Structure and stabilization mechanism of ATP4–∙Mg2+2–: a joint negative ion photoelectron spectroscopic and computational study

MgATP is a stable complex formed by the chelation of Mg²⁺ with deprotonated Adenosine-5'-triphosphate (ATP). In the cellular environment, MgATP plays a critical role in ATP hydrolysis, releasing substantial energy to support essential biological functions. To understand the structure and stabilization mechanism of MgATP, we conducted a joint negative ion photoelectron spectroscopic and computational study of the ATP4–∙Mg2+2– complex dianion, using ATP4–∙2H+2– as a reference. The experimentally determined adiabatic and vertical detachment energies (ADE and VDE) of ATP4–∙Mg2+2– at 20 K are 3.51 ± 0.05 eV and 3.82 ± 0.05 eV, respectively. The major spectral features of ATP4–∙Mg2+2– are attributed to two theoretically identified isomers with unfolded geometries, stabilized primarily by electrostatic interactions between Mg2+ and the triphosphate and ribose groups, with four deprotonated oxygens forming a pseudo-tetrahedral coordination. In contrast, ATP4–∙2H+2– exhibits fundamentally different stabilization mechanism. Although most of the fifteen identified ATP4–∙2H+2– isomers also adopt unfolded geometries, they are primarily stabilized by intramolecular hydrogen bonds with the triphosphate group and between triphosphate and ribose groups. The interaction between ATP4– and two protons is found to be much weaker than that with Mg2+, and ATP4–∙2H+2– exhibits substantial structural flexibility compared to ATP4–∙Mg2+2– due to the conformational constrain of the triphosphate chain by Mg2+. Thirteen ATP4–∙2H⁺2– isomers with unfolded geometries likely account for the major high-EBE spectral features. Notably, for the first time, a low electron-binding-energy and temperature-dependent spectral feature is observed and attributed to two folded isomers of ATP4–∙2H⁺2– that exist at 20 K but disappear at room temperature. This study provides valuable molecular-level insights into cellular MgATP that resides within the hydrophobic pocket of proteins.