Marine natural products: dactylol,a new sesquiterpene alcohol from a sea hare

A new sesquiterpene alcohol, dactylol, having an uncommon bicyclo [6.3.0] undecane skeleton has been isolated from the sea hare Aplysia dactylomela. Chemical degradation and NMR studies using a shift reagent established the structure and relative stereochemistry. The absolute configuration was derived from ORD/CD data of a substituted cyclopentanone degradation product.     

Enzyme activity to augment the characterization of tethered bilayer membranes

The rate of Ca2+ -triggered phospholipase A2 (PLA2) degradation of tethered bilayer membranes (tBLMs), composed of a synthetic lipid, beta-mercaptoethanol, and palmitoyloleoylphosphatidylcholine (POPC), is approximately 80 times greater than for those prepared with diphytanoylphosphatidylcholine (DPhyPC). Electrochemical impedance spectroscopy (EIS) and neutron reflectivity (NR) data indicate complete, water-free tBLMs that exhibit near ideal capacitive behavior and the presence of a water reservoir in the bilayer subspace proximal to the substrate (Au) surface for both tBLMs. Together these data indicate that the POPC and the DPhyPC tBLMs are structurally similar along the surface normal but markedly different at the outer leaflet/solution interface and that PLA2 is a sensitive probe of short length scale structural differences not revealed by EIS and NR.     

In-situ characterization of self-assembled monolayers of water-soluble oligo(ethylene oxide) compounds

In-situ spectroscopic ellipsometry (SE) was utilized to examine the formation of the self-assembled monolayers (SAMs) of the water-soluble oligo(ethylene oxide) [OEO] disulfide [S(CH(2)CH(2)O)(6)CH(3)](2) {[S(EO)(6)](2)} and two analogous thiols - HS(CH(2)CH(2)O)(6)CH(3) {(EO)(6)} and HS(CH(2))(3)O(CH(2)CH(2)O)(5)CH(3) {C(3)(EO)(5)} - on Au from aqueous solutions. Kinetic data for all compounds follow simple Langmuirian models with the disulfide reaching a self-limiting final state (d=1.2nm) more rapidly than the full coverage final states of the thiol analogs (d=2.0nm). The in-situ ellipsometric thicknesses of all compounds were found to be nearly identical to earlier ex-situ ellipsometric measurements suggesting similar surface coverages and structural models in air and under water. Exposure to bovine serum albumin (BSA) shows the self-limiting (d=1.2nm) [S(EO)(6)](2) SAMs to be the most highly protein resistant surfaces relative to bare Au and completely-formed SAMs of the two analogous thiols and octadecanethiol (ODT). When challenged with up to near physiological levels of BSA (2.5mg/mL), protein adsorption on the final state [S(EO)(6)](2) SAM was only 3% of that which adsorbed to the bare Au and ODT SAMs.     

Formation and finite element analysis of tethered bilayer lipid structures

Rapid solvent exchange of an ethanolic solution of diphytanoyl phosphatidylcholine (DPhyPC) in the presence of a mixed self-assembled monolayer (SAM) [thiolipid/β-mercaptoethanol (βME) (3/7 mol/mol) on Au] shows a transition from densely packed tethered bilayer lipid membranes [(dp)tBLMs], to loosely packed tethered bilayer lipid membranes [(lp)tBLMs], and tethered bilayer liposome nanoparticles (tBLNs) with decreasing DPhyPC concentration. The tethered lipidic constructs in the aqueous medium were analyzed by atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS). Finite element analysis (FEA) was applied to interpret spectral EIS features without referring to equivalent circuit modeling. Using structural data obtained earlier from neutron reflectometry and dielectric constants of lipid bilayers, we reproduced experimentally observed features of the electrochemical impedance (EI) spectra of complex surface constructs involving small pinhole defects, large membrane-free patches, and bound liposomes. We demonstrated by FEA that highly insulating (dp)tBLMs with low-defect density exhibit EI spectra in the shape of a perfect semicircle with or without low-frequency upward "tails" in the Cole-Cole representation. Such EI spectra were observed at DPhyPC concentrations of >5 × 10(-3) mol L(-1). While AFM was not able to visualize very small lateral defects in such films, EI spectra unambiguously signaled their presence by increased low frequency "tails". Using FEA we demonstrate that films with large diameter visible defects (>25 nm by AFM) produce EI spectral features consisting of two semicircles of comparable size. Such films were typically obtained at DPhyPC concentrations of <5 × 10(-3) mol L(-1). At DPhyPC concentrations of <1.0 × 10(-3) mol L(-1) the planar bilayer structures were replaced by ellipsoidal liposomes with diameters ranging from 50 to 500 nm as observed in AFM images. Despite the distinct surface morphology change, the EI curves exhibited two semicircle spectral features typical for the large size defects in planar tBLMs. FEA revealed that, to account for these EI features for bound liposome systems (50-500 nm diameter), one needs to assume much lower tBLM conductivities of the submembrane space, which separates the electrode surface and the phospholipid bilayer. Alternatively, FEA indicates that such features may also be observed on composite surfaces containing both bound liposomes and patches of planar bilayers. Triple semicircular features, observed in some of the experimental EI curves, were attributed to an increased complexity of the real tBLMs. The modeling demonstrated that such features are typical for heterogeneous tBLM surfaces containing large patches of different defectiveness levels. By integrating AFM, EIS, and FEA data, our work provides diagnostic criteria allowing the precise characterization of the properties and the morphology of surface supported bilayer systems.     

Subsolidus phase equilibria and properties in the system Bi2O3:Mn2O3±x:Nb2O5

Subsolidus phase relations have been determined for the Bi-Mn-Nb-O system in air (750-900 °C). Phases containing Mn²⁺, Mn³⁺, and Mn⁴⁺ were all observed. Ternary compound formation was limited to pyrochlore (A₂B₂O₆O′), which formed a substantial solid solution region at Bi-deficient stoichiometries (relative to Bi₂(Mn,Nb)₂O₇) suggesting that ≈14-30% of the A-sites are occupied by Mn (likely Mn²⁺). X-ray powder diffraction data confirmed that all Bi-Mn-Nb-O pyrochlores form with structural displacements, as found for the analogous pyrochlores with Mn replaced by Zn, Fe, or Co. A structural refinement of the pyrochlore 0.4000:0.3000:0.3000 Bi₂O₃:Mn₂O_3±x:Nb₂O₅ using neutron powder diffraction data is reported with the A and O′ atoms displaced (0.36 and 0.33 Å, respectively) from ideal positions to 96g sites, and with Mn²⁺ on A-sites and Mn³⁺ on B-sites (Bi_1.6Mn²⁺_0.4(Mn³⁺_0.8Nb_1.2)O₇, (?227), a=10.478(1) Å); evidence of A or O′ vacancies was not found. The displacive disorder is crystallographically analogous to that reported for Bi_1.5Zn_0.92Nb_1.5O_6.92, which has a similar concentration of small B-type ions on the A-sites. EELS spectra for this pyrochlore were consistent with an Mn oxidation between 2+ and 3+. Bi-Mn-Nb-O pyrochlores exhibited overall paramagnetic behavior with negative Curie-Weiss temperature intercepts, slight superparamagnetic effects, and depressed observed moments compared to high-spin, spin-only values. At 300 K and 1 MHz the relative dielectric permittivity of Bi_1.600Mn_1.200Nb_1.200O₇ was ≈128 with tan δ=0.05; however, at lower frequencies the sample was conductive which is consistent with the presence of mixed-valent Mn. Low-temperature dielectric relaxation such as that observed for Bi_1.5Zn_0.92Nb_1.5O_6.92 and other bismuth-based pyrochlores was not observed. Bi-Mn-Nb-O pyrochlores were readily obtained as single crystals and also as textured thin films using pulsed laser deposition.     

Crystal structure, magnetic, and dielectric properties of Aurivillius-type Bi3Fe0.5Nb1.5O9

Bi₃Fe_0.5Nb_1.5O₉ was synthesized using conventional solid state techniques and its crystal structure was refined by the Rietveld method using neutron powder diffraction data. The oxide adopts an Aurivillius-type structure with non-centrosymmetric space group symmetry A2₁am (a=5.47016(9) Å, b=5.43492(9) Å, c=25.4232(4) Å), analogous to other Aurivillius compounds that exhibit ferroelectricity. The Fe and Nb cations are disordered on the same crystallographic site. The [(Fe,Nb)O₆] octahedra exhibit tilting and distortion to accommodate the bonding requirements of the Bi cations located in the perovskite double layers. Magnetic measurements indicate non-Curie-Weiss-type paramagnetic behavior from 300 to 6 K. Measurements of dielectric properties and electrical resistivity exhibited changes near 250-260 °C and are suggestive of a ferroelectric transition.     

Phase relations, crystal chemistry, and dielectric properties in sections of the La2O3-CaO-MgO-TiO2 system

Subsolidus phase equilibria and crystal chemistry were studied for the La₂O₃-MgO-TiO₂ system and for the ternary sections LaMg_1/2Ti_1/2O₃-CaTiO₃-La₂O₃ and LaMg_1/2Ti_1/2O₃-CaTiO₃-La_0.833Mg_0.25Ti_0.75O₃ in the quaternary La₂O₃-CaO-MgO-TiO₂ system. Dielectric properties (relative permittivity and temperature coefficient of resonant frequency, τ_f) were measured at 5-10 GHz and mapped onto the phase equilibria relations to reveal the compositions of temperature-stable (τ_f=0) compounds and mixtures. Phase equilibria relations were obtained by X-ray powder diffraction analysis of approximately 80 specimens prepared by solid-state reactions in air at ∼1450°C. Six ternary phases were found to form in the La₂O₃-MgO-TiO₂ system, including the three previously reported compounds LaMg_1/2Ti_1/2O₃, La₅Mg_0.5Ti_3.5O₁₅, and “La₆MgTi₄O₁₈”; and the new phases La₁₀MgTi₉O₃₄, La₉Mg_0.5Ti_8.5O₃₁, and a perovskite-type solid solution (1−x)LaMg_1/2Ti_1/2O₃-xLa_2/3TiO₃ (0?x?0.5). The phase previously reported as “La₆MgTi₄O₁₈” was found to form off-composition, apparently as a point compound, at La₆Mg_0.913Ti_4.04O₁₈. Indexed experimental X-ray powder diffraction patterns are given for LaMg_1/2Ti_1/2O₃, La₅Mg_0.5Ti_3.5O₁₅, La₆Mg_0.913Ti_4.04O₁₈, La₁₀MgTi₉O₃₄, and La₉Mg_0.5Ti_8.5O₃₁. LaMg_1/2Ti_1/2O₃ exhibits a slightly distorted perovskite structure with ordered B-cations (P2₁/n; a=5.5608(2) Å, b=5.5749(3) Å, c=7.8610(5) Å, β=90.034(4)°). La₅Mg_0.5Ti_3.5O₁₅ (Pm1; a=5.5639(1), c=10.9928(5) Å) and La₆Mg_0.913Ti_4.04O₁₈ (R3m; a=5.5665(1), c=39.7354(9) Å) are n=5 and n=6 members, respectively, of the (111) perovskite-slab series A_nB_n−1O_3n. The new phases La₁₀MgTi₉O₃₄ (a=5.5411(2), b=31.3039(9), c=3.9167(1) Å) and La₉Mg_0.5Ti_8.5O₃₁ (a=5.5431(2), b=57.055(1), c=3.9123(1) Å) are n=5 and n=4.5 members, respectively, of the (110) perovskite-slab series A_nB_nO_3n+2, which exhibit orthorhombic subcells; electron diffraction revealed monoclinic superlattices with doubled c-parameters for both compounds. Extensive perovskite-type solid solutions form in the ternary sections LaMg_1/2Ti_1/2O₃-CaTiO₃-La₂O₃ and LaMg_1/2Ti_1/2O₃-CaTiO₃-La_0.833Mg_0.25Ti_0.75O₃. The La₂O₃-MgO-TiO₂ system contains two regions of temperature-stable (τ_f=0) compositions. The quaternary La₂O₃-CaO-MgO-TiO₂ system contains an extensive single-phase perovskite-type volume through which passes a surface of temperature-stable compositions with permittivities projected to be in the 40-50 range. Traces of this surface occur as lines of τ_f=0 perovskite-type phases in the ternary sections LaMg_1/2Ti_1/2O₃-CaTiO₃-La₂O₃ and LaMg_1/2Ti_1/2O₃-CaTiO₃-La_0.833Mg_0.25Ti_0.75O₃.