Globule to Helix Transition in Sodiated Polyalanines

International audienceThe structures of sodiated poly-alanine peptides containing 8-12 residues are investigated using infrared multiple photon dissociation (IRMPD) spectroscopy and classical and quantum modeling. Calculations indicate that the a-helical structure is the most stable conformation for the peptides whatever their size. The IRMPD spectra provide evidence for the coexistence of helical and globular shapes for Ala(8)Na(+), and possibly for Ala(9)Na(+) The turning point from globule to helix is thus found at Ala(8-9)Na(+). The N-H and O-H stretching region allows identifying a new spectroscopic pattern typical for alpha-helical structures of polyalanines

CO₂ Adsorption in Fe₂(dobdc): A Classical Force Field Parameterized from Quantum Mechanical Calculations

Carbon dioxide adsorption isotherms have been computed for the metal–organic framework (MOF) Fe₂(dobdc), where dobdc^4– = 2,5-dioxido-1,4-benzenedicarboxylate. A force field derived from quantum mechanical calculations has been used to model adsorption isotherms within a MOF. Restricted open-shell Møller–Plesset second-order perturbation theory (ROMP2) calculations have been performed to obtain interaction energy curves between a CO₂ molecule and a cluster model of Fe₂(dobdc). The force field parameters have been optimized to best reproduced these curves and used in Monte Carlo simulations to obtain CO₂ adsorption isotherms. The experimental loading of CO₂ adsorbed within Fe₂(dobdc) was reproduced quite accurately. This parametrization scheme could easily be utilized to predict isotherms of various guests inside this and other similar MOFs not yet synthesized

Chemical shift extremum of ¹²⁹Xe(aq) reveals details of hydrophobic solvation

Abstract The ¹²⁹Xe chemical shift in an aqueous solution exhibits a non-monotonic temperature dependence, featuring a maximum at 311 K. This is in contrast to most liquids, where the monotonic decrease of the shift follows that of liquid density. In particular, the shift maximum in water occurs at a higher temperature than that of the maximum density. We replicate this behaviour qualitatively via a molecular dynamics simulation and computing the ¹²⁹Xe chemical shift for snapshots of the simulation trajectory. We also construct a semianalytical model, in which the Xe atom occupies a cavity constituted by a spherical water shell, consisting of an even distribution of solvent molecules. The temperature dependence of the shift is seen to result from a product of the decreasing local water density and an increasing term corresponding to the energetics of the Xe-H₂O collisions. The latter moves the chemical shift maximum up in temperature, as compared to the density maximum. In water, the computed temperature of the shift maximum is found to be sensitive to both the details of the binary chemical shift function and the coordination number. This work suggests that, material parameters allowing, the maximum should be exhibited by other liquids, too