No oxygen isotope exchange between water and APS-sulfate at surface temperature: Evidence from quantum chemical modeling and triple-oxygen isotope experiments

The formation and hydrolysis of APS were identified as potential steps for oxygen exchange with water to occur. Quantum chemical modeling indicates that the combination of sulfate with ATP has effects on bond strength and symmetry of the sulfate. However, these small effects impart little influence on the integrity of the SO42− tetrahedron due to the high activation energy required for hydrolysis of SO42− (48.94 kcal/mol). Modeling also indicates that APS dissociation via hydrolysis is achieved through cleavage of the P-O bond instead of S-O bond, further supporting the lack of APS-H2O-oxygen exchange. The formation of APS in our in vitro experiments was verified by HPLC fluorescence spectroscopy, and triple-oxygen isotope data of the APS-sulfate indicate no oxygen isotope exchange occurred between APS-sulfate and water at 30 °C for an experimental duration ranging from 2 to 120 h. The study excludes APS formation as one of the causes for sulfate-oxygen isotope exchange with water during MDSR.