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Hybrid sugar-peptide copolymers
A new design concept for the synthesis of biomaterials fromcarbohydrates and peptides has been revealed by chemists at the University of California, Irvine (Angew. Chem. Int. Ed., published online Sept. 15, dx.doi.org/10.1002/anie.200501944). Zhibin Guan’s team used an approach known as interfacial polymerization to prepare a series of three galactaro-oligolysine hybrid copolymers from a galactose-derived monomer and three oligolysine peptide monomers: dilysine, trilysine, and tetralysine. The monosaccharide (red) and oligopeptide (blue) building blocks are linked in alternating sequence into one polymer chain (shown). Guan notes that the modular synthesis allows quick access to a diverse family of functionalized biomaterials that may find use in drug and gene delivery, DNA and protein microarrays, polymer therapeutics, and tissue engineering. His team’s initial series of hybrid copolymers are biodegradable and nonimmunogenic and can be used to efficiently transfer plasmid DNA into cell nuclei with minimal cytotoxicity. The group is currently working to design hybrid copolymers that can control the differentiation of embryonic stem cells.
Microfluidic biomaterial
Most materials used for bio-medical applications lack a way to modulate the concentration of soluble species within their bulk, a function served in living tissue by the vascular system. Abraham D. Stroock of Cornell University and coworkers have constructed a “microfluidic biomaterial” that consists of a network of channels in a calcium alginate hydrogel (J. Am. Chem. Soc. 2005, 127, 13788). The device is sturdy and impermeable enough to contain distinct flow paths, yet permeable enough for materials to diffuse through it. Using solutions of fluorescein or fluorescently labeled dextran, Stroock and coworkers show that soluble species can diffuse between the channels and the bulk, providing a way to deliver or extract them. A pressure-driven flow through the channels speeds up mass transfer through the device. Stroock hopes to use such a device as a physiologically accurate scaffold for tissue engineering by seeding cells into the bulk material.
Molecular wires have Ge in the middle
Transition-metal complexesin which two metal atoms are linked to one another through a conjugated digermanium chain have been synthesized by Alexander C. Filippou of Humboldt University of Berlin, in Germany, and coworkers (Angew. Chem. Int. Ed. 2005, 44, 5979). These digermylidyne complexes are reported to be the first group 14 analogs of carbon-based 'molecular wires' in which sp-hybridized carbon chains span two transition-metal centers. Filippou's group previously reported the synthesis of molybdenum and tungsten complexes containing triple bonds to germanium, tin, and lead. To make the new complexes, the researchers relied on some of the same chemistry to synthesize molybdenum and tungsten phosphine complexes containing triple-bonded germanium cyclopentadienyl ligands. Heating these compounds in the solid state cleaves the cyclopentadienyl groups, allowing the resulting metal fragments to dimerize and form the digermylidynes shown, in which the phosphorus ligands are bis(diethylphos-phino)ethane groups.
Organic salt turns DC to AC
A conducting organic salthas been demonstrated to possess the ability to convert direct current to alternating current. Materials with this ability display voltage-current characteristics that deviate from linearity and usually consist of semiconducting diodes in a series. The organic salt, named -(BEDT-TTF)2CsCo(SCN)4, where BEDT-TTF is bis-(ethylenedithio)tetra-thiafulvalene, consists of conducting layers of BEDT-TTF alternating with insulating layers of CsCo(SCN)4. Measurements of single crystals by Fumiaki Sawano and Ichiro Terasaki at Waseda University, Tokyo, and coworkers indicate that the organic salt “exhibits giant nonlinear resistance as a bulk phenomenon” (Nature 2005, 437, 522). For an organic salt to behave intrinsically in this way is unique, according to the researchers. Materials displaying nonlinear resistance are useful not only as components of practical devices such as DC-to-AC converters but also as systems to probe solid-state and statistical physics, the researchers write.
Bacterium blends light-harvesting strategies
In an unusual fusion of bio-chemical duties, the salt-loving red bacterium Salinibacter ruber harvests and processes light by using two structures previously thought to have evolved independently (Science 2005, 309, 2061). Sergei P. Balashov of the University of California, Irvine, and colleagues report that the bacterium contains a bacteriorhodopsin-like protein that binds two usually unrelated chromophores: retinal, the pigment rhodopsin proteins use to harness light to “pump” protons across cell membranes, and a carotenoid pigment known as salinixanthin (shown) that serves as a light-capturing “antenna,” a feature similar to those found in chlorophyll-based photosynthetic systems. The photons absorbed by the salinixanthin are transferred to the retinal chromophore, broadening the range of light wavelengths that can be used for proton pumping. The authors say this complex, which they call xanthorhodopsin, may have evolved to increase the efficiency of processing energy from light.
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