Active site of the solvated thiosulfate ion characterized by hydration structures and dynamics

Abstract

The reactivity of the terminated sulfur atom within the thiosulfate ion (S2O3(2-)) when it is involved in chemical reactions was investigated through the properties of the molecular hydration shell, obtained from the ab initio quantum mechanical change field molecular dynamics (QMCF MD) simulation. The average geometry indicated the significant effect of explicit water on the reduction of the S-S length, which was reflected in the splitting peaks of the spectrum for the stretching mode of this bond (ν(SS)). A further investigation on a simple model with various theoretical levels exhibited the hydrophobicity of the S-S bond. The evaluation of the molecular coordination number was sensitive to the radii of the atomic hydration spheres, which were obtained from the vague boundaries of the first peak in the atomic radial distribution functions. The number of actual contacts specified 6.8 water molecules interacting with the thiosulfate ion, and 2.4 extra waters located in the molecular hydration shell, forming a H-bonding network with the bulk water. The mean residence times for the water ligands distinguished the asymmetric strength of the hydration shell into a weaker sulfur and three stronger oxygen sites, instigating the terminated sulfur atom as the active site that is involved in chemical reactions.

Structure and water exchange of the hydrated thiosulfate ion in aqueous solution using QMCF MD simulation and large angle X-ray scattering

“…The hydrogen bond lifetime of S T was about half that of the oxygen at 1.16 ps and therefore it can be confirmed as a weak structure breaker. The results obtained in this study for both MRT and hydrogen bond correlation functions are in contrast to the conclusions presented by Trinapakul et al, 5 who reported unexpected results with the terminal thiosulfate sulfur having hydrophobic properties and with a S-S bond distance shorter in aqueous solution than in the solid state. This study clearly shows that significant hydrogen bonds are formed between water and the terminal thiosulfate sulfur even though they are weaker than the hydrogen bonds in bulk water.…”

“…Whether such an asymmetric hydration will affect the water exchange mechanism, as it does for the sulfite ion, is a point of particular interest in this study. The thiosulfate ion was proposed to be asymmetrically hydrated in a recent simulation study, 5 with the water molecules more strongly hydrogen bound to the oxygens (O thio ) than to the terminal sulfur (S T ). It was reported that the terminal sulfur atom has hydrophobic properties, and that the S C -S T bond is shorter in aqueous solution than in the solid state.…”

“…It was reported that the terminal sulfur atom has hydrophobic properties, and that the S C -S T bond is shorter in aqueous solution than in the solid state. 5 The weaker hydration of the S T atom is suggested to be the reason for its role as the active site in chemical reactions. 5 These statements are surprising and need to be verified by independent high level simulations and experimental structure determination in aqueous solution.…”

“…5 The weaker hydration of the S T atom is suggested to be the reason for its role as the active site in chemical reactions. 5 These statements are surprising and need to be verified by independent high level simulations and experimental structure determination in aqueous solution. Thiosulfate is regarded as a typical soft electron-pair donor forming strong complexes to typically soft electron-pair acceptors through its terminal sulfur atom.…”

Hydration properties determining the reactivity of nitrite in aqueous solution

“…11 From another point of view, we recently reported an anisotro-pic hydration shell of the thiosulfate ion, in which the solvent molecules determined the active site on its terminal sulfur atom. 16 These previous results raise the question of how solvent molecules determine the involvement of the nitrite ion in reactions.…”