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International audience; The understanding of the microstructure of associated liquids promoted by hydrogen-bonding andconstrained by steric hindrance is highly relevant in chemistry physics, biology and for many aspects ofdaily life. In this study we use a combination of X-ray diffraction, dielectric spectroscopy and moleculardynamics simulations to reveal temperature induced changes in the microstructure of different octanolisomers, i.e., linear 1-octanol and branched 2-, 3- and 4-octanol. In all octanols, the hydroxyl groupsform the bases of chain-, cyclic- or loop-like bonded structures that are separated by outwardlydirected alkyl chains. This clustering is analyzed through the scattering pre-peaks observed from X-rayscattering and simulations. The charge ordering which pilots OH aggregation can be linked to thestrength of the Debye process observed in dielectric spectroscopy. Interestingly, all methods used hereconverge to the same interpretation: as one moves from 1-octanol to the branched octanols, thecluster structure evolves from loose large aggregates to a larger number of smaller, tighter aggregates.All alcohols exhibit a peculiar temperature dependence of both the pre-peak and Debye process, whichcan be understood as a change in microstructure promoted by chain association with increased chainlength possibly assisted by ring-opening effects. All these results tend to support the intuitive picture ofthe entropic constraint provided by branching through the alkyl tails and highlight its capital entropicrole in supramolecular assembly.

We determined the intermolecular interaction potential, V(r), of dense lysozyme solutions, which governs the spatial distribution of the protein molecules and the location of its liquid–liquid phase separation (LLPS) region, in various crowding environments applying small-angle X-ray scattering in combination with liquid-state theory. We explored the effect of polyethylene glycol (PEG) on V(r) and the protein’s phase behavior over a wide range of temperatures and pressures, crossing from the dilute to the semidilute polymer regime, thereby mimicking all crowding scenarios encountered in the heterogeneous biological cell. V(r) and hence the protein–protein distances and the phase boundary of the LLPS region strongly depend on the polymer-to-protein size ratio and the polymer concentration. The strongest effect is observed for small-sized PEG molecules, leading to a marked decrease of the mean intermolecular spacing of the protein molecules with increasing crowder concentration. The effect levels off at int...

The hydration and hydrogen-bond topology of small water solvated molecules such as the naturally occurring organic osmolytes trimethylamine N-oxide (TMAO) and urea are under intense investigation. We aim at furthering the understanding of this complex hydration by combining experimental oxygen K-edge excitation spectra with results from spectra calculated via the Bethe-Salpeter equation based on structures obtained from ab initio molecular dynamics simulations. Comparison of experimental and calculated spectra allows us to extract detailed information about the immediate surrounding of the solute molecules in the solvated state. We quantify and localize the influence of the solute on the hydrogen bond network of the water solvent and find spectroscopic fingerprints of a clear directional asymmetry around TMAO with strong and local kosmotropic influence around TMAO's NO head group and slight chaotropic influence around the hydrophobic methyl groups. The influence of urea on the local water network is qualitatively similar to that of TMAO but weaker in magnitude. The strongest influence of both molecules on the shape of the oxygen K-edge spectra is found in the first hydration shells.

We present an X-ray Compton scattering study on aqueous trimethylamine N-oxide (TMAO) and guanidine hydrochloride solutions (GdnHCl) as a function of temperature. Independent from the concentration of the solvent, Compton profiles almost resemble results for liquid water as a function of temperature. However, the number of hydrogen bonds per water molecule extracted from the Compton profiles suggests a decrease of hydrogen bonds with rising temperature for all studied samples, and the differences between water and the solutions are weak. Nevertheless, the data indicate a reduced bond weakening with rising TMAO concentration up to 5 M of 7.2% compared to 8% for pure water. In contrast, the addition of GdnHCl appears to behave differently for concentrations up to 3.1 M with a weaker impact on the temperature response of the hydrogen bond structure.