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A phosphorothioate diastereomer is a stereochemical form of a phosphorothioate substitution that differs in configuration without changing connectivity. In a nucleic-acid phosphate joining different nucleotide environments, replacing a non-bridging oxygen with sulfur can make phosphorus a stereogenic center. The resulting $R_P$ and $S_P$ forms have the same elemental composition but a different spatial arrangement at that center.

They are diastereomers in the usual nucleic-acid context because the rest of the RNA already contains fixed stereochemistry, including its ribose units. Changing one phosphorus configuration while preserving those other centers does not produce the mirror image of the entire RNA. It is therefore misleading to call such a pair enantiomers merely because one phosphorus label changes from R to S. Assignment of the labels uses the Cahn-Ingold-Prelog priority rules, not the order in which a nucleotide sequence is written.

Two independently stereogenic substituted phosphates give four combinations: $(R_P,R_P)$, $(R_P,S_P)$, $(S_P,R_P)$ and $(S_P,S_P)$. Their molecular masses are the same. A mass measurement alone cannot determine either the configuration at one site or the proportions in an unresolved mixture.

A simple activity model shows why mixture composition can matter. Suppose equal amounts of two purified forms separately give product fractions $0.8$ and $0.2$ in a stipulated assay. If molecules act independently and the assay response is linear, a mixture containing fraction $f$ of the first form gives $p=0.8f+0.2(1-f)$. At $f=1/2$, $p=0.5$. These invented numbers illustrate averaging; competition for limiting components can invalidate that model.

The U6 metal rescue experiment supplies an actual chemical distinction. Sulfur substitutions with opposite phosphorus configurations at the same U6 position showed differing metal-rescue behavior, including differences between branching and exon ligation.^[ref] Position, configuration and measured reaction therefore belong in the description of a phosphorothioate reagent.

The U6 metal rescue experiment tests whether chemical groups in U6 small nuclear RNA position metal ions needed for RNA splicing. Its central perturbation is a phosphorothioate substitution: replace a selected non-bridging phosphate oxygen by sulfur. Changing the metal then tests whether altered ligand–metal compatibility can explain the resulting defect. This is a chemical interaction test within the assembled spliceosome.

In the 2013 yeast study, researchers introduced individual sulfur substitutions at twenty U6 positions. They activated and affinity-purified spliceosomes, then assayed their chemical reactions without ATP and soluble factors. This separated the assay from ATP-dependent activation and competing proofreading. Five tested substitutions strongly impaired branch formation in magnesium; cadmium supported rescue of branching. Exon-ligation rescue differed between substitutions, so rescue was not an interchangeable property of every altered phosphate.^[ref]

An original way to quantify the control logic is a ratio of ratios. Let $v_{O,A}$ and $v_{S,A}$ be measured activities for oxygen- and sulfur-containing RNA in metal A, with analogous values for metal B. Definewhen all denominators are positive. If switching metals multiplies every activity by the same amount, then $R=1$. Invented values $v_{O,A}=1$, $v_{S,A}=0.1$, $v_{O,B}=0.5$, $v_{S,B}=0.4$ give $R=8$: B preferentially relieves the sulfur-associated defect. This is an interaction metric, not a measured value from the paper or a direct metal-binding constant.

A preferential rescue still needs controls for folding, assembly and which reaction is being measured. The experiment's strength is connecting a particular RNA chemical group to catalytic output through a compensating perturbation. Omitting ATP after activation does not show that the complete cellular splicing process requires no ATP.