐Labeled Phosphorothioate Oligonucleotides

: 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 S8 ). 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.