  Site-specific information on how adenosine triphosphate in the aqueous phase
(ATP$_{(aq)}$) interacts with magnesium (Mg$^{2+}_{(aq)}$) is a prerequisite to
understanding its complex biochemistry. To gather such information, we apply
liquid-jet photoelectron spectroscopy (LJ-PES) assisted by electronic-structure
calculations to study ATP$_{(aq)}$ solutions with and without dissolved
Mg$^{2+}$. Valence photoemission data reveal spectral changes in the phosphate
and adenine features of ATP$_{(aq)}$ due to interactions with the divalent
cation. Chemical shifts in Mg 2p, Mg 2s, P 2p, and P 2s core-level spectra as a
function of the Mg$^{2+}$/ATP concentration ratio are correlated to the
formation of [MgATP]$^{-2}_{(aq)}$ and Mg$_2$ATP$_{(aq)}$ complexes,
demonstrating the element-sensitivity of the technique to Mg$^{2+}$-phosphate
interactions. In addition, we report and compare P 2s data from ATP$_{(aq)}$
and adenosine mono- and di-phosphate (AMP$_{(aq)}$ and ADP$_{(aq)}$,
respectively) solutions, probing the electronic structure of the phosphate
chain and the local environment of individual phosphate units in ATP$_{(aq)}$.
Finally, we have recorded intermolecular Coulombic decay (ICD) spectra
initiated by ionization of Mg 1s electrons to probe ligand exchange in the
Mg$^{2+}$-ATP$_{(aq)}$ coordination environment, demonstrating the unique
capabilities of ICD for revealing structural information. Our results provide
an overview of the electronic structure of ATP$_{(aq)}$ and
Mg$^{2+}$-ATP$_{(aq)}$ moieties relevant to phosphorylation and
dephosphorylation reactions that are central to bioenergetics in living
organisms.
