Search

Polycationic polymers are used extensively in biology to disrupt cell membranes and thus enhance the transport of materials into the cell. The highly polydisperse nature of many of these materials makes obtaining a mechanistic understanding of the disruption processes difficult. To design an effective mechanistic study, a monodisperse class of polycationic polymers, poly(amidoamine) (PAMAM) dendrimers, has been studied in the context of supported dimyristoylphosphatidylcholine (DMPC) lipid bilayers using atomic force microscopy (AFM). Aqueous solutions of amine-terminated generation 7 (G7) PAMAM dendrimers caused the formation of 15−40-nm-diameter holes in lipid bilayers. This effect was significantly reduced for smaller G5 dendrimers. For G3, no hole formation was observed. In addition to dendrimer size, surface chemistry had a strong influence on dendrimer−lipid bilayer interactions. In particular, acetamide-terminated G5 did not cause hole formation in bilayers. In all instances, the edges of bilayer defects proved to be points of highest dendrimer activity. A proposed mechanism for the removal of lipids by dendrimers involves the formation of dendrimer-filled lipid vesicles. By considering the thermodynamics, interaction free energy, and geometry of these self-assembled vesicles, a model that explains the influence of polymer particle size and surface chemistry on the interactions with lipid membranes was developed. These results are of general significance for understanding the physical and chemical properties of polycationic polymer interactions with membranes that lead to the transport of materials across cell membranes.

Apoptosis is defined by a distinct set of morphological changes observed during cell death including loss of focal adhesions, the formation of cell membrane buds or blebs, and a decrease in total cell volume. Recent studies suggest that these dramatic morphological changes, particularly apoptotic volume decrease (AVD), are an early prerequisite to apoptosis and precede key biochemical time-points. Here we use atomic force microscopy to observe early stage AVD of KB cells undergoing staurosporine-induced apoptosis. After a 3-h exposure to 1 μM staurosporine, a 32% decrease in total cell height and a 50% loss of total cell volume is observed accompanied by only a 15% change in cell diameter. The observed AVD precedes key biochemical hallmarks of apoptosis such as loss of mitochondrial membrane potential, phosphatidyl serine translocation, nuclear fragmentation, and measurable caspase-3 activity. This suggests that morphological volume changes occur very early in the induction of apoptosis.

Chemisorbed monolayers of octylgermane (C8H17GeH3) on gold have been formed by vapor deposition in ultrahigh vacuum. The monolayer has been characterized by X-ray photoelectron and reflection absorption infrared spectroscopies (XPS and RAIRS) and scanning tunneling microscopy (STM). XPS data indicate the monolayer can be oxidized by exposure to ozone. STM images exhibit a complex pattern which can be modeled as strain-mediated spinodal decomposition.

We have investigated poly(amidoamine) (PAMAM) dendrimer interactions with supported 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipid bilayers and KB and Rat2 cell membranes using atomic force microscopy (AFM), enzyme assays, flow cell cytometry, and fluorescence microscopy. Amine-terminated generation 7 (G7) PAMAM dendrimers (10−100 nM) were observed to form holes of 15−40 nm in diameter in aqueous, supported lipid bilayers. G5 amine-terminated dendrimers did not initiate hole formation but expanded holes at existing defects. Acetamide-terminated G5 PAMAM dendrimers did not cause hole formation in this concentration range. The interactions between PAMAM dendrimers and cell membranes were studied in vitro using KB and Rat 2 cell lines. Neither G5 amine- nor acetamide-terminated PAMAM dendrimers were cytotoxic up to a 500 nM concentration. However, the dose dependent release of the cytoplasmic proteins lactate dehydrogenase (LDH) and luciferase (Luc) indicated that the presence of the amine-terminated G5 PAMAM dendrimer decreased the integrity of the cell membrane. In contrast, the presence of acetamide-terminated G5 PAMAM dendrimer had little effect on membrane integrity up to a 500 nM concentration. The induction of permeability caused by the amine-terminated dendrimers was not permanent, and leaking of cytosolic enzymes returned to normal levels upon removal of the dendrimers. The mechanism of how PAMAM dendrimers altered cells was investigated using fluorescence microscopy, LDH and Luc assays, and flow cytometry. This study revealed that (1) a hole formation mechanism is consistent with the observations of dendrimer internalization, (2) cytosolic proteins can diffuse out of the cell via these holes, and (3) dye molecules can be detected diffusing into the cell or out of the cell through the same membrane holes. Diffusion of dendrimers through holes is sufficient to explain the uptake of G5 amine-terminated PAMAM dendrimers into cells and is consistent with the lack of uptake of G5 acetamide-terminated PAMAM dendrimers.

Although it is known that noncollagenous proteins of dental origin bind to the hydroxyapatite crystal surfaces, no measure of their binding strength has been calculated. This experiment used −COOH-capped generation 7 PAMAM dendrimers as nanoprobes of the biological hydroxyapatite nanorod surfaces. Dendrimer distribution was characterized using AFM. The results showed dendrimers to be spaced at intervals along the c-axis of the crystals. From these observations and assuming a fully ionized −COOH dendrimer, a mathematical model of the binding capacity of the crystal surface with the dendrimer was developed. The Monte Carlo method was used to simulate the binding process between the dendrimer and crystal surface, and the binding strength of the −COOH groups to the surface was calculated to be 90 ± 20 kJ/mol. These results support the CFM studies which have described alternating bands of charge domains on the crystal surface and that the binding strength will be dependent on both the intensity of the charge on the protein and the crystal surface.

The reaction of (Et3P)2Pd(μ-S)Ge[N(SiMe3)2]2 (2) with paraformaldehyde under photolytic conditions results in the insertion of formaldehyde into the Pd−Ge bond, with bond cleavage, to give (Et3P)2Pd(μ-S)(μCH2O)Ge[N(SiMe3)2]2 (3). Subsequent addition of CO to 3 results in migratory insertion into the Pd−C bond, followed by rapid reductive elimination to regenerate the Pd⁰ metal center and give a unique five-membered organic heterocycle, Ge[N(SiMe3)2]2SC(O)CH2O (6). The X-ray crystal structure of the dppe (dppe = bis(diphenylphosphino)ethane) derivative of 3, (dppe)Pd(μ-S)(μCH2O)Ge[N(SiMe3)2]2 (4), is reported.

Scanning tunneling microscopy data of a mixed monolayer comprised of a 40:60 ratio of H8Si8O12 and C6H13−H7Si8O12 clusters on gold are presented. The images display a composite monolayer surface with well-defined domain regions of the individual components. Holes present at face-centered cubic (fcc) sites of the starting Au/H7Si8O12 adsorbate layer indicate the location of active sites for impinging C6H13−H7Si8O12 clusters. Adsorption of a C6H13−H7Si8O12 cluster likely yields a mobile hydrogen atom available to recombine with and desorb an adjacent H8Si8O12 cluster. Hydrogen atom diffusion along substrate [121] directions is the proposed pattern formation mechanism of the mixed monolayer. Imaging of the spherosiloxane cluster domains identifies a novel terrace-based active site located in the fcc regions of the Au(111) 23 × √3 surface reconstruction.

Ultrahigh vacuum scanning tunneling microscopy data investigating octylsilane (C8H17SiH3) monolayer pattern formation on Au(111) are presented. The irregular monolayer pattern exhibits a 60 Å length scale. Formation of the octylsilane monolayer relaxes the Au(111) 23 × √3 surface reconstruction and ejects surface Au atoms. Au adatom diffusion epitaxially extends the Au(111) crystal lattice via step edge growth and island formation. The chemisorbed monolayer covers the entire Au surface at saturation exposure. Theoretical and experimental data suggest the presence of two octylsilane molecular adsorption phases:  an atop site yielding a pentacoordinate Si atom and a surface vacancy site yielding a tetracoordinate Si atom. Theoretical simulations investigating two-phase monolayer self-assembly dynamics on a solid surface suggest pattern formation results from strain-induced spinodal decomposition of the two adsorption phases. Collectively, the theoretical and experimental data indicate octylsilane monolayer pattern formation is a result of interfacial Au−Si interactions and the alkyl chains play a negligible role in the monolayer pattern formation mechanism.

Herein we report the CH insertion of a stable germylene, Ge[CH(SiMe3)2]2, into the α-CH bonds of the acetyl-containing ketones acetone, butanone, and cyclopropyl methyl ketone. This CH insertion reaction requires the presence of MgCl2, yet the reaction can be successfully carried out in hydrocarbon, aromatic, and ethereal solvents. In the absence of MgCl2, these acetyl-containing ketones do not undergo CH insertion but instead undergo an OH insertion to give a germyl vinyl ether. A number of other ketones, including the acetyl-containing pinacolone, have been observed to only undergo insertion into the OH bond even in the presence of MgCl2.

Ge[CH(SiMe3)2]2 (1) and Ge[N(SiMe3)2]2 (2) react with anthraquinone and naphthoquinone in a 2:1 ratio to form five-membered rings with the carbonyl oxygens and the ortho carbons of the adjacent aromatic ring. The reaction with 1,4-diacetylbenzene proceeds in a similar fashion with 1; however no reaction is observed with 2. Compounds 1 and 2 react with 1,2-diacetylbenzene to yield a novel bicyclo species. In a previous report describing equilibria between 1 and phenones, it appeared that 2 did not undergo reaction to generate the five-membered ring with the accompanying formation of a conjugated triene. However, these quinone results indicate that an equilibrium can exist between 2 and a phenone moiety. In the reaction with quinones, a second equivalent of germylene effectively traps the conjugated triene by undergoing a 1,4-addition reaction. Unlike the initial triene formation, this second insertion is irreversible.

The synthesis of three novel Pd germylene complexes is reported. (Ph3P)2PdGe[N(SiMe3)2]2 (1) was synthesized by ligand substitution of (Ph3P)4Pd, whereas (Et3P)PdGe[N(SiMe3)2]2 (2) and {dppePdGe[N(SiMe3)2]2}2 (3b) (dppe = (diphenylphosphino)ethane) were synthesized by photolytic reduction of their corresponding phosphine oxalato complexes followed by addition of the germylene ligand. In solution, 3b exists in equilibrium with the monomeric dppePdGe[N(SiMe3)2]2 (3a). The germylene ligand of 2 was found to be 1 order of magnitude more labile than the analogous Pt system and 2 orders of magnitude less labile than the analogous Ni system. The reactivity of these new palladium germylenes toward O2 is described.

Scanning tunneling microscopy (STM) data are presented in conjunction with reflection−absorption infrared spectroscopy and X-ray photoemission spectroscopy data investigating the adsorbate layer formation of H8Si8O12 clusters on a clean Au(111) 23×√3 surface. All three experimental techniques independently support desorption of approximately 10−15% of the H8Si8O12 clusters from the Au/H7Si8O12 adsorbate layer surface following evacuation of excess H8Si8O12 cluster pressure from the ultrahigh vacuum reaction chamber. Surprisingly, the STM data indicate that unlike all other reported molecular adsorbates having a strong chemical interaction with the Au(111) surface, the 23×√3 surface reconstruction is preserved following chemisorption of H8Si8O12 clusters. Additionally, at saturation coverage the clusters are preferentially bound to, and predominantly desorb from, specific sites on the Au(111) 23×√3 reconstructed surface. The preferential cluster adsorption/desorption behavior creates a unique pattern of holes and channels in the Au/H7Si8O12 adsorbate layer surface.

X-ray photoemission spectroscopy (XPS), low-energy electron diffraction (LEED), reflection−absorption infrared spectroscopy (RAIRS), and scanning tunneling microscopy (STM) have been used to characterize the discontinuous oxide films formed following exposure of gaseous H8Si8O12 hydridosilsesquioxane clusters to Si(111)-7 × 7. Collectively, the four surface characterization techniques support a reaction involving cluster decomposition on the Si(111)-7 × 7 surface. The decomposition of H8Si8O12 upon reaction with Si(111)-7 × 7 is in stark contrast with that of the reaction of H8Si8O12 with Si(100)-2 × 1 in which the cluster cage remains intact.

Carbon nanoprobes were utilized in tapping mode atomic force microscopy investigations to highlight the topographic differences between poly(amidoamine) (PAMAM) dendrimers, two-dimensional arrays of PAMAM dendrimers, and PAMAM core−shell tecto(dendrimers). The PAMAM core−shell tecto(dendrimers) used for this study consist of a shell of generation five (G5) dendrimers covalently linked to a G7 core. The volumes measured for the PAMAM core−shell tecto(dendrimers) suggest a 75−100% saturation of shell G5 dendrimers about the G7 core. This compares favorably with theoretical predictions that 12 G5 dendrimers should pack to form a shell about a G7 core. The effect of different imaging substrates was also explored. Tecto(dendrimers) imaged on hydrophobic surfaces experience a 6-fold increase in maximum peak height and a 3-fold decrease in diameter as compared to those imaged on mica. Measured volume is invariant.

Single and multicomponent mixed layers of silsesquioxane clusters on freshly evaporated gold surfaces have been investigated by X-ray photoelectron spectroscopy and reflection−absorption infrared spectroscopy. Approximately 5−10% of the cluster layers (e.g., H8Si8O12 and H10Si10O15) on gold desorb upon evacuation of the adsorbate from the reaction chamber. These open adsorption sites are an avenue for cluster displacement reactions that yield mixed monolayers (e.g., H8Si8O12/D8Si8O12 and H8Si8O12/C6H13−H7Si8O12) of several compositions on gold. This dynamic behavior is not observed for the C6H13−H7Si8O12 cluster layer on gold. Rather, this molecule acts as a poison to these reported displacement processes at the gold surface.

Substrate effects, volume reproducibility, and pH effects on the size and shape of ethylenediamine core poly(amidoamine) dendrimers (generations 6−9) were explored using tapping mode atomic force microscopy (AFM) measurements. A statistical analysis of the measurements indicated a 4% variation in volume for repeated measurement using the same tip. Volume determinations by numerical integration and a spherical cap estimation method were explored. The spherical cap model was shown to overestimate dendrimer volumes by a factor of ~2. As substrates were changed from mica to more hydrophobic surfaces, AFM-measured heights and diameters approached ideal-sphere dendrimer diameters. Acidification of dendrimer samples from generations 6−9 led to an observed 33% increase in volume, 26% increase in height, and 9% decrease in diameter. Expansion upon acidification can be attributed to maximization of charge separation and increased solvent retention within the dendrimers. Single dendrimer resolution within two-dimensional clusters was enhanced using carbon nanoprobes.

The chemisorption of the hydridosilsesquioxane clusters H8Si8O12 and H10Si10O15 onto a freshly evaporated gold surface has been observed by X-ray photoemission and reflection−absorption infrared spectroscopies. On the basis of these analytical techniques and supporting nonlocal density functional calculations, a single Si−Au bond at a cluster vertex is created through a surprising gold-mediated Si−H bond activation. The chemisorbed clusters are stable in a vacuum and when exposed to oxygen or water. Coverage-dependent peak frequency and infrared intensity shifts, characteristic of a chemically dynamic interface, are detected as a function of cluster overpressure. An equilibrium between the clusters and the gold surface is proposed, evidenced by a fairly constant rate of adsorption to gold for several coverage regimes.

Group 10 metal germylene complexes of general formula (R3P)2MGeQ2 are found to be highly photosensitive. Ambient fluoresent lighting causes complete decomposition of nickel germylene species in less than one minute in the presence of germylene traps. The formation of metallacycle (Et3P)2Pt(μ-η²-H2CO)Ge[N(SiMe3)2]2 and the photochemical cycloreversion reaction are also reported.

Many metal alloys are susceptible to corrosion, particularly after processing steps such as welding. Chemical vapor deposition (CVD) is an effective way to modify metal surfaces and impart specific physical and chemical properties. A hydrophobic, nanosegmented silicon oxide coating derived from the discrete cluster molecule H8Si8O12 has been shown to chemisorb to 302 and 304 stainless steel. To understand better how this cluster binds to steel, a comprehensive study of these clusters adsorbed on chromium oxide was undertaken. IR, XPS and valence-band spectroscopies show convincingly that the clusters are chemisorbed intact on this surface. The coating also readily forms on molybdenum, tungsten, iron and nickel oxides, promising general application to a wide variety of metal alloys. Copyright © 1999 John Wiley & Sons, Ltd.

A new transition metal germylene complex, (Ph3P)2NiGe[2,4,6-(CF3)3C6H2]2 (2), has been synthesized and structurally characterized. The metrical parameters of complex 2 are compared and contrasted with the previously synthesized complexes (Ph3P)2NiGe[N(SiMe3)2]2 (3) and (Et3P)2PtGe[N(SiMe3)2]2 (1). Nonlocal BP86 density functional calculations support the presence of Ni−Ge d−p π bonding in complexes 2 and 3.

The Si 2p and O 1s core-level photoemission peaks and the valence band peaks for silicon oxide films grown on crystalline silicon have been observed to shift to higher binding energy as a function of increasing film thickness. The physical basis for this shift has been ascribed to a variety of mechanisms including initial state effects, final state effects, and extrinsic effects. By constructing a structurally homogeneous silicon oxide film consisting of H10Si10O15 on silicon, initial state effects have been minimized and the magnitude of final state effects directly measured. Unlike previous studies utilizing the H8Si8O12 cluster, the cluster precursor utilized in these studies is large enough to allow direct observation of the theoretically predicted asymptotic behavior in the core-level shift as a function of cluster layer thickness prior to the onset of film charging.

A highly soluble organopolysilazane was generated from the reaction of CySiCl3 with Li3N in diglyme. On the basis of light scattering, elemental analysis, NMR spectra, and solubility data, this polymer is believed to be highly branched with a molecular weight of approximately 26 000 Da. The polymer is characterized by excellent solubility in all organic solvents, a highly economical method of synthesis, a hyperbranched topology, and potential applicability to a wide range of R group modification. The use of lithium nitride has allowed the generation of highly branched structures without the need to further treat the polymer with insolubilizing cross-linking agents.

A series of o-(trifluoromethyl)phenyl-substituted germanes have been synthesized to study the potential for weak Ge−F bonding interactions. ¹H−¹⁹F coupling constants in these compounds range between 2 and 5 Hz. These coupling magnitudes are consistent with previously reported literature examples in ¹⁹F and multinuclear NMR spectra for derivatives of the [2,4,6-(CF3)3C6H2]− ligand. Solid-state structural characterization of these germanes consistently reveals Ge−F contacts at 2.8−2.9 Å, midway between the longest single bond and the longest expected van der Waals contact. For the simple unsymmetrical catenate, [3,5-(CF3)2C6H3]2Ge(H)Ge(H)[2,4,6-(CF3)3C6H2]2 (4), there are close contacts to both germanium atoms from the ortho-CF3 groups. 4 is not otherwise sterically constrained to force distal, intramolecular contacts, and thus a weak Ge−F bonding interaction is invoked for the entire series of o-(trifluoromethyl)aryl-substituted tetravalent Ge compounds. The synthetic approaches to these germanes make extensive use of Ge(II) precursors, employing both catalytic hydrogenation and insertion reactions of germylenes.

The conversion of pentamer [(^tBuCH2)2TaN]5 (2) and ammonia to cubic TaN0.8 (5) proceeds via precipitate 1/n[TaC1.41H3.90N1.90]n (3) and amorphous TaN (4). Precursors other than (^tBuCH2)3Ta&dbd;CH^tBu (1) and NH3, which provided 2, such as Ta(NMe2)5 (7) and NH3, (Me2N)3Ta&dbd;N^tBu (9) and NH3, also afforded 5. Extended Hückel molecular orbital calculations were conducted to examine the electronic changes that appear upon oligomerization of hypothetical Me2TaN to (Me2TaN)∞, as a model for pentamer 2 formation. The relationship between pentamer 2 and the remaining nitrides, 3−5 was probed via magic angle spinning (MAS) and spin-echo solid-state ¹⁵N NMR experiments that probe the local environments of the various nitrogen atoms. In combination with ¹⁵N-labeling studies, the spin-echo spectra were used in an effort to analyze the possibility of a direct relationship between the 2-dimensional Ta2N2 “squares” of the pentamer (2) and the 3-dimensional Ta4N4 “cubes” of the ultimate product, cubic TaN0.8 (5).

Polycationic polymers are known to disrupt lipid bilayers. In this letter, we report the dependence of this disruption on the lipid structural phase. DMPC bilayers are exposed to two polycationic polymeric nanoparticles, PAMAM dendrimers and MSI-78. We find that regions of the bilayer that are in the gel phase are unaffected by the presence of polymers, whereas the liquid phase is disrupted.

Ge[CH(SiMe3)2]2 reacts with benzophenone to form a conjugated triene. Wilkinson's catalyst, Rh(PPh3)3Cl, is an effective precatalyst for the isomerization of one double bond and hydrogenation of a second double bond of this triene. This reaction demonstrates that germylene activation of the phenone moiety yields reactive double bonds amenable to catalytic transformation, even with bulky catalysts such as the presumed active form of Wilkinson's catalyst, Rh(PPh3)2Cl.

The germylene Ge[CH(Si(CH3)3)2]2 reacts with phenones to give conjugated trienes. This reaction is quantitative at room temperature in minutes and has some functional group tolerance.

The fluoroarylgermylene bis[2,4,6-tris(trifluoromethyl)phenyl]germanium(II) was synthesized by the reaction of 2 equiv of [2,4,6-tris(trifluoromethyl)phenyl]lithium with GeCl2·dioxane. This monomeric germylene has good thermal stability, despite having the least sterically encumbering set of stabilizing ligands of any alkyl germylene isolated to date.