12 research outputs found

    Comparison of the Thermal Denaturing of Human Serum Albumin in the Presence of Guanidine Hydrochloride and 1‚ÄĎButyl-3-methylimidazolium Ionic Liquids

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    The interaction of proteins with aqueous solutions of ionic liquids (ILs) has attracted considerable recent attention owing to the challenges of finding biocompatible water-free ILs. These systems remain of great interest because of the potential for using ILs as designer solvents for biocatalytic processes. Increasing evidence demonstrates that aqueous solutions of water-miscible ILs, such as the well-studied 1-alkyl-3-methylimidazolium ILs, disrupt the native fold of proteins and can drive the formation of non-native aggregates that could negatively impact catalytic function. Here, we present a study comparing the thermal unfolding of human serum albumin (HSA) in a 1 M solution of the protein denaturant guanidine hydrochloride with two 1 M aqueous solutions of 1-butyl-3-methylimidazolium ILs, namely the chloride and the acetate. Small-angle neutron scattering (SANS) measurements found qualitative agreement between the thermally driven unfolding process for the three denaturants, as well as with a Tris buffer solution. HSA irreversibly aggregates and unfolds in the three denaturant solutions upon heating to temperatures below that required to drive the same process in a simple Tris buffer solution. The results reveal subtle differences in the interaction of the ILs and guanidine hydrochloride with the protein, although the final states of the protein were similar in all cases. The results indicate that the ions of water-miscible ILs and guanidine hydrochloride have specific roles in disrupting protein structure and driving aggregation. The experimental approach employed has the potential to provide new insights into protein interactions with ionic liquids that may aid in the search for more biocompatible ionic liquids

    Alamethicin Disrupts the Cholesterol Distribution in Dimyristoyl Phosphatidylcholine‚ÄďCholesterol Lipid Bilayers

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    Cell membranes are complex mixtures of lipids, proteins, and other molecules that serve as active, semipermeable barriers between cells, as well as between their internal organelles, and the surrounding medium. Their compositions and structures are tightly regulated to ensure proper function. Cholesterol is a key component in mammalian cellular membranes, where it serves to maintain membrane fluidity and permeability. Here, the interaction of alamethicin, a 20 amino acid residue peptide that creates transmembrane pores in lipid bilayer membranes in a concentration-dependent manner, with bilayer membranes composed of dimyristoyl phosphatidylcholine (DMPC) and cholesterol (Chol) was studied. Small-angle neutron scattering (SANS) data demonstrate that a low concentration of alamethicin (peptide-to-lipid ratio of 1/200) disrupts a lateral inhomogeneity seen in peptide-free DMPC:Chol vesicles, which analysis of the SANS data indicates are Chol-rich and Chol-poor phases having different thicknesses. Alamethicin disrupts this structure, producing laterally homogeneous bilayers that are thinner than either phase of the peptide-free bilayers, and possess a strong asymmetry in the Chol content of the inner and outer bilayer leaflets. The results suggest that a secondary membrane disruption mechanism exists in parallel with the well-understood cytotoxic membrane permeabilization that results when alamethcin forms transmembrane pores. Specifically, the peptide can disrupt laterally organized lipidic structures in cell membranes, as well as significantly perturb the compositions of the inner and outer leaflets of the membrane. The existence of a secondary mechanism of action against cellular membranes for alamethicin raises the possibility that other membrane-active peptides function similarly

    Multicompartmental Microcapsules from Star Copolymer Micelles

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    We present the layer-by-layer (LbL) assembly of amphiphilic heteroarm pH-sensitive star-shaped polystyrene-poly­(2-pyridine) (PS<sub><i>n</i></sub>P2VP<sub><i>n</i></sub>) block copolymers to fabricate porous and multicompartmental microcapsules. Pyridine-containing star molecules forming a hydrophobic core/hydrophilic corona unimolecular micelle in acidic solution (pH 3) were alternately deposited with oppositely charged linear sulfonated polystyrene (PSS), yielding microcapsules with LbL shells containing hydrophobic micelles. The surface morphology and internal nanopore structure of the hollow microcapsules were comparatively investigated for shells formed from star polymers with a different numbers of arms (9 versus 22) and varied shell thickness (5, 8, and 11 bilayers). The successful integration of star unimers into the LbL shells was demonstrated by probing their buildup, surface segregation behavior, and porosity. The larger arm star copolymer (22 arms) with stretched conformation showed a higher increment in shell thickness due to the effective ionic complexation whereas a compact, uniform grainy morphology was observed regardless of the number of deposition cycles and arm numbers. Small-angle neutron scattering (SANS) revealed that microcapsules with hydrophobic domains showed different fractal properties depending upon the number of bilayers with a surface fractal morphology observed for the thinnest shells and a mass fractal morphology for the completed shells formed with the larger number of bilayers. Moreover, SANS provides support for the presence of relatively large pores (about 25 nm across) for the thinnest shells as suggested from permeability experiments. The formation of robust microcapsules with nanoporous shells composed of a hydrophilic polyelectrolyte with a densely packed hydrophobic core based on star amphiphiles represents an intriguing and novel case of compartmentalized microcapsules with an ability to simultaneously store different hydrophilic, charged, and hydrophobic components within shells

    Synergistic Role of Temperature and Salinity in Aggregation of Nonionic Surfactant-Coated Silica Nanoparticles

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    The adsorption of nonionic surfactants onto hydrophilic nanoparticles (NPs) is anticipated to increase their stability in aqueous medium. While nonionic surfactants show salinity- and temperature-dependent bulk phase behavior in water, the effects of these two solvent parameters on surfactant adsorption and self-assembly onto NPs are poorly understood. In this study, we combine adsorption isotherms, dispersion transmittance, and small-angle neutron scattering (SANS) to investigate the effects of salinity and temperature on the adsorption of pentaethylene glycol monododecyl ether (C12E5) surfactant on silica NPs. We find an increase in the amount of surfactant adsorbed onto the NPs with increasing temperature and salinity. Based on SANS measurements and corresponding analysis using computational reverse-engineering analysis of scattering experiments (CREASE), we show that the increase in salinity and temperature results in the aggregation of silica NPs. We further demonstrate the non-monotonic changes in viscosity for the C12E5‚Äďsilica NP mixture with increasing temperature and salinity and correlate the observations to the aggregated state of NPs. The study provides a fundamental understanding of the configuration and phase transition of the surfactant-coated NPs and presents a strategy to manipulate the viscosity of such dispersion using temperature as a stimulus

    Polymer Chain Shape of Poly(3-alkylthiophenes) in Solution Using Small-Angle Neutron Scattering

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    The chain shape of polymers affects many aspects of their behavior and is governed by their intramolecular interactions. Delocalization of electrons along the backbone of conjugated polymers has been shown to lead to increased chain rigidity by encouraging a planar conformation. Poly¬≠(3-hexylthiophene) and other poly¬≠(3-alkylthiophenes) (P3ATs) are interesting for organic electronics applications, and it is clear that a hierarchy of structural features in these polymers controls charge transport. While other conjugated polymers are very rigid, the molecular structure of P3AT allows for two different planar conformations and a significant degree of torsion at room temperature. It is unclear, however, how their chain shape depends on variables such as side chain chemistry or regioregularity, both of which are key aspects in the molecular design of organic electronics. Small-angle neutron scattering from dilute polymer solutions indicates that the chains adopt a random coil geometry with a semiflexible backbone. The measured persistence length is shorter than the estimated conjugation length due to the two planar conformations that preserve conjugation but not backbone correlations. The persistence length of regioregular P3HT has been measured to be 3 nm at room temperature and decreases at higher temperatures. Changes in the regioregularity, side chain chemistry, or synthetic defects decrease the persistence length by 60‚Äď70%

    Observing Framework Expansion of Ordered Mesoporous Hard Carbon Anodes with Ionic Liquid Electrolytes via in Situ Small-Angle Neutron Scattering

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    The reversible capacity of materials for energy storage, such as battery electrodes, is deeply connected with their microstructure. Here, we address the fundamental mechanism by which hard mesoporous carbons, which exhibit high capacities versus Li, achieve stable cycling during the initial ‚Äúbreak-in‚ÄĚ cycles with ionic liquid electrolytes. Using in situ small-angle neutron scattering we show that hard carbon anodes that exhibit reversible Li<sup>+</sup> cycling typically expand in volume up to 15% during the first discharge cycle, with only relatively minor expansion and contraction in subsequent cycles after a suitable solid electrolyte interphase (SEI) has formed. While a largely irreversible framework expansion is observed in the first cycle for the 1-methyl-1-propypyrrolidinium bis¬≠(trifluoromethanesulfonyl)¬≠imide (MPPY.TFSI) electrolyte, reversible expansion is observed in the electrolyte lithium bis¬≠(trifluoro-methanesulfonyl)¬≠imide (LiTFSI)/1-ethyl-3-methyl-imidazolium bis¬≠(trifluoromethanesulf-onyl)¬≠imide (EMIM.TFSI) related to EMIM<sup>+</sup> intercalation and deintercalation before a stable SEI is formed. We find that irreversible framework expansion in conjunction with SEI formation is essential for the stable cycling of hard carbon electrodes

    Small-angle neutron scattering study of specific interaction and coordination structure formed by mono-acetyl-substituted dibenzo-20-crown-6-ether and cesium ions

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    <p>This study uses small-angle neutron scattering (SANS) to elucidate the coordination structure of the complex of mono-acetyl-substituted dibenzo-20-crown-6-ether (ace-DB20C6) with cesium ions (Cs<sup>+</sup>). SANS profiles obtained for the complex of ace-DB20C6 and Cs<sup>+</sup> (ace-DB20C6/Cs) in deuterated dimethyl sulfoxide indicated that Cs<sup>+</sup> coordination resulted in a more compact structure than the free ace-DB20C6. The data were fitted well with SANS profiles calculated using Debye function for scattering on an absolute scattering intensity scale. For this theoretical calculation of the scattering profiles, the coordination structure proposed based on density functional theory calculation was used. Consequently, we conclude that the SANS analysis experimentally supports the proposed coordination structure of ace-DB20C6/Cs and suggests the following: (1) the complex of ace-DB20C6 and Cs<sup>+</sup> is formed with an ace-DB20C6/Cs molar ratio of 1/1 and (2) the two benzene rings of ace-DB20C6 fold around Cs<sup>+</sup> above the center of the crown ether ring of ace-DB20C6.</p

    Metal-Free cAMP-Dependent Protein Kinase Can Catalyze Phosphoryl Transfer

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    X-ray structures of several ternary product complexes of the catalytic subunit of cAMP-dependent protein kinase (PKAc) have been determined with no bound metal ions and with Na<sup>+</sup> or K<sup>+</sup> coordinated at two metal-binding sites. The metal-free PKAc and the enzyme with alkali metals were able to facilitate the phosphoryl transfer reaction. In all studied complexes, the ATP and the substrate peptide (SP20) were modified into the products ADP and the phosphorylated peptide. The products of the phosphotransfer reaction were also found when ATP-ő≥S, a nonhydrolyzable ATP analogue, reacted with SP20 in the PKAc active site containing no metals. Single turnover enzyme kinetics measurements utilizing <sup>32</sup>P-labeled ATP confirmed the phosphotransferase activity of the enzyme in the absence of metal ions and in the presence of alkali metals. In addition, the structure of the <i>apo</i>-PKAc binary complex with SP20 suggests that the sequence of binding events may become ordered in a metal-free environment, with SP20 binding first to prime the enzyme for subsequent ATP binding. Comparison of these structures reveals conformational and hydrogen bonding changes that might be important for the mechanism of catalysis

    Thermally Responsive Hyperbranched Poly(ionic liquid)s: Assembly and Phase Transformations

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    A library of linear and branched amphiphilic poly¬≠(ionic liquid)¬≠s based on hydrophobic cores and peripheral thermally sensitive shells was synthesized and studied with regard to their ability to form stimuli-responsive, organized assemblies in aqueous media. The thermally responsive derivatives of poly¬≠(ionic liquid)¬≠s were synthesized by neutralizing 32 terminal carboxyl groups of functionalized polyester cores by amine-terminated poly¬≠(<i>N</i>-isopropyl¬≠acrylamide)¬≠s (PNIPAM) (50% and 100%). We observed that these hyperbranched poly¬≠(ionic liquid)¬≠s possessed a narrow low critical solution transition (LCST) window with LCST for hyperbranched compounds being consistently lower than that for linear PNIPAM containing counterparts. We found that the poly¬≠(ionic liquid)¬≠s form spherical micellar assemblies with diverse morphologies, such as micelles and their aggregates, depending on the terminal compositions with reduced sizes for hyperbranched poly¬≠(ionic liquid)¬≠s. Increasing temperature above LCST promoted formation of network-like aggregates, large vesicles, and spherical micelles. Moreover, all PNIPAM-terminated compounds exhibited distinct unimolecular prolate nanodomain morphology in contrast to common spherical domains of initial cores. We proposed a multilength scale organized morphology to describe the thermoresponsive poly¬≠(ionic liquid)¬≠s micellar assemblies and discussed their morphological transformations during phase transitions associated with changes in hydrophobic‚Äďhydrophilic balance of poly¬≠(ionic liquid)¬≠s with distinct hydrophobic cores and variable peripheral shells

    CO<sub>2</sub>‚ÄĎReactive Ionic Liquid Surfactants for the Control of Colloidal Morphology

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    This article reports on a new class of stimuli-responsive surfactant generated from commercially available amphiphiles such as do¬≠decyl¬≠tri¬≠methyl¬≠am¬≠mmonium bromide (DTAB) by substitution of the halide counterion with counterions such as 2-cyano¬≠pyrrolide, 1,2,3-tri¬≠azolide, and <i>L</i>-proline that complex reversibly with CO<sub>2</sub>. Through a combination of small-angle neutron scattering (SANS), electrical conductivity measurements, thermal gravimetric analysis, and molecular dynamics simulations, we show how small changes in charge reorganization and counterion shape and size induced by complexation with CO<sub>2</sub> allow for fine-tunability of surfactant properties. We then use these findings to demonstrate a range of potential practical uses, from manipulating microemulsion droplet morphology to controlling micellar and vesicular aggregation. In particular, we focus on the binding of these surfactants to DNA and the reversible compaction of surfactant‚ÄďDNA complexes upon alternate bubbling of the solution with CO<sub>2</sub> and N<sub>2</sub>
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