Cooperative alpha-helix formation of beta-lactoglobulin induced by sodium n-alkyl sulfates

It is generally assumed that folding intermediates contain partially formed native-like secondary structures. However, if we consider the fact that the conformational stability of the intermediate state is simpler than that of the native state, it would be expected that the secondary structures in a folding intermediate would not necessarily be similar to those of the native state. beta-Lactoglobulin is a predominantly beta-sheet protein, although it has a markedly high intrinsic preference for alpha-helical structure. The formation of non-native alpha-helical intermediate of beta-lactoglobulin was induced by n-alkyl sulfates including sodium octyl sulfate, SOS; sodium decyl sulfate, SDeS; sodium dodecyl sulfate, SDS; and sodium tetradecyl sulfate, STS at special condition. The effect of n-alkyl sulfates on the structure of native beta-lactoglobulin at pH 2 was utilized to investigate the contribution of hydrophobic interactions to the stability of non-native alpha-helical intermediate. The addition of various concentrations of n-alkyl sulfates to the native state of beta-lactoglobulin (pH 2) appears to support the stabilized form of non-native alpha-helical intermediate at pH 2. The m values of the intermediate state of beta-lactoglobulin by SOS, SDeS, SDS and STS showed substantial variation. The enhancement of m values as the stability criterion of non-native alpha-helical intermediate state corresponded with increasing chain length of the cited n-alkyl sulfates. The present results suggest that the folding reaction of beta-lactoglobulin follows a non-hierarchical mechanism and hydrophobic interactions play important roles in stabilizing the non-native alpha-helical intermediate state.     

Gelation of native beta-lactoglobulin induced by electrostatic attractive interaction with xanthan gum

The mechanism and kinetics of the electrostatic gelation of native beta-lactoglobulin-xanthan gum mixtures in aqueous solution is reported. The total biopolymer concentration at which gelation was obtained was extremely low (0.1 wt %) compared to the usually tested concentrations for protein-polysaccharide mixed gels (4-12 wt %). This is, to our knowledge, the first time that oppositely charged proteins and polysaccharides are reported to form a gel without applying any treatment to denature the protein (e.g. heating, enzymatic hydrolysis) and at such low concentrations. Static light-scattering and viscoelastic measurements allowed determination of the gelation kinetics. It was found that the gelation process initiated following a similar path as that of an associative phase separation process, i.e., with the formation of primary and interpolymeric electrostatic complexes. However, interpolymeric complexes were able to form clusters and junction zones that resulted in the freeze-in of the whole structure at the point of gelation. The formed gel is therefore a coupled-gel, that is, a gel that has junction zones involving two different molecules. The structuration of xanthan gum, even at these low concentrations, may have played a role in the structuration process. Due to the electrostatic nature of the gels, there was an optimum pH and macromolecular ratio at which the stability of the gels was maximal. This was related to the existence of a stoichiometric electrical charge equivalence pH, where molecules carry equal but opposite charges and protein-polysaccharide interactions are at their maximum.     

Structural transformation of fluorinated carbon nanotubes induced by in situ electron-beam irradiation

We have investigated the structural transformation of fluorinated singlewalled nanotubes (SWNTs) induced by electron-beam irradiation during the transmission electron microscope observations. Heavily fluorinated SWNT bundles were systematically transformed into multiwall-like nanotubes by releasing fluorine atoms during electron-beam irradiation and even broken into two pieces of the capped graphitic structures. Such structural transformations at relatively low kinetic energy (< or = 300 keV) could be explained by the local strains induced by fluorination, where C-C bonds that were fluorine-attached became 1.53 A, a single bond similar to that of a diamond, from our density functional calculations. We propose a possible concerted pathway for the structural transformation of fluorinated SWNTs induced by electron-beam irradiation based on the experimental observations.     

Ultrafast hydration dynamics in melittin folding and aggregation: helix formation and tetramer self-assembly

Melittin, an amphipathic peptide from honeybee venom, consists of 26 amino acid residues and adopts different conformations from a random coil, to an alpha-helix, and to a self-assembled tetramer under certain aqueous environments. We report here our systematic studies of the hydration dynamics in these conformations using single intrinsic tryptophan (W19) as a molecular probe. With femtosecond resolution, we observed the solvation dynamics occurring in 0.62 and 14.7 ps in a random-coiled primary structure. The former represents bulklike water motion, and the latter reflects surface-type hydration dynamics of proteins. As a comparison, a model tripeptide (KWK) was also studied. At a membrane-water interface, melittin folds into a secondary alpha-helical structure, and the interfacial water motion was found to take as long as 114 ps, indicating a well-ordered water structure along the membrane surface. In high-salt aqueous solution, the dielectric screening and ionic solvation promote the hydrophobic core collapse in melittin aggregation and facilitate the tetramer formation. This self-assembled tertiary structure is also stabilized by the strong hydrophilic interactions of charged C-terminal residues and associated ions with water molecules in the two assembled regions. The hydration dynamics was observed to occur in 87 ps, significantly slower than typical water relaxation at protein surfaces but similar to water motion at membrane interfaces. Thus, the observed time scale of approximately 100 ps probably implies appropriate water mobility for mediating the formation of high-order structures of melittin in an alpha-helix and a self-assembled tetramer. These results elucidate the critical role of hydration dynamics in peptide conformational transitions and protein structural stability and integrity.     

Differences in electronic properties of fluorinated and trifluoromethylated fullerenes revealed by their propensity for dianion formation

The cross sections for electron transfer from sodium to C(60)F(n) (-) and C(60)(CF(3))(n) (-) anions in 50-keV collisions as a function of the number of functional groups n are reported. There are clear differences between derivatives of fluorine and trifluoromethyl due to the different electron withdrawing properties of F and CF(3). The role of inductive effects and pi electron delocalization on the electron affinity is discussed, assuming a correlation between the cross section and the electron affinity of the anion.     

α N-O turn induced by fluorinated α-aminoxy diamide: synthesis and conformational studies

Two α-aminoxy diamides with fluorinated side chains were synthesized. Their secondary structures characterization was carried out by ¹H NMR, and IR spectrometries as well as X-ray crystallography studies. α N-O turn secondary structures are adopted insusceptibly by side-chain-fluorinated α-aminoxy residues. Thus the fluorinated α-aminoxy diamide can be a potential residue as a biological tracer to be incorporated into aminoxy peptides.     

Structural micellar transition for fluorinated and hydrogenated sodium carboxylates induced by solubilization of benzyl alcohol

The solubility of benzyl alcohol in micellar solutions of sodium octanoate and sodium perfluorooctanoate was studied. From the isotherms of specific conductivity versus molality at different alcohol concentrations, the critical micelle concentration and the degree of ionization of the micelles were determined. The cmc linearly decreases upon increasing the amount of benzyl alcohol present in aqueous solutions with two distinct slopes. This phenomenon was interpreted as a clustering of alcohol molecules above a critical point, around 0.1 mol kg(-1). Attending to the equivalent conductivity versus square root of molality, the presence of a second micellar structure for the fluorinated compound was assumed. The thermodynamic parameters associated with the process of micellization were estimated by applying Motomura's model for binary surfactant mixtures, modified by Pérez-Villar et al. (Colloid Polym. Sci 1990, 268, 965) for the case of alcohol-surfactant solutions. A comparison of the hydrogenated and fluorinated compounds was carried out and discussed.     

Pattern formation in PMMA film induced by electric field

Micro‐sized patterns were created on thin poly(methyl methacrylate) (PMMA) films by the effect of external field, perpendicular to the film surface. The PMMA film, prepared by spin‐coating onto Si wafer, was heated to the fluid temperature (275 °C) and a linear pattern was created by the effect of electric field produced by a strip electrode. In another experiment, a round pattern was created as a result of local laser heating of the PMMA film under homogenous electrical field. The created patterns were analyzed by optical microscopy and profile meter. The dependence of the form and size of the created patterns on the intensity of the electric field, exposure time, and initial film thickness was examined. Wave guiding property of a linear pattern, produced by the above technique, was examined in a simple experiment. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1131–1135, 2009     

Anomalous "unquenching" of the fluorescence decay times of beta-lactoglobulin induced by the known quencher acrylamide

Picosecond time-resolved fluorescence, together with the addition of quenching agents, was employed to discriminate the fluorescence contributions of the two tryptophans of beta-lactoglobulin (Trp19 and Trp61) to the fluorescence decays of the protein. The fluorescence decays of beta-lactoglobulin at pH 3, 5 and 8 are best fitted using sums of three exponentials and show a dominant contribution (98%) of the components associated with the buried Trp19, which decays according to a double exponential function. The addition of acrylamide (0.05 M) causes an increase of the decay times associated with Trp19. This effect is observed at all pH values studied, but the effect is stronger at pH 3 and pH 5, than at pH 8. The unexpected increase of the decay times of Trp19 and the variation of the respective amplitudes were rationalized in terms of alterations of Trp19 mobility. The hindrance of Trp19 upon acrylamide binding was also monitored and supported by fluorescence anisotropy measurements.     

A novel approach to fluorinated polyurethane by macromonomer copolymerization

For the first time, through macromonomer radical copolymerization, a novel fluorinated polyurethane (FPU) was synthesized based on partly acrylate-endcapped polyurethane macromonomers with hexafluorobutyl acrylate (HFBA). Partly acrylate-endcapped polyurethane (PU) macromonomers were synthesized using isophronediisocyanate (IPDI), dimethylol propionic acid (DMPA), polyethylene adipate glycols (PEA) etc. The novel fluorinated polymer, which bore PU side chains and fluorinated side chains, was confirmed by F¹⁹ NMR spectroscopy, X-ray photoelectron spectroscopy (XPS), elemental analysis, scanning electron spectroscopy (SEM) etc. Copolymerization of polyurethane macromonomers with hexafluorobutyl acrylate (HFBA) was briefly investigated. The surface tension of FPU solution was measured and showed sharply decrease compared to that of pure polyurethane. Results from SEM showed a uniform size distribution of phase micro-domains on the fracture surface of FPU.     

Permeability,permselectivity, and penetrant‐induced plasticization in fluorinated polyimides studied by positron lifetime measurements

The ortho‐positronium (o‐Ps) lifetime τ₃ and its intensity I₃ in various fluorinated polyimides were determined by the positron annihilation technique and were studied with the spin–lattice relaxation time T₁ and the propylene permeability, solubility, diffusivity, and permselectivity for propylene/propane in them. τ₃, I₃, and the distribution of τ₃ changed when the bulky moieties in the polyimides were changed. The polyimides, having both large τ₃ and I₃ values, exhibited a short T₁ and a high permeability with a low permselectivity. The propylene permeability and diffusivity were exponentially correlated with the product of I₃ and the average free‐volume hole size estimated from τ₃. In highly plasticized states induced by the sorption of propylene, the permeability increased with the propylene pressure in excellent agreement with the change in the free‐volume hole properties probed by o‐Ps. The large and broad distribution of the free‐volume holes and increased local chain mobility for the 2,2‐bis(3,4‐decarboxyphenyl) hexafluoropropane dianhydride‐based polyimides are thought to be important physical properties for promoting penetrant‐induced plasticization. These results suggest that o‐Ps is a powerful probe of not only the free‐volume holes but also the corresponding permeation mechanism and penetrant‐induced plasticization phenomenon. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 308–318, 2003     

Improved regioselectivity in pyrazole formation through the use of fluorinated alcohols as solvents: synthesis and biological activity of fluorinated tebufenpyrad analogs

The preparation of N-methylpyrazoles is usually accomplished through reaction of a suitable 1,3-diketone with methylhydrazine in ethanol as the solvent. This strategy, however, leads to the formation of regioisomeric mixtures of N-methylpyrazoles, which sometimes are difficult to separate. We have determined that the use of fluorinated alcohols such as 2,2,2-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) as solvents dramatically increases the regioselectivity in the pyrazole formation, and we have used this modification in a straightforward synthesis of fluorinated analogs of Tebufenpyrad with acaricide activity.     

Kinetics of channel formation in bilayer lipid membranes (BLMs) and tethered BLMs: monazomycin and melittin

The kinetics of channel formation by the polyene-like antibiotic monazomycin, both in a bilayer lipid membrane (BLM) and in a tethered BLM (tBLM), and by the peptide melittin in a tBLM, is investigated. Stepping the applied potential from a value at which channels are not formed to one at which they are formed yields current vs time curves that are sigmoidal on a BLM, while they show a maximum on a tBLM; in the latter case, sigmoidal curves are obtained by plotting the charge against time. These curves are interpreted on the basis of a general kinetic model, which accounts for the potential-dependent penetration of adsorbed monomeric molecules into the lipid bilayer, followed by their aggregation with channel formation by a mechanism of nucleation and growth. In the case of monazomycin, which is present in the solution in the form of relatively hydrophilic clusters and is adsorbed as such on top of the lipid bilayer, penetration into the bilayer following a potential jump is assumed to be preceded by a potential-independent disaggregation of the adsorbed clusters into adsorbed monomers.     

Drop formation instabilities induced by entrapped gas bubbles

During the formation of a liquid drop out of a capillary tube, instabilities can appear which cause an “instantaneous” variation in the drop volume. An analytical treatment of this phenomenon, which takes into account the presence of entrapped gas bubbles to explain the experimental observations is presented. A nondimensional number (called the Bubble Stability Number) has been derived, describing the influence of different experimental parameters (surface tension, radius of the capillary tube, external pressure, and volume of the entrapped gas) in determining the region of stable growth. Applications of this theory to systems which can present this type of instability are also discussed.     

Membrane formation by water vapor induced phase inversion

The membrane formation by the phase inversion process was studied by coagulating a polysulfone/N-methyl-2-pyrrolidone solution with water vapor as a coagulant. The phase separation occurred when the relative humidity in the membrane casting atmosphere was higher than about 65%. The pore size was strongly affected by the relative humidity as well as the concentration of the polymer solution. It increased as both the relative humidity and the polymer concentration were decreased. The membranes produced showed a uniform structure composed of closed pores. The pure water flux measurement confirmed the closeness of the pores. The information on the late stage phase separation was obtained in situ by an optical microscope due to the slow phase separation. The pores seemed to grow very much at the late stage by coarsening which was observed to occur mainly by coalescence of polymer-lean droplets. As the relative humidity was lower, the coarsening continues longer ending up to a larger droplet size. The coarsening seems to enhance the interconnectivity of pores when the polymer concentration was low enough.     

Particulate formation induced by infrared laser dielectric breakdown

Laser induced dielectric breakdown has been utilized to initiate, sustain and study a number of chemical reactions. A 1 joule per line TEA CO₂ laser has been used as the source to induce dielectric breakdown. Product analysis was carried out by mass spectrometry and infrared spectroscopy. The reaction and method are discussed in terms of efficiency, selectivity and scope.     

Mineral formation on dentin induced by nano-bioactive glass

The object of this study was to evaluate the effect of bioactive glass (BG) size on mineral formation on dentin surfaces. Totally demineralized dentin discs were treated using BG suspensions with different particle sizes:i.e., microscale bioactive glass (m-BG), submicroscale bioactive glass (sm-BG) and nanoscale bioactive glass (n-BG). Field-emission scanning electron microscopy and 3D profile measurement laser microscopy were used to observe the surface morphology and roughness. It was found that all BG particles could promoted mineral formation on dentin surfaces, while plug-like depositions were observed on the dentin discs treated by n-BG and they were more acid-resistant. The present results may imply that n-BG has potential clinical application for dentin hypersensitivity treatment.