Gas-phase chemistry of diphosphate anions as a tool to investigate the intrinsic requirements of phosphate ester enzymatic reactions: The [(MMHP2O7)-M-1-H-2](-) Ions

摘要

Experimental studies on gaseous inorganic phosphate ions are practically nonexistent, yet they can prove helpful for a better understanding of the mechanisms of phosphate ester enzymatic processes. The present contribution extends our previous investigations on the gas-phase ion chemistry of diphosphate species to the [(MMHP2O7)-M-1-H-2](-) ions where M-1 and M-2 are the same or different and correspond to the Li, Na, K, Cs, and Rb cations. The diphosphate ions are formed by electrospray ionization of 10(-4) m solutions of Na5P3O10 in CH3CN/H2O (1/1) and MOH bases or M salts as a source of M+ cations. The joint application of mass spectrometric techniques and quantum-mechanical calculations makes it possible to characterize the gaseous [(MMHP2O7)-M-1-H-2](-) ions as a mixed ionic population formed by two isomeric species: linear diphosphate anion coordinated to two M+ cations (group 1) and [PO3...(MM2)-M-1... HPO4](-) clusters (group U). The relative gas-phase stabilities and activation barriers for the isomerization I -> II, which depend on the nature of the M+ cations, highlight the electronic susceptibility of P-O-P bond breaking in the active site of enzymes. The previously unexplored gas-phase reactivity of [(MMHP2O7)-M-1-H-2](-) ions towards alcohols of different acidity was investigated by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR/MS). The reaction proceeds by addition of the alcohol molecule followed by elimination of a water molecule.