The Effects of Sulphate and Tartrate Ions on the Molecular Organization of Water: Towards Understanding the Hofmeister Series (VI)

Using the 1-propanol (1P) probing methodology we have developed earlier, we characterized the effects of sulphate and tartrate anions on the molecular organization of H2O. The results indicate that these two large anions belong to a new class of ``hydrophobe-like hydration center{''}. That is, sulphate and tartrate ions act as ``hydration centers{''} with the hydration number 14 +/- 3 for both, and leave the bulk H2O, away from hydration shells, unperturbed in the absence of the probing 1-propanol. As the mole fraction of the probe increases, however, the hydrogen bond probability of bulk H2O away from hydration shells appears to decrease smoothly, as occurs with ``hydrophobes{''} in H2O. We plot the negative hydration number against the power to reduce the hydrogen bond probability of bulk H2O for the two large anions. We also plotted the characteristic indices for ``hydrophiles{''} and ``hydration centers{''} whose characteristics we determined in the same manner earlier. H2O defines the origin on this map. We found that a typical Hofmeister ranking for each anion matches reasonably well with that of the distance from the origin for each ion, in decreasing order starting from ions plotted in the north-west quadrant (representing the ``hydrophobe-like{''} behavior) of the map and then in increasing order from the origin towards the south on the ordinate, the ``hydrophile-like{''} behavior. These findings could be useful in understanding the Hofmeister series, pointing to the importance of the contribution made by the effect of each ion on H2O, in addition to helping understand direct ion-protein interactions.