To confirm the hypothesis, ATP-Mg complex was prepared according to Guillain et al. (see SI for details for ATP and ADP, and spectra for ADP-Mg, Fig ) and analyzed under the same nh-PHIP conditions as ATP standard (Figure A). Formation of ATP-Mg was successful as there were no ATP hydride signals in H spectrum (Figure ). Upon mixing ATP-Mg and EDTA methanol-d4 solutions, no ATP signals were observed in the 1 H hydride spectrum. ATP signals returned (Figure , D) after mixing ATP-Mg and EDTA water solutions prior to dilution into methanol-d4. This indicates that when ATP is in a complex with Mg 2+ , it cannot form a complex with nh-PHIP catalyst. EDTA, does not act as a strong metal ion chelator in methanol (as it did not break the ATP-Mg complex when methanol was the main solvent) and only works well in water solutions and is, therefore, compatible with the nh-PHIP iridium catalyst system. Similarly, to the ATP-Mg standard solution (Figure ), when EDTA water solution was added to the reconstituted RBC extract, new peaks appeared in the RBC nh-PHIP spectrum. These new signals were confirmed by spiking to belong to the ATP-iridium (and ADP-iridium) complex, but yet again they did not overlap with the ATP standard spectrum (see SI Figure and Figure ). Adding more EDTA to the RBC extract did not yield any improvement; instead, whenthe concentration of EDTA in the NMR tube exceeded the concentration of Ir-catalyst, the intensity of the whole nh-PHIP spectrum decreased (see SI Figure ). Chelex treatment of the RBC extract also resulted in new signals in the nh-PHIP spectrum, which were confirmed to belong to the ATP (see SI, Figure ). Similarly, to the EDTA-treated RBC nh-PHIP spectrum, the ATP signals did not overlap with the ATP standard solution spectrum.