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Use solid-phase synthesis to form self-assembling peptides with highly fluorinated cores, based on the coiled coil domain of yeast bZIP transcriptional activator GCN4. The primary driving forces for self-assembly are hydrophobic interactions and the replacement of CH₃ groups with CF₃ groups. This modification uses the low polarizability of the fluorine atoms to increase the hydrophobicity of the core. Such peptides are more stable than their hydrocarbon counterparts. B. Bilgiçer, A. Fichera, and K. Kumar* designed cores consisting of six fluorinated leucine residues and one asparagine residue to dictate oligomerization and orientation specificity. This work introduces a paradigm of self-assembling molecules based on orthogonal solubility properties of liquid phases—a useful feature, considering the importance of liquid-phase separation in biological systems. (J. Am. Chem. Soc. 2001, 123, 4393–4399; DLN)

New supramolecular machines operate on a silica surface. Molecular components can be self-assembled into larger complexes to form supramolecular machines such as pseudorotaxanes that can undergo conformation changes, or quasi-mechanical motion, when stimulated. Research reported by J. F. Stoddart, J. I. Zink, and co-workers shows that supramolecular machines can be deposited on a silica surface and remain active. They used a complex composed of a [2]pseudorotaxane ring, a naphthalene derivative “thread”, and a photosensitizer, encapsulated into a nanoporous optically transparent matrix derived from sol–gel chemistry. The resulting light-activated supramolecular machine undergoes threading and dethreading reactions, although the reaction rate is one-tenth of that in solution. To reduce possible steric constraints present in the pores, these supramolecular units were also anchored onto a silicon surface. In this case, the authors observed that the photochemically induced dethreading decreased somewhat after each threading reaction, leading to a loss of machine activity. (Angew. Chem., Int. Ed. 2001, 40, 2447–2451; LD)

Here’s a convenient three-step synthesis for thiopyrylium tetraflouroborate. Substituted thiopyrylium salts are well documented, but the preparation of unsubstituted thiopyrylium salts requires several steps and often produces very low yields. H. Poleschner and K. Seppelt* have devised a novel synthesis based on cyclization of an unsaturated thioether.

HMPT is hexamethylphosphorus triamide. This salt is remarkably stable in water and provides a means for synthesizing other salts in this family. (Eur. J. Org. Chem. 2001, 2477–2480; RM)

This simple reagent reacts efficiently with alcohols to form a protecting group. T. Mukaiyama and co-workers found that 2-(4-methoxybenzyloxy)-3-nitropyridine reacts with a variety of alcohols at room temperature in 5 min using a triflate catalyst to give the corresponding p-methoxybenzyl (PMB) ether.

Tf is trifluoromethanesulfonyl. Yields were typically 80–90%, and the reaction proceeded cleanly for primary, secondary, and tertiary alcohols. This method worked well for acid-sensitive groups (acetal, silyloxy) and for base-sensitive groups (acryloyloxy, halo, keto), suggesting its potential value in synthesis of complex structures. (Chem. Lett. 2001, 424–425; WJP)

Synchrotron facilities are experiencing a serious shortage of trained staff at all beamlines according to Gerd Rosenbaum, director for the Southeastern Regional Collaborative Access Team at Argonne National Labs. The impending boom in structural genomics places increased demands on the five U.S. and two planned European synchrotrons. These instruments emit highly focused light beams across a broad range of wavelengths, allowing scientists to determine atomic structure and behavior of materials. The remarkable progress in mapping the human ge nome has pushed proteins to the forefront of materials of current interest.

Work at synchrotrons is multidisciplinary, involving accelerator physicists, beam scientists, engineers, and chemists to provide expert support. Most of the time spent is to support the users by understanding what the users want to achieve and showing them what the equipment can do. (Nature 2001, 410, 721; DLN)

Three polypeptide chains mounted on a template produce a molecular tripod with a controlled cavity space between polymer chains. H. Mihara and co-authors have produced helical bundle peptides designed to create a hydrophobic core. They control helical geometry and hydrophobicity by specific choice of amino acid residues in the chains. By attaching these chains to a trifunctional starting template, the three polypeptide chains were directed into a tripodlike geometry.

The authors found that a fluorescent probe (8-anilino-1-naphthalenesulfonic acid) binds to the peptide structure within the cavity created by the three polypeptide chains. The degree of probe binding was estimated by its fluorescence intensity. The authors were able to determine the optimum amino acid compositions to maximize probe binding. Such structures can be used as models for protein receptor sites, and they have potential for sensor devices. (Biopolymers 2001, 59, 65–71; DAS)

Use “precipiton” technology to facilitate reaction product separations. T. Bosanac, J. Yang, and C. S. Wilcox* have devised an intriguing separation approach based on a change in a reaction product’s solubility, which they control by structural isomerization of an ancillary portion of the desired product. They define a precipiton as a molecular fragment that is intentionally attached to a reactant molecule and that can be isomerized after reaction to facilitate precipitation or phase transfer of the attached product. The authors found that cis-stilbenes were quite soluble in common organic solvents, whereas the trans isomers were virtually insoluble. They attached several alkenoates to the cis-stilbene precipiton via an ester linkage to form 1, which was treated with nitrile oxides to form the desired oxazoline product 2 by [3 + 2] cycloaddition.

Ph₂S-mediated isomerization to the insoluble trans-stilbene isomer 3 resulted in its precipitation from solution, followed by transesterification to free the oxazoline product 4 from its precipiton fragment. Product yields were 70–90%. This method appears useful as a route to functionalized oxazolines. The authors note that they cannot yet regenerate the high-solubility form of the precipiton efficiently. (Angew. Chem., Int. Ed. 2001, 40, 1875–1879; WJP)

Use a membrane reactor to recycle homogeneous catalysts. The trick is to attach the catalyst to a soluble polymer so that the polymer-bound catalyst is retained by the membrane and is effectively recycled in situ. This system provides the advantages of homogeneous catalysis, such as high reaction rate and selectivity without diffusion limitations, combined with the simple catalyst recovery of heterogeneous catalysis. J. Wöltinger and co-authors report highly efficient enantioselective reduction of -tetralone using an oxaborolidine catalyst attached to either poly(methylhydrosiloxane-co-dimethylsiloxane) copolymer or polystyrene in a Chemzyme reactor.

The reactor allows small molecules such as starting materials and products to pass through the membrane, while the larger polymer-bound catalyst molecules are retained. The authors found they could run the reactor for >2 weeks and maintain almost complete conversion (>99%). Catalyst retention is better with the polystyrene-bound catalyst (99.94% vs 98.50% retention with the polysiloxane-bound catalyst). The average ee in both cases was 96.8%, only slightly lower than that reported for Corey’s original diphenylprolinol catalyst. (Org. Process Res. Dev. 2001, 5, 241–248; WW)

Liquid marbles can be made to roll freely on a solid surface. Liquid marbles are produced by forming drops of a water–glycerol mixture that are then coated with a hydrophobic powder, typically consisting of fluorinated silanes. According to P. Aussillous and D. Quéré*, this procedure produces nearly spherical composite droplets that behave like soft solids and show dramatically reduced adhesion to solid surfaces. These “liquid marbles” are elastic, so they can bounce and roll without leaking and even float on a pool of water. Also, the hydrophobic powder—present at the liquid–air interface—allows the coated drop to move across a glass plate without wetting the surface. Experiments with liquid marbles confirm the theory that smaller droplets should roll faster than larger ones on an inclined plane. When the plate is inclined above 24°, the high velocities of the liquid marbles cause them to deform from spheres to peanut or doughnut (torus) shapes. Although the formation of toroidal drops has been previously proposed, this report is the first instance in which these deformed drops have been observed. The concept of liquid marbles may be an interesting solution to the problem of depositing small amounts of liquid onto solid (or even liquid) substrates. (Nature 2001, 411, 924–927; LD, DLN)

Carbon nanotubes can be “soldered” to one another. F. Banhart has found that nanotubes can be connected by irradiating the junctions where they cross each other with an electron beam.

The figure illustrates a nanotube junction before and after the soldering process, in which amorphous carbon is deposited onto the junction. To target the nanotube junctions precisely, this process was conducted in a scanning electron microscope with a field emitter that can focus the e-beam onto a spot <1 nm diam. Welds with varying amounts of graphitic solder were observed around the bonded junction, depending on the extent to which the newly bonded structure was exposed to oxygen. The authors stressed that this really is a type of soldering technique, because the tubes do not coalesce but are held together by a small aggregate of the amorphous carbon. Simple T-junctions were reported, but one can well imagine intricate structures produced using this remarkable technique. (Nano Lett. 2001, 1, 329–332; DAS)

Expanded graphite can be used to produce electrically conductive composites with good mechanical properties. G.-H. Chen and co-workers produced expanded graphite by treating graphite flake with concentrated H₂SO₄ and HNO₃, using a known process. In situ polymerization of styrene in the presence of the expanded graphite produced a composite material that changed from electrical insulator to semiconductor at 1.8 wt% of expanded graphite. Unfilled polystyrene has a conductivity of 10^–16 S/cm, which increases steadily to 10^–2 S/cm at a 5% level of expanded graphite. By comparison, ~50% addition of graphite powder is required to reach the same conductivity level. Microscopy studies suggest that the graphite disperses in the form of nanosheets (10–30 nm thick) within the polymer matrix. The tensile strength of the composite also increased with expanded graphite content (up to 25% improvement with 5% loading). The impact strength decreased with filler level, although this reduction could be nearly eliminated by mechanically rolling the composite material. The role of expanded graphite as a conducting filler should be of interest in several industrial processes. (Polym. Int. 2001, 50, 980–985; DAS)