Nanocrystal induced organization of a langmuir phospholipid monolayer

Assembly of amorphous clusters under floating monolayers: a comparison of in situ and ex situ techniques

We report synchrotron X-ray scattering studies of biomimetic crystallization of hydroxyapatite (the primary constituent of bone), using monolayers of fatty acid molecules floating on simulated body fluid (SBF) as well as aqueous solutions of calcium phosphate. A ∼10 Å thick film of amorphous material is observed to form immediately at the molecular monolayer, consistent with the proposed formation of "Posner clusters". This layer becomes denser but not significantly thicker as the subphase concentration and the temperature approach physiological conditions. The amorphous films do not crystallize within 24 h, in contrast to prior reports of more rapid crystallization using electron microscopy on ex situ samples. However, crystallization occurs almost immediately after our films are transferred onto solid substrates. These results illustrate the importance of in situ measurements for model biomineralization experiments.

From specular reflectivity to time-resolved grazing incidence X-ray scattering out of the specular plane (GISAXS): equilibrium and nonequilibrium states of organic/inorganic monolayers at liquid surfaces

The grazing incidence diffuse X-ray scattering out of the specular plane technique (GISAXS) is presented, and its capabilities are compared to the more classical X-ray specular reflectivity technique (XSR). Three experimental illustrations are given to prove the efficiency of GISAXS. First, the structure of a DPPC phospholipid monolayer is analyzed. Second, the time-resolved kinetics of lipid desorption from a monolayer interacting with a mineral gel subphase is studied. Third, the structural evolution of biomembranes at extremely low temperatures is illustrated. The GISAXS technique appears to compete efficiently with neutron reflection techniques by taking advantage of remarkably short acquisition times crucial in kinetics experiments.

Structural and electric properties of two semifluorinated alkane monolayers compressed on top of a controlled hydrophobic monolayer substrate

We investigate the dynamic behavior upon lateral compression of two mixed films made with one of the two semifluorinated alkanes F(CF2)8(CH2)18H and F(CF2)10(CH2)10H and the natural alpha-helix alamethicin peptide. Surface pressure, surface potential versus molecular area isotherms, and grazing-incidence x-ray diffraction were applied to characterize this system. We show that both mixed films demix vertically to form two asymmetric flat bilayers where the lower layer is made of alamethicin and the upper layer is made of semifluorinated molecules. The structure matching of the semifluorinated alkanes (where the hydrophilic group is missing) with a suitable organization of the underlying alamethicin monolayer allows for a continuous compression of the upper semifluorinated layers while the density of the lower alamethicin monolayer remains constant. Comparing data of the two studied mixed films enables us to evaluate the effect of chain length on the in-plane organization of the molecules and on the electric properties of the upper layers.

Observation of the desorption of mineral nanoparticles from a hybrid Langmuir monolayer: an in situ grazing incidence X-ray (GIXOS) study

The kinetics and mechanism of the desorption of mineral clay particles originally adsorbed to a phospholipid monolayer has been studied. The desorption was induced by the injection of citric acid solution into the aqueous subphase. To follow the related kinetic processes, grazing incidence diffuse X-ray scattering out of the specular plane (GIXOS) was used with a remarkable time resolution of the order of one minute. From the variations of the electron density profile normal to the surface, the Langmuir desorption isotherm and the related kinetic constants were determined.

Grazing Incidence X-ray Scattering to Probe the Self-Assembly of Phthalocyanine Nanorods on a Liquid Surface

The structural properties of Langmuir monolayers on aqueous substrates of a metal free phthalocyanine, 2,9,16,23-tetrakis(phenylthio)-29H,31H-phthalocyanine and an Aluminum centered phthalocyanine, Aluminum 2,9,16,23-tetrakis(phenylthio)-29H,31H-phthalocyanine chloride are reported here. Their structure is investigated under progressive lateral compression by grazing incidence diffuse X-ray scattering out of the specular plane to determine specular reflectivity-like information where the phase change of the molecules from "flat-lying" on the surface to "edge-standing" perpendicular to the surface was directly observed. Furthermore grazing incident X-ray diffraction is used to investigate the in-plane ordering of the system where it has been found that at high density states the systems can be considered as monolayers consisting of arrays of side-by-side cofacially aggregated cylindrical rodlike entities.

Observation of zinc phthalocyanine aggregates on a water surface using grazing incidence X-ray scattering

Reported here are the structural properties of a zinc 2,9,16,23-tetra-tert-butyl-29H,31H-phthalocyanine Langmuir monolayer on a water surface under progressive lateral compression investigated by grazing incidence X-ray scattering. Grazing incidence diffuse X-ray scattering out of the specular plane (GIXOS) is exploited to determine specular reflectivity-like information where the phase change of flat-lying molecules on the surface to edge-standing molecules perpendicular to the surface is directly observed. Furthermore, grazing incident X-ray diffraction (GIXD) is used to investigate the in-plane ordering of the system where it has been found that in the high-density state (approximately 0.35 nm2 per molecule) the system can be considered to be a monolayer consisting of arrays of side-by-side lying cofacially aggregated cylindrical rodlike entities.

Thermodynamic and structural properties of phospholipid langmuir monolayers on hydrosol surfaces

Measurements of Langmuir pressure/area isotherms, rheology, grazing incidence X-ray diffraction (GIXD), and grazing incidence diffuse X-ray scattering out of the specular plane (GIXOS) have been used to investigate the influence of a hydrosol containing charged mineral nanoparticles on the thermodynamic and structural properties of a DPPC monolayer. The mineral adsorption layer that is formed via electrostatic interaction underneath the lipid layer alters the thermodynamic properties of the phospholipid monolayer in terms of maximal achievable compression, compressibility, and phase behavior. Modifications appear in the latter case as a coolinglike effect. Rheology measurements of the bulk viscoelastic properties revealed a stabilizing effect of the transient bulk network on the surfactant layer. The lipid chain lattice is found to be reorganized and adapted to the internal atomic structure of the mineral particles. A model for the superposition of Bragg rods from the lipid chains and the minerals is applied to separate these scattering contributions. In the vicinity of the mineral particles, the (2) reflection for DPPC on a liquid substrate was found, indicating strongly suppressed fluctuations at the surface. An estimation of the Debye-Waller factor associated with the lipid layer organization is used to quantify the damping of fluctuations within the lipid matrix due to the rigidifying and stabilizing effect of the mineral particles.