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IRG: Functional Biomolecular Materials
Assembly of a Vectorially-Oriented Four-Helix Bundle at the Air/Water Interface
J. Strzalka1, S. Ye1, A. Tronin1, X. Chen2, C.C. Moser2, P.L. Dutton2, B.M. Ocko3 & J.K. Blasie1
Departments of Chemistry1 and Biochemistry & Biophysics2, University of Pennsylvania, and Department of Physics3, Brookhaven National Laboratory.
Bundles of a-helices provide a scaffold for binding prosthetic groups at selected locations within the structure to mimic functions exhibited by biological proteins. The first family of these artificial peptides designed de novo was based on amphipathic di-helices, which self-assembled in aqueous solution forming 4-helix bundles of antiparallel topology. To realize any device applications, the peptides must be vectorially oriented in an ensemble, e.g., at an interface. The di-helices were made amphiphilic via attachment of C16 hydrocarbon chains to their N-termini. Specular x-ray reflectivity showed that these modified di-helices can be vectorially-oriented at an air-water interface with their helical axes normal to the interface. However, off-specular x-ray reflectivity indicated that these di-helices did not associate to form 4-helix bundles, possibly because they were constrained to be of parallel topology. To achieve 4-helix bundles vectorially-oriented at an interface, we relaxed the latter constraint via a 1:1 association of the amphiphilic di-helices with water soluble counterparts. Specular x-ray reflectivity demonstrated 4-helix bundle formation only when the association between di-helices is directed via designed attractive electrostatic interactions between the polar faces of the amphipathic helices.
4 Helix Bundle Association of B+B-C16 and B+B- at the Air-Water Interface
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Electron Density Profiles for
B+B-C16 |
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IRG: Multifunctional Complex Oxides
Large Magnetoresistance in Perovskite Spin Glasses
(I-W. Chen, P. Davies, T. Egami, J. Kikkawa)
Large magnetoresistance (MR) refers to a significant change of the electrical resistance in response to a magnetic field. Such effect has been most extensively studied in manganese oxides such as La1-xSrxMnO3, and in double perovskites such as Sr2FeMoO6. These oxides all possess long-range magnetic order. In the case of La1-xSrxMnO3 mixed valency is further required to facilitate the double exchange mechanism that gives rise to simultaneous metallic conductivity and ferromagnetic ordering. We have now found that a large magnetoresistance also exists in magnetic spin glasses, such as the ones obtained in the perovskite Sr1-xLaxRu1-xFexO3 series (see phase diagram Fig. A below). The magnetoresistance is almost linear with the field, giving a two-fold decrease of the resistance at large field (Fig. B below). X-ray near edge fine structure studies confirmed that there is no mixed valency in these materials. Moreover, the magnetic phase diagram and the magnetoresistance are apparently independent of the dimensionality of the structure as the same phenomena were observed in the layered perovskite Sr2-xLaxRu1-xFexO4 series. These results suggest that the magnetoresistance is mediated not by phase transition or domain alignment, but by local spin alignment at the nm scale between perovskite unit cells.
Figure A) Magnetic phase diagram of Sr1-xLaxRu1-xFexO3 determined using AC/DC magnetization data. FM = ferromagnetic, PM = paramagnetic, SG = spin glass. Inset shows temperature dependence of AC susceptibility under field-cooled (100 Oe) and zero-field cooled conditions. Hysteresis below the freezing temperature signifies a spin glass.
Figure B) Field dependence of magnetoresistance (MR) of Sr1-xLaxRu1-xFexO3 for x = 0.1-0.3 at 10K. MR is normalized by the resistance at zero field.
IRG: Microscale Soft Materials
Preparation, Stability, and In Vitro Performance of
Vesicles Made with Diblock Copolymers
James C-M. Lee, Harry Bermudez, Bohdana M. Discher,Maureen A. Sheehan,
You-Yeon Won, Frank S. Bates, Dennis E. Discher
In a collaboration with Frank Bates and his student in the Minnesota MRSEC, we found that Vesicles made completely from diblock copoly-mers— polymersomes—can be stably prepared by a wide range of techniques common to liposomes. Processes such as film rehydration, sonication, and extrusion can generate many-micron giants as well as monodisperse, ~100 nm vesicles of PEO-PEE (polyethyl-eneoxide-polyethylethylene) or PEO-PBD (polyethyl-eneoxide-polybutadiene). These thick-walled vesicles of polymer can encapsulate macromolecules just as lipo-somes can but, unlike many pure liposome systems, these polymersomes exhibit no in-surface thermal tran-sitions and a subpopulation even survive autoclaving. Suspension in blood plasma has no immediate ill-effect on vesicle stability, and neither adhesion nor stimulation of phagocytes are apparent when giant polymersomes are held in direct, protracted contact. Proliferating cells, in addition, are unaffected when cultured for an ex-tended time with an excess of polymersomes. The effects are consistent with the steric stabilization that PEG-lipid can impart to liposomes, but the present single-component polymersomes are far more stable mechanically and are not limited by PEG-driven micell-ization. The results potentiate a broad new class of tech-nologically useful, polymer-based vesicles. (Biotechnol. Bioeng. 73: 135–145, 2001)
Phagocytic challenge employing freshly isolated blood cells, red cells (r) or neutrophils (n), in 20 – 50% autologous plasma. Control yeast cells (y) or polymersomes (p) are subjected to the challenge. Video sequences (A, B) show neutrophils rapidly adhere to, spread on, and engulf yeast cells but neither recognize nor are stimulated by PEO-PEE polymersomes. Events are quantitated in (C): polymersomes are inert to white cells for at least 30 min of direct contact.
Field-induced structures in suspensions of magnetic and non-magnetic particles
Yodh et al have explored magnetic field induced ordering and micro-phase separation of aqueous ferrofluids, and aqueous mixtures of ferrofluid and other non-magnetic particles. The ferrofluid is a surfactant stabilized aqueous suspension of magnetite (Fe3O4) particles with an average diameter 20nm (including the ~5nm surfactant layer); the non-magnetic particles are charge stabilized PMMA spheres with diameters of 42nm, 108nm and 220nm; the rods are fd-virus with length 880nm and diameter 6.6nm. In the presence of a magnetic field applied perpendicular to the sample, the ferrofluid alone exhibits the usual transition from an isotropic phase to a phase of ferrofluid columns at ~600G; in higher fields, ~1100G, the columns become more sheet-like, although a true lamellar phase is not observed. Surprisingly, the addition of non-magnetic latex spheres or rods to the suspension lowered the isotropic-columnar transition fields, induced lamellar phases at low field, and generated a range of self-assembled coexisting structures consisting of columns and lamellae. In the figure below we illustrate these differences through a series of images as a function of applied magnetic field (normal to the plane) and as a function of PMMA particle concentration. We are currently exploring various theoretical scenarios to understand the role of non-magnetic particles in bringing about these structures.
IRG: Carbon Nanotube-based Materials
Electronic Properties of Nanoscopic Peapods (Luzzi, Yazdani, Mele and Johnson)
Nanoscopic peapods, which consist of encapsulated arrays of C60 molecules nested inside single wall nanotubes (SWNTs), represent a new class of nanoscale materials having tunable properties. We report the first electronic measurements of this system using a scanning tunneling microscope (STM). Our results demonstrate that the encapsulated C60 modify the local electronic structure of the SWNT cage. Furthermore, our measurements and calculations show that a periodic array of C60 molecules gives rise to a new hybrid electronic band, which derives its character from both the SWNT states and the C60 molecular orbitals. The peapod samples were produced using molecular self-assembly techniques. The new findings point to the future design of other hybrid nanoscale structures that could be tailored for a particular electronic function. Much like the dopant added to silicon, which turns beach sand into today's computer chips, the encapsulated molecules could make nanotubes more attractive as the material of choice for future nanocircuits.
Figure a) High-resolution electron micrograph of a peapod (C60 molecules within a carbon nanotube). b and c) Scanning tunneling micrographs at positive and negative tip bias of a peapod. d) Signal profiles along the horizontal lines of b and c showing the modified electronic signature of the peapod.
Seed
Polymer Membranes and New Morphologies in Thin Film Polymer Blends (Composto)*
Although polymer blends have been utilized since the 1940’s, polymer scientists are only now learning how to manipulate and control the phase morphology and compositional layering of polymer blend thin films that underlie technologies ranging from paints to flexible displays. In a series of studies, we have identified three distinct evolutionary stages of structural and compositional development in thin film polymer blends (Phys. Rev. E. 61, 1659, 2000; J. Chem. Phys. 113, 10386, 2000; Europhys. Lett. 50, 6322, 2000). Of particular interest are the membrane structures, shown below, having pore sizes that can be tuned from the nm to micron length scale depending on annealing time and composition. More recently, new morphologies, including encapsulated tubular phases, have been discovered by systematically varying film thickness (Polymer 42, 9155, 2001) and lateral confinement (Macromolecules 33, 3274, 2000; Phys. Rev. Lett. 87, 98302/1, 2001).
Nanoporous membrane (left) consisting of interconnected poly(styrene-co-acrylonitrile)
rich phase (light) and a poly(methylmethacrylate) rich phase which has been
etched away (dark). A microporous membrane (right) displaying the continuous
poly(styrene-co-acrylonitrile) rich matrix and round, disk-like pores (dark).
The nanoporous membranes are formed during the early stage (5-120 min. for a
500nm thick 50/50 film at 170C), whereas the microporous membrane forms during
the intermediate stage (2h – 60h). Annealing time, thickness and composition
controls the pore diameter and areal density.
* Partial MRSEC support
last modified:
April, 2002
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