920 research outputs found
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Selective and highly efficient dye scavenging by a pH-responsive molecular hydrogelator
A structurally simple low molecular weight hydrogelator derived from isophthalic acid forms robust pH-responsive hydrogels capable of highly efficient and selective dye adsorption
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Amyloid formation: interface influence
The causes of pathological conditions
such as Alzheimer’s and Parkinson’s
diseases are becoming better
understood. Proteins that misfold from
their native structure to form aggregates
of β-sheet fibrils — termed amyloid — are
known1,2 to be implicated in these ‘amyloid
diseases’. Understanding the early steps
of fibril formation is critical, and the
conditions, mechanism and kinetics of
protein and peptide aggregation are being
widely investigated through a variety of
in vitro studies.
Kinetic aspects of the dispersion of the
protein or peptide in solution are thought
to influence the fibrillization process by
mass-transfer effects. In addition, mixing also
leads to shear forces, which can influence
fibril growth by perturbing the equilibrium
between the isolated and aggregated proteins,
causing existing fibrils to fragment and create
new nuclei3. Writing in the Journal of the
American Chemical Society, David Talaga
and co-workers have now highlighted4 an
additional factor that can influence the
fibrillization of amyloid-forming proteins —
the presence of hydrophobic interfaces
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Self-assembly, nematic phase formation and organocatalytic behaviour of a proline-functionalized lipopeptide
The self-assembly of the amphiphilic lipopeptide PAEPKI-C16 (P = proline, A = alanine, E = glutamic acid, K = lysine, I = isoleucine, C16 = hexadecyl) was investigated using a combination of spectroscopic, microscopic and scattering methods and compared to C16-IKPEAP with the same (reversed) peptide sequence and the alkyl chain positioned N-terminally and which lacks a free N-terminal proline residue. The catalytic activity of these peptides were then compared using a model aldol reaction system. For PAEPKI-C16, Cryo-TEM images showed the formation of micrometer length fibers, which by Small-angle X-ray scattering (SAXS) were found to have a radius of 2.5 - 2.6 nm. Spectroscopic analysis shows these fibers are built from -sheets. This behaviour is in complete contrast to that of C16-IKPEAP which forms spherical micelles with peptides in a disordered conformation [Hutchinson, J. A. et al. J. Phys. Chem. B 2019, 123, 613]. For PAEPKI-C16, the spontaneous alignment of fibers was observed upon increasing pH, which was accompanied by observed birefringence and anisotropy of SAXS patterns. This shows the formation of a nematic liquids and unprecedented nematic hydrogel formation was also observed these lipopeptides at sufficiently high concentrations. SAXS shows retention of an ultrafine (1.7 nm core radius) fibrillar network within the hydrogel. PAEPKI-C16 with free N-terminal proline shows enhanced anti:syn diastereoselectivity and better conversion compared to C16-IKPEAP. The cytotoxicity of PAEPKI-C16 was also lower than C16-IKPEAP for both fibroblast and cancer cell lines. These results highlight the sensitivity of lipopeptide properties to the presence of a free proline residue. The spontaneous nematic phase formation by PAEPKI-C16 points to the highly anisotropy of its ultrafine fibrillar structure and the formation of such a phase at low concentration in aqueous solution may be valuable for future applications
Direct test of defect mediated laser induced melting theory for two dimensional solids
We investigate by direct numerical solution of appropriate renormalization
flow equations, the validity of a recent dislocation unbinding theory for laser
induced freezing/melting in two dimensions. The bare elastic moduli and
dislocation fugacities which are inputs to the flow equations are obtained for
three different 2-d systems (hard disk, inverse power and the
Derjaguin-Landau-Verwey-Overbeek potentials) from a restricted Monte Carlo
simulation sampling only configurations {\em without} dislocations. We conclude
that (a) the flow equations need to be correct at least up to third order in
defect fugacity to reproduce meaningful results, (b) there is excellent
quantitative agreement between our results and earlier conventional Monte Carlo
simulations for the hard disk system and (c) while the qualitative form of the
phase diagram is reproduced for systems with soft potentials there is some
quantitative discrepancy which we explain.Comment: 11 pages, 14 figures, submitted to Phys. Rev.
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Thermally regulated reversible formation of vesicle-like assemblies by hexaproline amphiphiles
Peptides composed of hexaproline and glutamic acid (P6E) or lysine (P6K) as C-terminal units show thermally promoted aggregation, affording vesicle-like assemblies upon heating to 80 ºC. The aggregation is analyzed by dynamic light scattering (DLS), with number averaged diameters of ca. 600 and 300 nm respectively for P6E and P6K. NMR studies reveal that upon heating the amount of NMR-visible species is reduced to ca. 50% and that an important conformational change is experienced by the molecules in solution. Circular dichroism (CD) shows that at 20º C the peptides present a polyproline II (PP-II) conformation which is disorganized upon heating. Scanning electron microscopy for samples which were fast frozen at 80 ºC reveals vesicle-like assemblies. Using pyrene as a fluorescence probe, a critical aggregation concentration of ca. 30 m was estimated for P6E while that of P6K was above 0.6 mM. The aggregation process is found to be fully reversible and could serve as a basis for development of stimuli responsive carriers
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Tuning thermal properties and microphase separation in aliphatic polyester ABA copolymers
Four alkyl substituted β-lactones were investigated as monomers in ring opening polymerisation to produce a family of poly(3-hydroxyalkanoate)s. Homopolymers were synthesised using a robust aluminium salen catalyst, resulting in polymers with low dispersity (Đ < 1.1) and predictable molecular weights. ABA triblock copolymers were prepared using poly(L-lactic acid) as the A block and the afore- mentioned poly(3-hydroxyalkanoate) as the B block via a sequential addition method. Characterisation of these copolymers determined they were well controlled with low dispersities and predictable molecular weight. DSC analysis determined copolymers prepared from β-butyrolactone or β-valerolactone yielded polymers with tunable and predictable thermal properties. Copolymers prepared from β-heptanolactone yielded a microphase separated material as indicated by SAXS, with two distinct Tgs. The polymers could be readily cast into flexible films and their improved tensile properties were explored
Micellization of Sliding Polymer Surfactants
Following up a recent paper on grafted sliding polymer layers (Macromolecules
2005, 38, 1434-1441), we investigated the influence of the sliding degree of
freedom on the self-assembly of sliding polymeric surfactants that can be
obtained by complexation of polymers with cyclodextrins. In contrast to the
micelles of quenched block copolymer surfactants, the free energy of micelles
of sliding surfactants can have two minima: the first corresponding to small
micelles with symmetric arm lengths, and the second corresponding to large
micelles with asymmetric arm lengths. The relative sizes and concentrations of
small and large micelles in the solution depend on the molecular parameters of
the system. The appearance of small micelles drastically reduces the kinetic
barrier signifying the fast formation of equilibrium micelles.Comment: Submitted to Macromolecule
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Self-assembly of telechelic tyrosine end-capped PEO and poly(alanine) polymers in aqueous solution
The self-assembly in aqueous solution of three novel telechelic conjugates comprising a central hydrophilic polymer and short (trimeric or pentameric) tyrosine end-caps has been investigated. Two of the conjugates have a central poly(oxyethylene) (polyethylene oxide, PEO) central block with different molar masses. The other conjugate has a central poly(l-alanine) (PAla) sequence in a purely amino-acid based conjugate. All three conjugates self-assemble into β-sheet based fibrillar structures, although the fibrillar morphology revealed by cryogenic-TEM is distinct for the three polymers—in particular the Tyr5-PEO6k-Tyr5 forms a population of short straight fibrils in contrast to the more diffuse fibril aggregates observed for Tyr5-PEO2k-Tyr5 and Tyr3-PAla-Tyr3. Hydrogel formation was not observed for these samples (in contrast to prior work on related systems) up to quite high concentrations, showing that it is possible to prepare solutions of peptide–polymer-peptide conjugates with hydrophobic end-caps without conformational constraints associated with hydrogelation. The Tyr5-PEO6k-Tyr5 shows significant PEO crystallization upon drying in contrast to the Tyr5-PEO2k-Tyr5 conjugate. Our findings point to the remarkable ability of short hydrophobic peptide end groups to modulate the self-assembly properties of polymers in solution in model peptide-capped “associative polymers”. Retention of fluidity at high conjugate concentration may be valuable in potential future applications of these conjugates as bioresponsive or biocompatible materials, for example exploiting the enzyme-responsiveness of the tyrosine end-group
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