Toward an Understanding of the Hydrolysis of Mg−PP_i. An ab Initio Study of the Isomerization Reactions of Neutral and Anionic Mg−Pyrophosphate Complexes

Abstract

Ab initio calculations were performed to study the stability of various pyrophosphate species in the gas phase: H 4 P 2 O 7 , H 3 P 2 O 7 -, H 2 P 2 O 7 2-, HP 2 O 7 3-, P 2 O 7 4-, and their complexes with Mg 2+ . It is found that the metal cation allows the existence of highly charged anions in the gas phase. We also study the isomerization reactions Mg‚H 2 P 2 O 7 f (H 2 PO 4 ‚Mg‚PO 3 ), (Mg‚HP 2 O 7 ) -f (HPO 4 ‚Mg‚PO 3 ) -, and (Mg‚P 2 O 7 ) 2f (PO 4 ‚Mg‚PO 3 ) 2-, at the self-consistent-field (SCF) and second-order perturbation (MP2) levels of the theory, using a 6-31+G** basis set with diffuse and polarization functions. Other basis sets, including one of valence triple ζ plus polarization (vTZP) quality, were employed to check for the convergence of the results. It is found that the same mechanism occurs for the isomerizations of the three species: one of the P-O bridging bonds of the reactant is longer than the other, and the route to the products proceeds through its elongation. This asymmetry is induced by the metal cation in the case of the evenly charged anions. In all cases the metal cation coordinates the transition states and the leaving groups. The structures found for the complexes (H 2 PO 4 ‚Mg‚PO 3 ), (HPO 4 ‚Mg‚PO 3 ) -, and (PO 4 ‚Mg‚PO 3 ) 2are different from those reported previously, the metal cation being enclosed by the two phosphates. The activation barrier increases with the charge of the anion, from ∆G°q ) 5.6 kcal/mol for the neutral complex Mg‚H 2 P 2 O 7 , to ∆G°q ) 10.4 kcal/mol for the monoanion (Mg‚HP 2 O 7 ) -, to ∆G°q ) 13.5 kcal/mol for the dianion (Mg‚P 2 O 7 ) 2-. The positive value found for the energy of the isomerization (Mg‚P 2 O 7 ) 2f (PO 4 ‚Mg‚PO 3 ) 2-, ∆G°q ) 1.8 kcal/mol, predicts the synthesis to be spontaneous in the gas phase, opposite of what occurs in the aqueous solution. This result supports the view that the hydration energy makes a large contribution to the energy of hydrolysis. The gas-phase hydrolysis reaction H 2 O + Mg 2+ + H 2 P 2 O 7 2f Mg 2+ + H 2 PO 4-+ H 2 PO 4is also studied as a multistep reaction, involving the isomerization of H 2 O + (Mg‚H 2 P 2 O 7 ) f H 2 O + (PO 3 ‚Mg‚H 2 PO 4 ) as an intermediate step. It is found that the equilibrium in the gas phase yields H 2 PO 4 ‚Mg‚H 2 PO 4 as the final species; an energy input is required for separating the metal cation from the phosphate anions.

The Diphosphate Monoanion in the Gas Phase: A Joint Mass Spectrometric and Theoretical Study

“…Clustering and proton transfer products have been observed with substrates characterised by a gasphase acidity value close to that of H 4 P 2 O 7 and amounting to 323 kcal mol À1 . [12,15] The H 3 P 2 O 7 À ions react with (CH 3 ) 3 SiCl and (CH 3 ) 3 SiN 3 yielding [H 2 P 2 O 7 -(CH 3 ) 3 Si] À ions, in agreement with previous observations [22] showing that nucleophiles with a proton affinity lower than that of Cl À (DH8 acid HCl = 333.4 kcalmol…”

Quantum chemical analysis of ATP, GTP and related compounds in gas phase

“…H NMR experiments [24,25] show that the molecule exists in water in an anti-configuration, and the triphosphate tail is fully deporotonated under neutral conditions especially when complexed with Mg 2+ . Mg 2+ has a strong catalytic activity related to its ability to elongate one of the anhydride P-O bonds, with an estimated reaction barrier of 13.5 kcal mol -1 for a pyrophosphate dianion [26][27][28].…”

“…There have been several theoretical studies on the hydrolysis of pyrophosphate in the gas phase [26,[29][30][31] in addition to several other attempts made to model the break of anhydride P-O-P linkage and to understand the underlying mechanism. These approaches range from simple molecular orbital calculations to sophisticated quantum mechanical calculations of isolated active site of protein such as myosin [26,27,30,[32][33][34][35][36][37].…”

“…These approaches range from simple molecular orbital calculations to sophisticated quantum mechanical calculations of isolated active site of protein such as myosin [26,27,30,[32][33][34][35][36][37]. In the field of quantum chemistry, many investigations have been carried out at the Hartree Fock level.…”

Gaseous H5P2O8− Ions: A Theoretical and Experimental Study on the Hydrolysis and Synthesis of Diphosphate Ion

“…[13] Multiply charged anions such as HP 2 O 7 3À and P 2 O 7 4À were hypothesized to be unstable in the gas phase, though Mg 2 + complexation allows them to exist. [14] For the acid-catalyzed hydrolysis of pyrophosphate the potential energy surface is found to be dissociative if the bridging oxygen atom is protonated. [15] A possible catalytic role of the Mg 2 + ion in the hydrolysis reaction of Mg-pyrophosphate complexes was investigated theoretically.…”