Latest Science Highlights
Isotope and Temperature Effects in Liquid Water Probed by Soft X RaysNThe geometric structure of liquid water has been investigated in detail by many techniques, but many details are still under debate, such as the actual number of hydrogen bonds (at a given time) between the various water molecules. Even less is known about the electronic structure. Since it is the intermittent bonding between water molecules that gives liquid water its peculiar characteristics, the electronic structure plays a crucial role in understanding the properties of the liquid state. Consequently, information essential for insight into chemical and biological processes in aqueous environments is lacking. To address this need, researchers from Germany and the U.S. have used soft x-ray spectroscopy at the ALS to gain detailed insight into the electronic structure of liquid water. Their spectra show a strong isotope and a weak temperature effect, and, for the first time, a splitting of the primary emission line in x-ray emission spectra. By making use of the internal "femtosecond clock" of the core-hole lifetime, a detailed picture of the electronic structure can be painted that involves fast dissociation processes of the probed water molecules. |
Characterization of Selective Binding of Alkali Cations with CarboxylateDuring its lifetime, a cell spends a considerable fraction of its energy pumping sodium and calcium out and potassium in. This balancing process is similar to that found in the coils of the DNA double helix, where specific ions nestle and help stabilize this macromolecule. These are only two examples of selective ion interactions in biology; there are many others also vital to life. The existence of these interactions has been known since the early 20th century, when Franz Hofmeister observed that some salts (ionic compounds) aided the solution of proteins in egg, some caused proteins to destabilize and precipitate, and others ranged in activity between the two extremes. Hofmeister then ranked "salt-out" (destabilizing) ions versus "salt-in" (stabilizing) ions according to the magnitude of their effects (the "Hofmeister effects"). However, despite enormous effort, why certain interactions are preferred over others is not completely understood. Recently, a team of researchers from UC Berkeley used the model systems of acetate and formate (two simple carboxylic acids) with a series of cations to test predictions made in the literature for preferential interactions. Near-edge x-ray absorption fine structure (NEXAFS) spectroscopy was used as this technique is highly sensitive to the chemical environments around a molecule. Experiments at ALS Beamline 8.0.1 confirmed strengthening of the interaction between the cations and the carboxylate group in the following order: potassium, sodium, and lithium. |
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