Functional lipid pairs as building blocks of phase-separated membranes.

Biological membranes exhibit a great deal of compositional and phase heterogeneity due to hundreds of chemically distinct components. As a result, phase separation processes in cell membranes are extremely difficult to study, especially at the molecular level. It is currently believed that the lateral membrane heterogeneity and the formation of domains, or rafts, are driven by lipid-lipid and lipid-protein interactions. Nevertheless, the underlying mechanisms regulating membrane heterogeneity remain poorly understood. In the present work, we combine inelastic X-ray scattering with molecular dynamics simulations to provide direct evidence for the existence of strongly coupled transient lipid pairs. These lipid pairs manifest themselves experimentally through optical vibrational (a.k.a. phononic) modes observed in binary (1,2-dipalmitoyl-sn-glycero-3-phosphocholine [DPPC]-cholesterol) and ternary (DPPC-1,2-dioleoyl-sn-glycero-3-phosphocholine/1-palmitoyl-2-oleoyl-glycero-3-phosphocholine [DOPC/POPC]-cholesterol) systems. The existence of a phononic gap in these vibrational modes is a direct result of the finite size of patches formed by these lipid pairs. The observation of lipid pairs provides a spatial (subnanometer) and temporal (subnanosecond) window into the lipid-lipid interactions in complex mixtures of saturated/unsaturated lipids and cholesterol. Our findings represent a step toward understanding the lateral organization and dynamics of membrane domains using a well-validated probe with a high spatial and temporal resolution.

Hidden diversity of vacancy networks in Prussian blue analogues.

Prussian blue analogues (PBAs) are a diverse family of microporous inorganic solids, known for their gas storage ability1, metal-ion immobilization2, proton conduction3, and stimuli-dependent magnetic4,5, electronic6 and optical7 properties. This family of materials includes the double-metal cyanide catalysts8,9 and the hexacyanoferrate/hexacyanomanganate battery materials10,11. Central to the various physical properties of PBAs is their ability to reversibly transport mass, a process enabled by structural vacancies. Conventionally presumed to be random12,13, vacancy arrangements are crucial because they control micropore-network characteristics, and hence the diffusivity and adsorption profiles14,15. The long-standing obstacle to characterizing the vacancy networks of PBAs is the inaccessibility of single crystals16. Here we report the growth of single crystals of various PBAs and the measurement and interpretation of their X-ray diffuse scattering patterns. We identify a diversity of non-random vacancy arrangements that is hidden from conventional crystallographic powder analysis. Moreover, we explain this unexpected phase complexity in terms of a simple microscopic model that is based on local rules of electroneutrality and centrosymmetry. The hidden phase boundaries that emerge demarcate vacancy-network polymorphs with very different micropore characteristics. Our results establish a foundation for correlated defect engineering in PBAs as a means of controlling storage capacity, anisotropy and transport efficiency.

Order, Disorder, and Dynamics in Brownmillerite Sr2Fe2O5.

The room-temperature structure of brownmillerite-type Sr2Fe2O5 remains controversial, despite numerous published crystallographic studies utilizing X-ray, neutron, and electron diffraction data collected on single-crystalline and powder samples. The main disagreements concern the ordering of twisted FeO4 tetrahedral chains within and between the layers stacked along the b axis, and the impact of this ordering on oxide-ionic conductivity. Here, we present new data along with a reinterpretation of previously published diffraction images, including the reassignment of satellite reflections, which harmonize the results of past studies in a unified description of tetrahedral chain ordering in Sr2Fe2O5 at length scales relevant to X-ray and neutron diffraction. Implications for the prevailing model of oxide ion transport in this material are also discussed.

Measuring the Vibrational Density of States of Nanocrystal-Based Thin Films with Inelastic X-ray Scattering.

Knowledge of the vibrational structure of a semiconductor is essential for explaining its optical and electronic properties and enabling optimized materials selection for optoelectronic devices. However, measurement of the vibrational density of states of nanomaterials is challenging. Here, using the example of colloidal nanocrystals (quantum dots), we show that the vibrational density of states of nanomaterials can be accurately and efficiently measured with inelastic X-ray scattering (IXS). Using IXS, we report the first experimental measurements of the vibrational density of states for lead sulfide nanocrystals with different halide-ion terminations and for CsPbBr3 perovskite nanocrystals. IXS findings are supported with ab initio molecular dynamics simulations, which provide insight into the origin of the measured vibrational structure and the effect of nanocrystal surface. Our findings highlight the advantages of IXS compared to other methods for measuring the vibrational density of states of nanocrystals such as inelastic neutron scattering and Raman scattering.

Revealing the mechanism of passive transport in lipid bilayers via phonon-mediated nanometre-scale density fluctuations.

The passive transport of molecules through a cell membrane relies on thermal motions of the lipids. However, the nature of transmembrane transport and the precise mechanism remain elusive and call for a comprehensive study of phonon excitations. Here we report a high resolution inelastic X-ray scattering study of the in-plane phonon excitations in 1,2-dipalmitoyl-sn-glycero-3-phosphocholine above and below the main transition temperature. In the gel phase, for the first time, we observe low-frequency transverse modes, which exhibit a phonon gap when the lipid transitions into the fluid phase. We argue that the phonon gap signifies the formation of short-lived nanometre-scale lipid clusters and transient pores, which facilitate the passive molecular transport across the bilayer plane. Our findings suggest that the phononic motion of the hydrocarbon tails provides an effective mechanism of passive transport, and illustrate the importance of the collective dynamics of biomembranes.

Anharmonicity due to electron-phonon coupling in magnetite.

We present the results of inelastic x-ray scattering for magnetite and analyze the energies and widths of the phonon modes with different symmetries in a broad range of temperature 125 < T < 293 K. The phonon modes with X(4) and Δ(5) symmetries broaden in a nonlinear way with decreasing T when the Verwey transition is approached. It is found that the maxima of phonon widths occur away from high-symmetry points, which suggests the incommensurate character of critical fluctuations. Strong phonon anharmonicity induced by electron-phonon coupling is discovered by a combination of these experimental results with ab initio calculations which take into account local Coulomb interactions at Fe ions. It (i) explains observed anomalous phonon broadening and (ii) demonstrates that the Verwey transition is a cooperative phenomenon which involves a wide spectrum of phonons coupled to the electron charge fluctuations condensing in the low-symmetry phase.

Giant Kohn anomaly and the phase transition in charge density wave ZrTe3.

A strong Kohn anomaly in ZrTe3 is identified in the mostly transverse acoustic phonon branch along the modulation vector q_{P} with polarization along the a;{*} direction. This soft mode freezes to zero frequency at the transition temperature T_{P}, and the temperature dependence of the frequency is strongly affected by fluctuation effects. Diffuse x-ray scattering of the incommensurate superstructure shows a power-law scaling of the intensity and the correlation length that is compatible with an order parameter of dimension n=2.

Lattice dynamics of MgO at high pressure: theory and experiment.

The longitudinal acoustic and optical phonon branches along the Gamma-X direction of MgO at 35 GPa have been determined by inelastic x-ray scattering using synchrotron radiation and a diamond-anvil cell. The experimentally observed phonon branches are in remarkable agreement with ab initio lattice dynamics results. The derived thermodynamic properties, such as the specific heat CV and the entropy S are in very good accord with values obtained from a thermodynamically assessed data set involving measured data on molar volume, heat capacity at constant pressure, bulk modulus and thermal expansion.