GAS-PHASE CHEMISTRY OF DIPHOSPHATE ANIONS AS A TOOL TO INVESTIGATE THE INTRINSIC REQUIREMENTS OF PHOSPHATE ESTER ENZYMATIC REACTIONS: THE [M1M2HP2O7]- IONS

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 [M1M2HP2O7]- ions where M1 and M2 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 10-4M 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 [M1M2HP2O7]- ions as a mixed ionic population formed by two isomeric species: linear diphosphate anion coordinated to two M+ cations (group I) and[PO3···MH2PO4]- clusters II and the [PO3···M1M2···HPO4]- clusters (group II). 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 [M1M2HP2O7]- 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.