Impact of Lipid Oxidization on Vertical Structures and Electrostatics of Phospholipid Monolayers Revealed by Combination of Specular X‑ray Reflectivity and Grazing-Incidence X‑ray Fluorescence

The influence of phospholipid oxidization of floating monolayers on the structure perpendicular to the global plane and on the density profiles of ions near the lipid monolayer has been investigated by a combination of grazing incidence X-ray fluorescence (GIXF) and specular X-ray reflectivity (XRR). Systematic variation of the composition of the floating monolayers unravels changes in the thickness, roughness and electron density of the lipid monolayers as a function of molar fraction of oxidized phospholipids. Simultaneous GIXF measurements enable one to qualitatively determine the element-specific density profiles of monovalent (K⁺ or Cs⁺) and divalent ions (Ca²⁺) in the vicinity of the interface in the presence and absence of two types of oxidized phospholipids (PazePC and PoxnoPC) with high spatial accuracy (±5 Å). We found the condensation of Ca²⁺ near carboxylated PazePC was more pronounced compared to PoxnoPC with an aldehyde group. In contrast, the condensation of monovalent ions could hardly be detected even for pure oxidized phospholipid monolayers. Moreover, pure phospholipid monolayers exhibited almost no ion specific condensation near the interface. The quantitative studies with well-defined floating monolayers revealed how the elevation of lipid oxidization level alters the structures and functions of cell membranes

Three-Legged 2,2′-Bipyridine Monomer at the Air/Water Interface: Monolayer Structure and Reactions with Ni(II) Ions from the Subphase

The behavior of compound <b>2</b> [1,3,5-tri­(2,2′-bipyridin-5-yl)­benzene] with three bipyridine units arranged in a star geometry is investigated in the presence and absence of Ni­(ClO₄)₂. Its properties at the air–water interface as well as after transfer onto a solid substrate are studied by several techniques including Brewster angle microscopy, X-ray reflectivity, neutron reflectivity, X-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, and atomic force microscopy combined with optical microscopy. It is found that compound <b>2</b> within the monolayers formed stays almost vertical at the interface and that at high Ni²⁺/<b>2</b> (Ni²⁺/<b>2</b> = 4000, 20′000) ratios two of the three bipyridine units of <b>2</b> are complexed, resulting in supramolecular sheets that are likely composed of arrays of linear metal–organic complexation polymers