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RESEARCH NUGGETS

IRG: Functional Biomolecular Materials

Incorporating Novel Prosthetic Groups into Artificial Proteins

D. Metcalf, F. Cochran, S. Wu, W. Wang, H. Kono, W. F. DeGrado, M. J. Therien, J. G. Saven

Cofactors are compounds that confer function to many biological proteins. Designing artificial proteins with synthetic non-biological cofactors could lead to peptide-based systems with novel properties involving nonlinear optical responses and light-induced electron transport and/or proton translocation. Importantly, the protein scaffolding can be used to control the solubility, position and orientation of the cofactor within the peptide, as well as the peptide's supramolecular assembly into nanophase materials whose macroscopic behavior arises from such novel properties. Engineering peptide-cofactor molecular structures requires careful design of peptide sequences, especially when non-biological cofactors are involved, such as the bridged diporphyrin compounds currently being synthesized at Penn. Excitation and electron transfer can be controlled in such synthetic bridged cofactors by varying the donor and acceptor groups and the chemical properties of the bridge, including its length. The target structure of the four-helix bundle complex, as shown in the figure, was identified using new computational design methods developed at Penn, wherein the peptide backbone structure, metal coordination, and noncovalent peptide-cofactor interactions are each optimized. The cofactor binding and structural properties of the complex are currently being investigated.

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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 La_1-xSr_xMnO₃, and in double perovskites such as Sr₂FeMoO₆. These oxides all possess long-range magnetic order. In the case of La_1-xSr_xMnO₃ 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 Sr_1-xLa_xRu_1-xFe_xO₃ 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 Sr_2-xLa_xRu_1-xFe_xO₄ 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 Sr_1-xLa_xRu_1-xFe_xO₃ 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 Sr_1-xLa_xRu_1-xFe_xO₃ for x = 0.1-0.3 at 10K. MR is normalized by the resistance at zero field.

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IRG: Microscale Soft Materials

Nematic Elastomers at the Mesoscale

Nematic Elastomers combine the order-disorder properties of rod-shaped liquid crystal molecules with the resilience of rubbery solids. Micron-scale rods in highly porous, collapsable gels provide a uniquely visualizable platform for studying some of the unusual effects in these materials. Novel responses include soft elasticity modes that theoretically arise with collective rotations of the director, s. Interestingly, many cell biological systems - cytoskeletons in particular - are composed of hard-rod structures embedded in a soft matrix, lending motivation for synthetics and insight into natural processes. The emerging work on nematic elastomers is a collaboration of Lubensky, Yodh, and Discher groups in Penn’s Soft Materials MRSEC-group.

Theory of Nematic Gel Phase Behavior
Underlying the rich equilibrium phase diagrams of nematic gels is a mixing free energy combined with an elastic free energy of anisotropic gaussian chains and an Onsager theory for rods:

Simulated Shear alignment of a membranous Nematic Elastomer
( Length/Diam ~11 )

Fd-viral rods magnetically aligned in a porous ‘Tanaka’ del, collapsed in a ‘bad’ solvent

and then stretched

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IRG: Carbon Nanotube-based Materials

Mapping the Electronic States of Fullerene Peapods
(Fischer, Johnson, Kane, Luzzi, Mele and Winey in collaboration with Prof. A. Yazdani, University of Illinois, Urbana-Champaign)

We reported the first spectroscopic study showing that encapsulation of a molecular species (C₆₀) in a carbon nanotube can tune the electronic properties on the outer tube wall. The finding will have impact for the development of new nanometer scale electronic devices and architectures based on carbon nanotubes. Our results were published as the cover story of Science (D. Hornbaker et al., Science 295, 828-831 (2002))

We studied the electronic properties of fullerene "peapods", a linear array of C₆₀ molecules enclosed within the carbon nanotube. Using material produced by David Luzzi at the University of Pennsylvania, low temperature scanning tunneling spectroscopy of these structures was carried out by Prof. Ali Yazdani at the University of Illinois. The experiments showed that encapsulation of C₆₀ produces profound changes in the unoccupied electronic states, though relatively minor changes in the spectrum of filled electronic states. Theory developed by E.J. Mele (Penn) shows that the modifications result from a hybridization of propagating electronic states on the carbon nanotube with the lowest unoccupied molecular orbital on the buckyball. This produces a new type of impurity band on the tube wall, and a hybridization gap which produces a reduction in the differential tunneling conductance. The theory provides a strikingly complete description of the experimental findings, and provide an example of a system where the electronic properties of the nanotube have been engineered at the nanometer scale.

The graphic on the left shows the calculated band structure of the unmixed (tube (dashed) and ball (blue)) and hybridized (red) system reported in D. Hornbaker et al.

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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

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