 Phosphorothioate substitution

A phosphorothioate substitution replaces an oxygen of a phosphate group with sulfur. In a nucleic acid backbone, the usual experimental substitution replaces a non-bridging oxygen, while retaining the oxygen bridges connecting neighboring nucleotides. It changes backbone chemistry without deleting a nucleotide or changing its nucleobase sequence. The altered group is a phosphate analogue rather than a disulfide bond.

The two non-bridging positions matter. For a phosphate joining different nucleotide environments, replacing one oxygen or the other can give different configurations at phosphorus. Such phosphorothioate diastereomers can interact differently with an RNA-folding environment or a metal ion. The 2013 U6 metal rescue experiment tested both $R_P$ and $S_P$ substitutions at selected sites, rather than treating every sulfur replacement as equivalent.^[ref]

An original counting example shows the extra information required. At three independently stereogenic, substituted phosphates, two possible configurations at each site give $2^3=8$ configuration combinations. All eight can have the same nucleotide sequence and elemental formula. Measuring molecular mass alone therefore cannot tell which configuration combination was supplied. If every oxygen-to-sulfur replacement is made once at a selected site, each adds approximately $32-16=16$ unified atomic mass units; three add about $48$, regardless of those configuration choices. These rounded atomic masses are illustrative, not exact isotope masses.

Replacing oxygen with sulfur also changes metal–ligand interactions. A loss of RNA activity in magnesium followed by preferential rescue with a different metal can test whether the original oxygen helped position a catalytic ion. It can also reflect altered folding or assembly, so the comparison needs controls. The substitution is a targeted chemical perturbation whose interpretation depends on position, configuration and the reaction being measured.