High-energy-resolution quasielastic neutron scattering has been used to elucidate the diffusion of water molecules in proximity to single bilayer lipid membranes supported on a silicon substrate. By varying sample temperature, level of hydration, and deuteration, we identify three different types of diffusive water motion: bulk-like, confined, and bound. The motion of bulk-like and confined water molecules is fast compared to those bound to the lipid head groups (7-10 H2O molecules per lipid), which move on the same nanosecond time scale as H atoms within the lipid molecules.
Molecular dynamics simulations have been used to determine the diffusion of water molecules as a function of their position in a fully hydrated freestanding 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine (DMPC) bilayer membrane at 303 K and 1 atm. The diffusion rate of water in a ∼10 Å thick layer just outside the membrane surface is reduced on average by a factor of ∼2 relative to bulk. For water molecules penetrating deeper into the membrane, there is an increasing reduction in the average diffusion rate with up to one order of magnitude decrease for those deepest in the membrane. A comparison with the diffusion rate of selected atoms in the lipid molecules shows that ∼6 water molecules per lipid molecule move on the same time scale as the lipids and may therefore be considered to be tightly bound to them. The quasielastic neutron scattering functions for water and selected atoms in the lipid molecule have been simulated and compared to observed quasielastic neutron scattering spectra from single-supported bilayer DMPC membranes.
We compare the freezing/melting behavior of water hydrating single-supported bilayers of a zwitterionic lipid DMPC with that of an anionic lipid DMPG. For both membranes, the temperature dependence of the elastically scattered neutron intensity indicates distinct water types undergoing translational diffusion: bulk-like water probably located above the membrane and two types of confined water closer to the lipid head groups. The membranes differ in the greater width ΔT of the water freezing transition near the anionic DMPG bilayer (ΔT ∼ 70 K) compared to zwitterionic DMPC (ΔT ∼ 20 K) as well as in the abruptness of the freezing/melting transitions of the bulk-like water.
We have demonstrated the solid-state formation of a uranyl peroxide (UP) species from hydrated uranyl fluoride via a uranyl hydroxide intermediate, the first observation of a UP species formed in a solid-state reaction. Water vapor pressure is shown to be a driving factor of both the loss of fluorine and the subsequent formation of peroxo units. We have ruled out a photochemical mechanism for formation of the UP species by demonstrating that the same reaction occurs in the dark. A radiolytic mechanism is unlikely because of the low radioactivity of the sample material, suggesting the existence of a novel UP formation mechanism.
We
confirm that synthetic uranyl hydroxide hydrate metaschoepite
[(UO)24O(OH)6]·5H2O is unstable
against dehydration under dry conditions, and we present a structural
and vibrational spectroscopic study of synthetic metaschoepite and
its ambient temperature dehydration product. Complementary structural
(X-ray diffraction and neutron diffraction) and vibrational spectroscopic
techniques (Raman spectroscopy, infrared spectroscopy, and inelastic
neutron scattering) are used to probe different components of these
species. Analysis of the dehydration product suggests that it contains
both pentagonally coordinated and hexagonally coordinated uranyl ions,
necessitating that some uranyl ions undergo a coordination change
during the dehydration of pentagonally coordinated metaschoepite.
Vibrational spectra of metaschoepite and its dehydration product are
interpreted with power spectra generated from ab initio molecular
dynamics trajectories, allowing assignment of all major features.
We identify the uranyl symmetric stretching modes of the four distinct
uranyl ions in synthetic metaschoepite and clarify the assignment
of lower energy Raman modes in both structures. The coanalysis of
experimental and computational data reveals a strong coupling between
the uranyl stretching modes and hydroxide bending modes in the anhydrous
structure, leading to the presence of several high-energy combination
bands in the inelastic neutron scattering data.
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