Stabilization of excess charge in isolated adenosine 5 '-triphosphate and adenosine 5 '-diphosphate multiply and singly charged anions

Multiply charged anions (MCAs) represent highly energetic species in the gas phase but can be stabilized through formation of molecular clusters with solvent molecules or counterions. We explore the intramolecular stabilization of excess negative charge in gas-phase MCAs by probing the intrinsic stability of the [adenosine 5′-triphosphate-2H] ([ADP-2H]), [adenosine 5′-diphosphate-2H] ([ADP-2H]), and HPO dianions and their protonated monoanionic analogues. The relative activation barriers for decay of the dianions via electron detachment or ionic fragmentation are investigated using resonance excitation of ions isolated within a quadrupole trap. All of the dianions decayed via ionic fragmentation demonstrating that the repulsive Coulomb barriers (RCB) for ionic fragmentation lie below the RCBs for electron detachment. Both the electrospray ionization mass spectra (ESI-MS) and total fragmentation energies for [ATP-2H], [ADP-2H], and HPO indicate that the multiply charged HPO phosphate moiety is stabilized by the presence of the adenosine group and the stability of the dianions increases in the order HPO < [ADP-2H] < [ATP-2H]. Fully optimized, B3LYP/6-31+G* minimum energy, structures illustrate that the excess charges in all of the phosphate anions are stabilized by intramolecular hydrogen bonding either within the phosphate chain or between the phosphate and the adenosine. We develop a model to illustrate that the relative magnitudes of the RCBs and hence the stability of these ions is dominated by the extent of intramolecular hydrogen bonding