Lifshitz points in blends of AB and BC diblock copolymers

We consider micro- and macro-phase separation in blends of AB and BC flexible diblock copolymers. We show that, depending on architecture, a number of phase diagram topologies are possible. Microphase separation or macrophase separation can occur, and there are a variety of possible Lifshitz points. Because of the rich parameter space, Lifshitz points of multiple order are possible. We demonstrate Lifshitz points of first and second order, and argue that, in principle, up to 5th-order Lifshitz points are possible

Fibrillisation of hydrophobically modified amyloid peptide fragments in an organic solvent

The self-assembly of a hydrophobically modified fragment of the amyloid beta(A beta) peptide has been studied in methanol. The peptide FFKLVFF is based on A beta(16-20) extended at the N terminus by two phenylalanine residues. The formation of amyloid-type fibrils is confirmed by Congo Red staining, thioflavin T fluorescence and circular dichroism experiments. FTIR points to the formation of beta-sheet structures in solution and in dried films and suggests that aggregation occurs at low concentration and is not strongly affected by further increase in concentration, i.e. the peptide is a strong fibril-former in methanol. UV fluorescence experiments on unstained peptide and CD point to the importance of aromatic interactions between phenylalanine groups in driving aggregation into beta-sheets. The CD spectrum differs from that usually observed for beta-sheet assemblies formed by larger peptides or proteins and this is discussed for solutions in methanol and also trifluoroethanol. The fibril structure is imaged by transmission electron microscopy and scanning electron microscopy on dried samples and is confirmed by small-angle X-ray scattering experiments in solution

PepMat 2016: The second conference on peptide-based materials for biomedicine and nanotechnology

Postprint (published version

Surface structure of thin asymmetric PS-b-PMMA diblock copolymers investigated by atomic force microscopy

Asymmetric poly(styrene-b-methyl methacrylate) (PS-b-PMMA) diblock copolymers of molecular weight M-n = 29,700g mol(-1) (M-PS = 9300 g mol(-1) M-PMMA = 20,100 g mol(-1), PD = 1.15, chi(PS) = 0.323, chi(PMMA) = 0.677) and M-n = 63,900 g mol(-1) (M-PS = 50,500 g mol(-1), M-PMMA = 13,400 g mol(-1), PD = 1.18, chi(PS) = 0.790, chi(PMMA) = 0.210) were prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization. Atomic force microscopy (AFM) was used to investigate the surface structure of thin films, prepared by spin-coating the diblock copolymers on a silicon substrate. We show that the nanostructure of the diblock copolymer depends on the molecular weight and volume fraction of the diblock copolymers. We observed a perpendicular lamellar structure for the high molar mass sample and a hexagonal-packed cylindrical patterning for the lower molar mass one. Small-angle X-ray scattering investigation of these samples without annealing did not reveal any ordered structure. Annealing of PS-b-PMMA samples at 160 degrees C for 24 h led to a change in surface structure

Structure variation and evolution in microphase-separated grafted diblock copolymer films

The phase behavior of grafted d-polystyrene-block-poly(methyl methacrylate) diblock copolymer films is examined, with particular focus on the effect of solvent and annealing time. It was observed that the films undergo a two-step transformation from an initially disordered state, through an ordered metastable state, to the final equilibrium configuration. It was also found that altering the solvent used to wash the films, or complete removal of the solvent prior to thermal annealing using supercritical CO2, could influence the structure of the films in the metastable state, though the final equilibrium state was unaffected. To aid in the understanding to these experimental results, a series of self-consistent field theory calculations were done on a model diblock copolymer brush containing solvent. Of the different models examined, those which contained a solvent selective for the grafted polymer block most accurately matched the observed experimental behavior. We hypothesize that the structure of the films in the metastable state results from solvent enrichment of the film near the film/substrate interface in the case of films washed with solvent or faster relaxation of the nongrafted block for supercritical CO2 treated (solvent free) films. The persistence of the metastable structures was attributed to the slow reorganization of the polymer chains in the absence of solvent

Modulating self-assembly of a nanotape-forming peptide amphiphile with an oppositely charged surfactant

A peptide amphiphile (PA) C16-KTTKS, containing a pentapeptide headgroup based on a sequence from procollagen I attached to a hexadecyl lipid chain, self-assembles into extended nanotapes in aqueous solution. The tapes are based on bilayer structures, with a 5.2 nm spacing. Here, we investigate the effect of addition of the oppositely charged anionic surfactant sodium dodecyl sulfate (SDS) via AFM, electron microscopic methods, small-angle X-ray scattering and X-ray diffraction among other methods. We show that addition of SDS leads to a transition from tapes to fibrils, via intermediate states that include twisted ribbons. Addition of SDS is also shown to enhance the development of remarkable lateral ‘‘stripes’’ on the nanostructures, which have a 4 nm periodicity. This is ascribed to counterion condensation. The transition in the nanostructure leads to changes in macroscopic properties, in particular a transition from sol to gel is noted on increasing SDS (with a further reentrant transition to sol on further increase of SDS concentration). Formation of a gel may be useful in applications of this PA in skincare applications and we show that this can be controlled via development of a network of fine stranded fibrils

Self-assembly of peptide bioconjugates: selected recent research highlights

This Topical Review briefly discusses selected highlights of recent research on self-assembling peptide amphiphiles (PAs) and polymer–peptide conjugates. Subjects covered include new polymer chemistries used to prepare polymer–peptide conjugates, PA self-assembly landscapes and kinetics, developments in the application of bioactive PAs and the relationship between self-assembly and bioactivity, novel PA/biopolymer composites, functional π-stacking peptide conjugates, use of enzymes to tune self-assembly, and developments in high throughput methods and the design and application of sequenced peptides

Anomalous structural and mechanical properties of solids confined in quasi one dimensional strips

We show using computer simulations and mean field theory that a system of particles in two dimensions, when confined laterally by a pair of parallel hard walls within a quasi one dimensional channel, possesses several anomalous structural and mechanical properties not observed in the bulk. Depending on the density $\rho$ and the distance between the walls $L_y$, the system shows structural characteristics analogous to a weakly modulated liquid, a strongly modulated smectic, a triangular solid or a buckled phase. At fixed $\rho$, a change in $L_y$ leads to many reentrant discontinuous transitions involving changes in the number of layers parallel to the confining walls depending crucially on the commensurability of inter-layer spacing with $L_y$. The solid shows resistance to elongation but not to shear. When strained beyond the elastic limit it fails undergoing plastic deformation but surprisingly, as the strain is reversed, the material recovers completely and returns to its original undeformed state. We obtain the phase diagram from mean field theory and finite size simulations and discuss the effect of fluctuations.Comment: 14 pages, 13 figures; revised version, accepted in J. Chem. Phy

Fmoc–RGDS based fibrils: atomistic details of their hierarchical assembly

We describe the 3D supramolecular structure of Fmoc–RGDS fibrils, where Fmoc and RGDS refer to the hydrophobic N-(fluorenyl-9-methoxycarbonyl) group and the hydrophilic Arg-Gly-Asp-Ser peptide sequence, respectively. For this purpose, we performed atomistic all-atom molecular dynamics simulations of a wide variety of packing modes derived from both parallel and antiparallel ß-sheet configurations. The proposed model, which closely resembles the cross-ß core structure of amyloids, is stabilized by p–p stacking interactions between hydrophobic Fmoc groups. More specifically, in this organization, the Fmoc-groups of ß-strands belonging to the same ß-sheet form columns of p-stacked aromatic rings arranged in a parallel fashion. Eight of such columns pack laterally forming a compact and dense hydrophobic core, in which two central columns are surrounded by three adjacent columns on each side. In addition to such Fmoc¿Fmoc interactions, the hierarchical assembly of the constituent ß-strands involves a rich variety of intra- and inter-strand interactions. Accordingly, hydrogen bonding, salt bridges and p–p stacking interactions coexist in the highly ordered packing network proposed for the Fmoc–RGDS amphiphile. Quantum mechanical calculations, which have been performed to quantify the above referred interactions, confirm the decisive role played by the p–p stacking interactions between the rings of the Fmoc groups, even though both inter-strand and intra-strand hydrogen bonds and salt bridges also play a non-negligible role. Overall, these results provide a solid reference to complement the available experimental data, which are not precise enough to determine the fibril structure, and reconcile previous independent observations.Peer ReviewedPostprint (published version

Silica templating of a self-assembling peptide amphiphile that forms nanotapes

The peptide amphiphile C16-KTTKS templates silica polymerization, enabling the production of silica nanotape structures, imaged via electron microscopy (TEM and SEM). X-ray scattering shows that the nanotapes comprise stacked layers, as for the parent peptide amphiphile, but with a substantially increased layer spacing resulting from silica polymerization