Alexander Y. Fadeev

Capture and imaging of a prehairpin fusion intermediate of the paramyxovirus PIV5

During cell entry, enveloped viruses fuse their viral membrane with a cellular membrane in a process driven by energetically favorable, large-scale conformational rearrangements of their fusion proteins. Structures of the pre- and postfusion states of the fusion proteins including paramyxovirus PIV5 F and influenza virus hemagglutinin suggest that this occurs via two intermediates. Following formation of an initial complex, the proteins structurally elongate, driving a hydrophobic N-terminal “fusion peptide” away from the protein surface into the target membrane. Paradoxically, this first conformation change moves the viral and cellular bilayers further apart. Next, the fusion proteins form a hairpin that drives the two membranes into close opposition. While the pre- and postfusion hairpin forms have been characterized crystallographically, the transiently extended prehairpin intermediate has not been visualized. To provide evidence for this extended intermediate we measured the interbilayer spacing of a paramyxovirus trapped in the process of fusing with solid-supported bilayers. A gold-labeled peptide that binds the prehairpin intermediate was used to stabilize and specifically image F-proteins in the prehairpin intermediate. The interbilayer spacing is precisely that predicted from a computational model of the prehairpin, providing strong evidence for its structure and functional role. Moreover, the F-proteins in the prehairpin conformation preferentially localize to a patch between the target and viral membranes, consistent with the fact that the formation of the prehairpin is triggered by local contacts between F- and neighboring viral receptor-binding proteins (HN) only when HN binds lipids in its target membrane.

Wetting study of imidazolium ionic liquids.

In this work, we present a systematic contact angles study of a series of 1-alkyl, 3-methyl-imidazolium ionic liquids (ILs) on well-defined polar and nonpolar monolayer surfaces supported on Si wafers. The advancing and receding contact angles of ILs were used to determine the surface energy of the monolayer surfaces using Neumanns equation-of-state and Zismans critical surface tension approaches. In parallel, the contact angles of conventional probe fluids (molecular liquids) including water, formamide, methylene iodide, ethylene glycol, and hexadecane were determined on the same surfaces. The results obtained showed a great deal of similarity in wetting behavior of ionic vs molecular probe fluids: the contact angles of both sets of liquids followed the same patterns in accord with the surface tension of the fluid. A good agreement was found between the surface energy determined by different sets of liquids.

Elution behavior of polyethylene in polar mobile phases on a non-polar sorbent

Linear polyethylene standards in the range of 1-500 kg/mol, dissolved in 1,2,4-trichlorobenzene, were injected into a column packed with oligo(dimethylsiloxane) modified silica gel. Fifteen polar solvents (cyclohexanone, cyclohexylacetate, cyclohexanol, nonylalcohol, dimethylformamide, dimethyl sulfoxide, ethylene- and diethylene glycol monobutyl ether, benzylalcohol, hexylacetate, bis(2-ethyl-hexyl)phthalate, N,N-dimethylacetamide, propylene carbonate, dipropylene glycol and N-methyl-pyrrolidone) were evaluated as mobile phases. Depending on the type of mobile phase evaluated, different elution behaviors are observed for polyethylene: (1) polyethylene was eluted in the size exclusion mode, (2) polyethylene was eluted together with the sample solvent peak at constant elution volume, (3) polyethylene was partially or fully retained on the column. The retained polymer was easily removed from the column by injecting a small volume of trichlorobenzene. The use of ethylene glycol monobutyl ether as the mobile phase enabled separation of the polyethylene from polypropylene. In this case polypropylene is eluted in the size exclusion mode, while polyethylene is eluted at a constant elution volume or remains in the column.

Gas chromatography study of retention of organic compounds on silica with an attached layer of hydrophobic groups

Abstract Silica adsorbents with bonded C16H33, n-C6F13(CH2)3, tert.-C6F13(CH2)3 or n-C3F7(CH2)3 groups were prepared by the reaction between hydroxylated silica (Silochrome) and corresponding N-(organodimethylsilyl)morpholines. These adsorbents, as well as silica with bonded phenylpolysiloxane which was prepared by multilayer attachment of phenyltrichlorosilane, were investigated by means of gas chromatography at zero surface coverage. Thermodynamic functions (Gibbs energies, entropies and heats of adsorption) were shown to change significantly with chemical nature and structure of attached groups. On the basis of the investigation of the adsorption of cyclic compounds we proposed that the mechanism of interaction between non-polar organic compounds and silanized silicas is absorption to the bonded layer. Dispersive and specific components of intermolecular interaction in adsorption have been estimated by several techniques.

LAMELLAR NANOSTRUCTURE IN ‘SOMASIF’-BASED ORGANOCLAYS

Thermally induced lamellar structure changes due to phase transition and degradation in organoclays based on a synthetic ‘Somasif’ mineral and two organic surfactants, di-methyl dihydro-ditallow ammonia chloride (DMDTA) and tri-butyl-hexadecyl phosphonium bromide (HTBP) were investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, in situ simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) over the temperature range 30–280°C. Results indicated that the surfactant layer in ‘Somasif’-based organoclays underwent thermally induced melting-like order-disorder transition followed by desorption of surfactant molecules, resulting in drastic changes in the character of the layer periodicity. The transition temperature (Ttr), determined from the endothermic transition in DSC, was found to depend strongly on the type and the content of surfactant incorporated. Temperature-resolved SAXS indicated complex intercalated layered structures, containing multiple lamellar stack populations of two different organic layer thicknesses. A weak scattering peak (s0), located at exactly the half angular position of the strong first scattering maximum s1 (s0 = 0.5s1), was found in all tested ‘Somasif’ clays. The presence of this peak can be attributed to a slight breaking of the translational symmetry in the layered structure, causing the 1D repeat period in real space to be doubled. In other words, some portions of layers are grouped into pairs and a single pair forms the new repeat unit. This arrangement is reminiscent of the Peierls-like distortion.

Synthesis of chiral mesoporous silicas with oligo(saccharide) surfaces and their use in separation of stereoisomers

Novel chiral mesoporous silicas (SBA-15 motif) with chemically bonded oligo(saccharides) (1, 3, and 7 glucose units) were obtained through the cocondensation of organosilicon derivatives of the oligo(saccharides) and silica precursors in the presence of polymer surfactant template under mild acidic conditions. The pore order and structure of the materials prepared were characterized by transmission electron microscopy and nitrogen adsorption. The direct application of the oligo(saccharide)-grafted SBA-15 stationary phases in the HPLC separations of stereoisomers was demonstrated for the first time.

Covalent functionalization of silica surface using "inert" poly(dimethylsiloxanes).

Methyl-terminated poly(dimethylsiloxanes) (PDMSs) are typically considered to be inert and not suitable for surface functionalization reactions because of the absence of readily hydrolyzable groups. Nevertheless, these siloxanes do react with silica and other oxides, producing chemically grafted organic surfaces. Known since the 1970s and then forgotten and recently rediscovered, this reaction provides a versatile yet simple method for the covalent functionalization of inorganic surfaces. In this work, we have explored the reactions of linear methyl-terminated and cyclic PDMS and bis-fluoroalkyl disiloxanes for the surface functionalization of mesoporous silica (Dpore ≈ 30-35 nm). The optimal reaction conditions included 24 h of contact of neat siloxane liquids and silica at 120-250 °C (depending on the siloxane). A study of the reactions of silicas with different extents of hydration demonstrated the critical role of water in facilitating the grafting of the siloxanes. The proposed reaction mechanism involved the hydrolysis of the adsorbed siloxanes by the Lewis acidic centers (presumably formed by water adsorbed onto surface defects) followed by the coupling of silanols to the surface to produce grafted siloxanes. For rigorously dehydrated silicas (calcination ∼1000 °C), an alternative pathway that did not require water and involved the reaction of the siloxanes with the strained siloxane rings was also plausible. According to FTIR and chemical analysis, the reactions of bis-fluoroalkyl disiloxanes and cyclic PDMS (D3-D5) produced covalently-attached monolayer surfaces, and the reactions of high-MM methyl-terminated PDMS produced polymeric grafted silicas with a PDMS mass content of up to 50%. As evidenced by the high contact angles of ∼130°/100° (adv/rec) and the negligible amount of water adsorption over the entire range of relative pressures, including saturation (p/p0 → 1), the siloxane-grafted porous silicas show uniform, high-quality hydrophobic surfaces. An overall comparison of siloxanes with classical silane coupling agents (i.e., silanes with readily hydrolyzable functionalities such as chloro, amino, etc.) demonstrated that the reactions of siloxanes produced surfaces of similar quality and, although requiring higher temperatures, used noncorrosive, less hazardous reagents, thereby providing an environmentally benign alternative to the chemical functionalization of metal oxide surfaces.

Self-assembled monolayers of organophosphonic acids supported on teeth

Abstract Controlling the surface properties of the tooth, e.g. adsorption and adhesion to its surface is of interest for use in dental applications. This work investigates self-assembled monolayers (SAMs) of organophosphonic acids supported on the surface of human teeth. The SAMs were prepared via a solution-phase reaction of C18H37P(O)(OH)2 with enamel chips. ATR suggests the formation of SAMs with highly ordered alkyl groups (based on the CH2 stretching). Water contact angles of the enamel-supported SAMs are in the range ∼100–70° (advancing/receeding), which is consistent with the formation of hydrophobic surfaces. According to atomic force microscopy imaging, the SAM growing process appears as the formation of islands on the surface of the tooth. At the late stages of the reaction, islands merge yielding larger domains covering almost the entire surface of the tooth. The size of these domains undergoes a complex change during the course of the deposition and after 100 h of reaction it becomes ∼40–60 nm in lateral dimension. The surfaces prepared demonstrate good stability in the presence of water and organic solvents, showing the potential uses of SAMs of organophosphonic acid for permanent modification of the surface properties of the tooth.

Adsorption study of alkyl-silicas and methylsiloxy-silicas

We report the synthesis and adsorption study of the lyophobic porous silicas. Four adsorbents were prepared and tested: (1) octyl-silica, (2) hexadecyl-silica, (3) bis(trimethylsiloxy)-silica, and (4) oligo(dimethylsiloxane)-silica. Octyl- and hexadecyl-silicas were prepared via the reaction of silica with (CH3)2NSi(CH3)2CnH(2n+1) (n=8 and 16), the reactions were carried under the optimized conditions yielding high bonding densities of alkyl groups approximately 2.9-3.0 groups/nm2 and highly uniform non-polar adsorbents. Bis(trimethylsiloxy)-silica was prepared via the reaction silica with ClSi(CH3)2(CH2)10Si(CH3)[OSi(CH3)3]2. Oligo(dimethylsiloxane)-silica was prepared via the reaction of silica with ClSi(CH3)2-[OSi(CH3)2]2-Cl. Adsorption of small organic compounds (n-alkanes, alkylbenzene, benzene, diethyl ether) was investigated using two methods, classical static adsorption and gas chromatography. Thermodynamic parameters (heat, Gibbs energy, and entropy) of the adsorption of organic compounds were studied as a function of the nature of adsorbate and of the nature of the bonded layer as well. The results obtained suggest penetration of the adsorbate molecules into the bonded layer and the importance of this process in the retention mechanism in gas chromatography. Energy of the dispersion interactions with the surface decreases in the following order: n-C16H33(CH3)2Si- > n-C8H17(CH3)2Si- > [(CH3)3SiO]2Si(CH3)-(CH2)10(CH3)2Si- > -[[(CH3)2SiO]2]x-(CH3)2Si-. Energy of the electrostatic and hydrogen bonding interactions with the surface, as assessed from the adsorption of benzene and diethyl ether molecules, decreases in the opposite direction, indicating that alkyl-silicas are less polar adsorbents than methylsiloxy-silicas.

Activated silica supports for preparation of chromatographic sorbents. A comparative study of silicas containing attached epoxy, tosyloxy and halogen groups

A comparative study of silica sorbents with attached epoxy, tosyloxy and halogen groups is presented. Epoxy-activated sorbents are shown to have the lowest concentration of attached functional groups among the activated sorbents studied. The low content of epoxy groups is suggested to originate from the partial destruction of epoxy rings during the modification of silica. New methods of epoxidation without the use of epoxysilanes and tosylation without the use of tosyl chloride are proposed. Haloalkyl-activated sorbents are shown to have the highest content of attached functional groups in a bonded layer. It is suggested that haloalkyl-activated sorbents are the most promising for the preparation of sorbents with high loadings of immobilized ligands.