How Does Mg 2+ (aq) Interact with ATP (aq) ? Biomolecular Structure through the Lens of Liquid-Jet Photoemission Spectroscopy.

Liquid-jet photoemission spectroscopy (LJ-PES) allows for a direct probing of electronic structure in aqueous solutions. We show the applicability of the approach to biomolecules in a complex environment, exploring site-specific information on the interaction of adenosine triphosphate in the aqueous phase (ATP (aq) ) with magnesium (Mg 2+ (aq) ), highlighting the synergy brought about by the simultaneous analysis of different regions in the photoelectron spectrum. In particular, we demonstrate intermolecular Coulombic decay (ICD) spectroscopy as a new and powerful addition to the arsenal of techniques for biomolecular structure investigation. We apply LJ-PES assisted by electronic-structure calculations to study ATP (aq) solutions with and without dissolved Mg 2+ . Valence photoelectron 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 [Mg(ATP) 2 ] 6- (aq) , [MgATP] 2- (aq) , and [Mg 2 ATP] (aq) complexes, demonstrating the element sensitivity of the technique to Mg 2+ -phosphate interactions. The most direct probe of the intermolecular interactions between ATP (aq) and Mg 2+ (aq) is delivered by the emerging ICD electrons following ionization of Mg 1s electrons. ICD spectra are shown to sensitively probe ligand exchange in the Mg 2+ -ATP (aq) coordination environment. In addition, we report and compare P 2s data from ATP (aq) and adenosine mono- and diphosphate (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) . Our results provide a comprehensive view of the electronic structure of ATP (aq) and Mg 2+ -ATP (aq) complexes relevant to phosphorylation and dephosphorylation reactions that are central to bioenergetics in living organisms.