Resumen

MCCARTHY, William J.; ADAMOWICZ, Ludwik; SAINT-MARTIN, Humberto  y  ORTEGA-BLAKE, Iván. A comparative ab initio Study of the Isomerizations and Hydrolyses of neutral and anionic M-Pyrophosphate Complexes, with M = Ca, Zn. Rev. Soc. Quím. Méx [online]. 2002, vol.46, n.2, pp.145-158. ISSN 0583-7693.

In this work ab initio calculations were performed on anhydrous and monohydrated gas-phase calcium and zinc dications chelated with various anionic pyrophosphate species: H₂P₂O₇^2-, HP₂O₇^3- and P₂O₇^4-. The cleavage of the M-pyrophosphate into a metaphosphate and an orthophosphate coordinated by the dication was also investigated. The studied isomerization reactions were [M · H_NP₂O₇]^(2-N)- → [H_NPO₄·M·PO₃]^(2-N)- where N = 0, 1, 2, and M = Ca, Zn. These reactions were also studied in the presence of one water molecule and also with a continuum model to mimic the solvation effects. It was found that the interaction with a water molecule stabilizes the pyrophosphates relative to their metaphosphate isomers. Moreover, upon interacting with the dianionic complexes, the water molecule readily breaks and produces a hydroxide anion: H₂O + [M · P₂O₇]^2- → [HO · M · HP₂O₇]^2-. The latter dianionic complexes resulted virtually isoenergetic with the hydrolyzed [M · (HPO₄)₂]^2- complexes. The results were compared to previous data of magnesium-containing complexes. This comparative analysis shows that the differences in the structures imposed by the various cations may play an important role in the allosteric transformations of the substrate, that ultimately promote the hydrolysis. Though not conclusive, evidence was found that Zn²⁺ has a similar oxidizing power on M-pyrophosphates as Mg²⁺, whereas the oxidizing power of Ca²⁺ is significantly lower. Nevertheless, the electronic density on the M-pyrophosphates and on the M-phosphates turned out to be very similar for the three cations, and thus the main difference in their biochemical role when interacting with phosphates stems from their different coordination properties, that could even lead to the presence of a hydroxide anion in the first hydration shells of Mg- and Zn-pyrophosphates.

Palabras llave : Pyrophosphate hydrolysis; metal cations; nucleophilic substitution; coordination properties; ab initio calculations.

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