Modulation of adenosine diphosphate-magnesium ion radical pair spin populations in an external magnetic field

Life's dependence on adenosine triphosphate (ATP) makes its synthesis one of the most important biochemical processes in nature. During ATP synthesis, the (Mg 2+)-adenosine diphosphate (ADP) complex is formed. It has been posited that magnetic field effects (MFEs) may arise through intermediate access of a radical pair state. Prior studies have invoked radical pair mechanisms to explain MFEs and magnetic isotope effects (MIEs) in enzymes involved in ATP synthesis. However, the microscopic conditions required for such effects to occur-and to be reproducibly measured-have not been clearly established. In this work, we investigate the microscopic conditions required for MFEs in the ADP-Mg dimer and identify the criteria for magnetic field sensitivity. Thermal suppression of singlet pathways was observed in the low-moderate magnetic field regime. Corresponding triplet populations exhibited a robust bimodality, with a dominant orientation-independent maximum near 2.2 mT alongside a secondary orientation-dependent maximum at 50 mT. Modelling of MIEs from Mg revealed minimal differences in the spin statistics, suggesting that MIEs are a secondary factor rather than a governing determinant. Embedding these field-dependent spin populations within a time evolution framework demonstrated that MFEs shift the balance between productive and unproductive radical pair recombination, yielding an up to 5-fold increase in effective ATP yields. These results establish a concrete physical basis for magnetic sensitivity of radical pair dynamics in ATP synthesis, and delineate field regimes in which spin-state populations can, in principle, couple to bioenergetic chemistry, while also elucidating the conditions key to observing MFEs in ATP synthesis.