Paul B. Howes

Influence of palmitic acid and hexadecanol on the phase transition temperature and molecular packing of dipalmitoylphosphatidyl-choline monolayers at the air–water interface

Palmitic acid (PA) and 1-hexadecanol (HD) strongly affect the phase transition temperature and molecular packing of dipalmitoylphosphatidylcholine (DPPC) monolayers at the air–water interface. The phase behavior and morphology of mixed DPPC/PA as well as DPPC/HD monolayers were determined by pressure-area-isotherms and fluorescence microscopy. The molecular organization was probed by synchrotron grazing incidence x-ray diffraction using a liquid surface diffractometer. Addition of PA or HD to DPPC monolayers increases the temperature of the liquid-expanded to condensed phase transition. X-ray diffraction shows that DPPC forms mixed crystals both with PA and HD over a wide range of mixing ratios. At a surface pressure (π) of 40 mN/m, increasing the amount of the single chain surfactant leads to a reduction in tilt angle of the aliphatic chains from nearly 30° for pure DPPC to almost 0° in a 1:1 molar ratio of DPPC and PA or HD. At this composition we also find closest packing of the aliphatic chains. Furth...

Bacterial S-Layer Protein Coupling to Lipids: X-Ray Reflectivity and Grazing Incidence Diffraction Studies

The coupling of bacterial surface (S)-layer proteins to lipid membranes is studied in molecular detail for proteins from Bacillus sphaericus CCM2177 and B. coagulans E38-66 recrystallized at dipalmitoylphosphatidylethanolamine (DPPE) monolayers on aqueous buffer. A comparison of the monolayer structure before and after protein recrystallization shows minimal reorganization of the lipid chains. By contrast, the lipid headgroups show major rearrangements. For the B. sphaericus CCM2177 protein underneath DPPE monolayers, x-ray reflectivity data suggest that amino acid side chains intercalate the lipid headgroups at least to the phosphate moieties, and probably further beyond. The number of electrons in the headgroup region increases by more than four per lipid. Analysis of the changes of the deduced electron density profiles in terms of a molecular interpretation shows that the phosphatidylethanolamine headgroups must reorient toward the surface normal to accommodate such changes. In terms of the protein structure (which is as yet unknown in three dimensions), the electron density profile reveals a thickness lz approximately 90 A of the recrystallized S-layer and shows water-filled cavities near its center. The protein volume fraction reaches maxima of >60% in two horizontal sections of the S-layer, close to the lipid monolayer and close to the free subphase. In between it drops to approximately 20%. Four S-layer protein monomers are located within the unit cell of a square lattice with a spacing of approximately 131 A.

Synchrotron X-Ray Study of Lung Surfactant-Specific Protein SP-B in Lipid Monolayers

This work reports the first x-ray scattering measurements to determine the effects of SP-B(1-25), the N-terminus peptide of lung surfactant-specific protein SP-B, on the structure of palmitic acid (PA) monolayers. In-plane diffraction shows that the peptide fluidizes a portion of the monolayer but does not affect the packing of the residual ordered phase. This implies that the peptide resides in the disordered phase, and that the ordered phase is essentially pure lipid, in agreement with fluorescence microscopy studies. X-ray reflectivity shows that the peptide is oriented in the lipid monolayer at an angle of approximately 56 degrees relative to the interface normal, with one end protruding past the hydrophilic region into the fluid subphase and the other end embedded in the hydrophobic region of the monolayer. The quantitative insights afforded by this study lead to a better understanding of the lipid/protein interactions found in lung surfactant systems.

Cholesterol Monohydrate Nucleation in Ultrathin Films on Water

The growth of a cholesterol crystalline phase, three molecular layers thick at the air-water interface, was monitored by grazing incidence x-ray diffraction and x-ray reflectivity. Upon compression, a cholesterol film transforms from a monolayer of trigonal symmetry and low crystallinity to a trilayer, composed of a highly crystalline bilayer in a rectangular lattice and a disordered top cholesterol layer. This system undergoes a phase transition into a crystalline trilayer incorporating ordered water between the hydroxyl groups of the top and middle sterol layers in an arrangement akin to the triclinic 3-D crystal structure of cholesterol x H(2)O. By comparison, the cholesterol derivative stigmasterol transforms, upon compression, directly into a crystalline trilayer in the rectangular lattice. These results may contribute to an understanding of the onset of cholesterol crystallization in pathological lipid deposits.

Langmuir and Langmuir-Blodgett films of amphiphilic hexa-peri-hexabenzocoronene: new phase transitions and electronic properties controlled by pressure.

We present the synthesis as well as the structural and electronic properties of an amphiphilic derivative of hexaalkylhexa-peri-hexabenzocoronene (HBC), which contains one alkyl substituent that is terminated with a carboxylic acid group. The molecules form well-defined Langmuir films when spread from a solution at the air-water interface. Grazing-incidence X-ray diffraction (GIXD) and X-ray reflectivity studies of the Langmuir monolayer reveal two crystallographic phases at room temperature which depend on the surface pressure applied to the film. Scattering from very well-ordered (zeta = 200-400 A) pi-stacked lamellae of HBC molecules tilted approximately 45 degrees relative to the surface normal is observed in the low-pressure phase. In this phase, the HBC molecules pack in a rectangular two-dimensional unit cell with a = 22.95 A and b = 4.94 A. In the high-pressure phase, coherence from the pi stack is lost. This is a consequence of stress induced by the crystallization of the substituent alkyl chains into a hexagonal lattice, which has a trimerized superstructure in one direction: a = 3 x b = 15.78 A, b = 5.26 A, gamma = 120 degrees, A = 71.9 A2 = 3 x 23.9 A2. Thin monolayer films can be transferred to solid supports by the Langmuir-Blodgett (LB) technique. Atomic force microscopy (AFM) with atomic resolution reveals the crystalline packing of alkyl chains in the high-pressure phase. Kelvin force microscopy (KFM) shows a clear potential difference between the high- and low-pressure phases. This is discussed in terms of orbital delocalization (band formation) in the highly coherent low-pressure phase, which is in contrast to the localized molecular orbitals present in the high-pressure phase. The highly coherent pi stack is expected to sustain a very high charge-carrier mobility.

Coupling of protein sheet crystals (S-layers) to phospholipid monolayers

The coupling of bacterial S-layer proteins to phospholipid membranes has been studied in molecular detail with respect to, particularly, the lipid headgroups. Emphasis has been laid on two of the best characterized protein species, the S-layer protein from Bacillus sphaericus CCM2177 and from Bacillus coagulans E38–66/V1. A combination of fluorescence microscopy, surface sensitive scattering techniques (grazing-incidence X-ray diffraction as well as X-ray and neutron reflectometry) and infrared spectroscopy (FT-IRRAS), applied to surface monolayers of lipids onto which the protein has been reconstituted as continuous molecular crystal sheets, provides a wealth of information which has been utilized to propose detailed molecular models.

The growth of indium on the Si(111) surface studied by X-ray reflectivity and Auger electron spectroscopy

Abstract The growth of indium on the 7 × 7 reconstructed (111) surface of silicon has been studied as a function of substrate temperature by X-ray reflectivity and Auger electron spectroscopy. A correlation is seen between features in the X-ray reflectivity curves and breaks in the Auger signal versus time plot. The oscillation of the X-ray signal indicates that below 400°C two consecutive pseudomorphic indium layers are formed before three-dimensional islanding occurs. Kinematic analysis predicts the shape of the X-ray signal versus time curve and yields the perpendicular displacement of the indium layers from the substrate. It is proposed that the first layer of indium atoms are bonded vertically above the top most layer of silicon atoms in T1 sites.

Oriented Crystalline Monolayers and Bilayers of 2×2 Silver(I) Grid Architectures at the Air-Solution Interface: Their Assembly and Crystal Structure Elucidation

Oriented crystalline monolayers, approximately 14 A thick, of a 2 x 2 Ag+ grid complex, self-assembled at the air-solution interface starting from an water-insoluble ligand 3,6-bis[2-(6-phenylpyridine)]pyridazine spread on silver-ion-containing solutions, were examined by grazing-incidence X-ray diffraction and specular X-ray reflectivity using synchrotron radiation. The monolayer structure was refined, including a determination of the positions of the counter-ions, with the SHELX-97 computer program. The monolayers were transferred from the interface onto various solid supports and visualized by scanning force microscopy, and characterized by X-ray photoelectron spectroscopy in terms of molecular structure. On surface compression, the initial self-assembled monolayer undergoes a transition to a crystalline bilayer in which the two layers, almost retaining the original arrangement, are in registry. Such a phase transition is of relevance to the understanding of crystal nucleation.

DNA damage of macrophages at an air-tissue interface induced by metal nanoparticles

Abstract An air-tissue interface model was used to assess nanoparticulate-induced DNA damage to airway macrophages. Human Mono Mac 6 cells and rat alveolar macrophages were cultured on a collagen membrane and the deposition of metal nanoparticles in nitrogen enhanced using electrostatic charge. Cells were exposed to nanoparticles of iron, gold, silver for up to 10 min, then cultured in medium for 24 hours. Damage to DNA was assessed using the Comet assay. Nanoparticle dose delivered to cells varied with metal. Significant DNA damage to macrophages was induced by all three metal nanoparticles. Transmission electron microscopy showed deposition of discrete nanoparticles of gold and silver, but not iron. We conclude that an air-tissue model is a useful method for modelling DNA damage to airway cells from inhalation of metal nanoparticles.

The effect of Sb on the nucleation and growth of Ag on Ag(100)

Abstract We have investigated the homoepitaxial growth of Ag(100) at room temperature by means of surface X-ray diffraction. The out-of-phase specular reflection shows intensity oscillations during growth indicating layer-by-layer growth. From transverse in-plane profile scans the island-size distribution is determined at various stages during the growth. Pre-deposition of Sb on the clean surface improves the layer-by-layer growth by enhancing the interlayer transport. The lineshape of the peak profiles indicates that the preferred terrace size has disappeared in this case, showing that the randomly deposited Sb atoms act as nucleation centers for the formation of Ag islands.