Water adsorption and hydrolysis in the Si2O4, P2O5 and P4O10 systems - essential roles of the phosphate system in biosynthesis
Journal article, 2002
Successive water addition to the mononuclear species SiO2(g) and PO2(OH)(g), as well as the corresponding binuclear Si2O4(g) and P2O5(g) molecules, is studied by means of density functional theory. Hydrolysis of (HO)3SiOSi(OH)3(g) is found to be slightly endothermic, and only an asymmetric energy minimum is found for this silicic acid dimer. Four stable conformations are determined for the pyrophosphoric acid system (HO)2OPOPO(OH)2(g). Depending on the choice of reference structure and basis set, the hydrolysis energetics ranges from 14–26 kJ/mol exothermic to 3–7 kJ/mol endothermic. In general, the hydrolysis reaction is best described as a near zero-energy process. Significant differences in the water chemistry of the final monomeric products, Si(OH)4(g) and PO(OH)3(g), appear in the first solvation shell. Connecting the P–OH and P=O groups by water bridges results in a greater tendency for proton delocalization in PO(OH)3(g), than is observed when water is used to connect two Si–OH groups in Si(OH)4(g). Taking the properties of pyrophosphoric acid as model for the P–O–P bridge in adenosine triphosphate (ATP), the results of the present study support the notion that this molecule and its hydrolysis products provide a nano-scale buffer, which is essential for sustaining biosynthesis by controlling the proton and water activities.