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Is water responding symmetrically to charge?

Date Do, 28.05.2015 - Do, 28.05.2015
Time 16.30
Speaker Prof. Sylvie Roke, Laboratory for fundamental BioPhotonics, EPFL
Location EPF Lausanne, CH G1 495
Program Being positive is best in all human affairs. For charges in an aqueous environment, being negative might be better. A number of biological, chemical and physical phenomena show indirect evidence that water may have a preference for negative charge, for example:

Lipid membranes, macromolecules and their interfaces are either neutral or negatively charged – why did biology choose for negative membranes? The hydrophobic/water surface carries a negative charge The point of zero charge of most interfaces occurs at very low pH The dynamics of water reorientation and H-bond exchange is different around positive and negative ions Hofmeister effects are more pronounced for negatively charged ions than for positively charged ones

Here, we have quantified the differences between the hydration-shell and hydrophobic/water interfacial structure induced by two ions of virtually identical size, shape, and chemical structure, but with opposite charge. We find that the water structures around the two isolated ions, as well as the influences of these two ions on the (weakly charged) hydrophobic/water interface are dramatically different: Anions are preferentially hydrated and induce greater orientation order on water near hydrophobic interfaces. Cations have far fewer and weaker pi-H bonds than the anions and strongly reduce the orientational order of water near a hydrophobic interface. The observations can be explained by the cooperative (anions), and non-cooperative (cations) interplay of H-bonding and electrostatic interactions. Our results imply a preference for water interfaces to be negatively charged, which explains nature’s choice to prefer negative rather than positive charge.
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