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 photo-electron 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, which are 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 a 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 within 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 constraint of the triphosphate chain by Mg2+. Thirteen [ATP4−·2H+]2− isomers with unfolded geometries likely account for the major high-EBE (electron-binding-energy) spectral features. Notably, for the first time, a low EBE and temperature-dependent spectral feature is observed and attributed to two folded isomers of [ATP4−·2H+]2−, which exist at 20 K but disappear at room temperature. This study provides valuable molecular-level insights into cellular MgATP that resides within the hydrophobic pockets of proteins.