Table of Contents

Molecularly imprinted polymers may result in better microelectronic chips, because the polymers (MIPs) can be efficiently molded into place in arrays of silicon microstructures. M. Yan* and A. Kapua use a two-step microfabrication technique, micromolding in capillaries, to prepare the polymers on silicon wafers. This first step begins by producing a soft elastomeric poly(dimethylsiloxane) stamp from a silicon wafer master mold (created by photolithography) that contains the desired patterns. In the second step, the stamp is placed on a silicon wafer, and the MIP is molded into the channels produced at the interface. In this way, micrometer-scale patterns of MIPs could be easily generated (MIP monoliths of ~20 µm were fabricated). As an example, the authors used the technique to prepare microfabricated MIPs against the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D). These were tested in a radioligand assay and found to be selective for the template herbicide. The technique may be very useful in constructing molecularly imprinted on-chip devices. (Polym. Prepr. 2000, 41, 264–265; OR)

Evidence points to a surprising stability for carbonic acid. The elusive H₂CO₃ molecule has long been considered kinetically unstable. R. Ludwig and A. Kornath review recent theoretical work predicting that H₂CO₃ can exist in its free form, along with experiments demonstrating its synthesis and stability in gas and solid phases. Calculations have proved that pure H₂CO₃ has a surprising kinetic stability, even though it decomposes quickly in aqueous solution. Calculations gave a half-life of 180,000 years for pure H₂CO₃, indicating that it is still stable at room temperature. However, this value decreased dramatically to 2 min, assuming catalysis by two water molecules. The authors suggest that H₂CO₃ stability in pure form may allow preparative-scale synthesis. They also point to other candidates such as H₂SO₃, which may be more stable than we currently believe. (Angew. Chem., Int. Ed. 2000, 39, 1421–1423; WJP)

Here’s an easy way to achieve acetamidoglycosylation of alcohols and carbohydrates. 2-Amino sugars are biologically important molecules, and over the years the methods to introduce this functionality have met with varying success. The method reported by D. Y. Gin and co-authors allows direct incorporation of the C₂-N-acetylamino functionality onto glycal donors, as well as glycosidic coupling with various glycosyl acceptors—all in one pot. This procedure uses a reagent mixture of thianthrene-5-oxide and triflic anhydride combined with N-trimethylsilylacetamide in a mixture of CHCl₃ and CH₂Cl₂.

Bn is benzyl; TIPS is triisopropylsilyl; Tf is trifluoromethanesulfonyl. In addition to good yields of this three-step, one-pot procedure, glycal-protecting groups, such as benzyl, triisopropylsilyl, and allyl ethers, as well as acid-labile isopropylidene ketals, can be used. The reagent combination selectively reacts with the glycol enol ether functionality in the presence of simple alkenes and therefore should be a useful synthetic method. (Angew. Chem., Int. Ed. 2000, 39, 204–207; RM)

This polymer’s molecular weight is pH-dependent. A boron-based condensation polymer can be “turned” on and off by changing the pH of the aqueous polymerization medium, according to T. Shimizu and co-authors. They found that reaction of a bisglucuronamide (1) with a bis(boronic acid) (2) resulted in efficient formation of a boronate polymer complex at high pH (>10).

The polymer molecular weight was estimated at 2 ¥ 10⁶ Da, indicating a degree of polymerization >2000. When the pH was lowered, the authors were able to follow the gradual degradation of the polymer structure by monitoring the spin-state of the boron atom using ¹¹B-NMR. At almost neutral pH (6–7), ~50% of the polymer remained; and at a weakly acidic pH (~3), the polymer was com pletely hydrolyzed. (Chem. Commun. 2000, 881–882; OR)

This sol–gel method makes single silica nano tubes. According to M. Harada and M. Adachi*, single silica nanotubes or bundles of a few tubes can be made by the surfactant-mediated template mechanism. The authors have demonstrated that single silica nanotubes can be prepared when the surfactant packing parameter p is in the range of 0.33 to 0.50; p = V_c/(S l_c), and V_c and l_c represent the volume and effective length, respectively, of the hydrocarbon chain attached to a surfactant polar head whose area is S. They used the laurylamine hydrochloride–tetraethoxysilane system to form the sol–gel. The essential features of this nanotube formation method are that the combined molecules self-organize into cylindrical assemblies in a quasi-equilibrium state, and deformation of the generated aggregates does not occur during the condensation of silicon precursors. The authors suggest that this procedure may work to form nanotubes from other materials such as titania. Such silica nanotubes have potential application in electronics, optics, advanced catalysis, energy storage, and energy conversion. (Adv. Mater. 2000, 12, 839–841; XSZ)

Measure the ratio of single- to double-stranded DNA with time-resolved fluorescence assay. Analyzing the decay kinetics for the cyanine dye PicoGreen (PG) bound to mixtures of single-stranded (ssDNA) and double-stranded DNA (dsDNA) in solution can directly reveal single- to double-stranded ratios, according to J. C. Scaiano and co-authors. They tested the DNA of calf thymus and salmon testes by intercalating the PG dye and observed that with dsDNA, the dye’s fluorescence decays monoexponentially with a lifetime of 4.5 ns, whereas the PG–ssDNA complex decays biexponentially (1.16 and 3.09 ns) with essentially equal weights (51 and 49%). By using a triple-exponential decay framework, decay profiles were effectively analyzed by a single parameter fit (i.e., the pre-exponential factor for dsDNA) to directly reveal the solution ssDNA:dsDNA ratio. The authors further suggest extension of this technology to gel biotechniques such as single-cell gel electrophoresis (also known as comet assay). (Chem. Commun. 2000, 689–690; GAB)

Ketones can be reduced directly to hydrocarbons using a one-pot process and a hetero geneous catalyst. B. Török*, G. London, and M. Bartók report that a bifunctional, 5% platinum on K-10 montmorillonite catalyst operating at 170 °C and 750 psig hydrogen pressure neatly converts cyclic and acyclic ketone carbonyl groups to methylene groups in 60–98% yield. The authors propose that platinum serves to hydrogenate the ketone to an alcohol, which is dehydrated to an olefin over the solid acid clay and then hydrogenated a second time.

Steric crowding around the carbonyl groups appeared to be a limiting factor in those reductions reported in <96% yield. (Synlett 2000, 631–632; DAS)

Use thermal cross-linking to enhance the compressive strength of poly(benzobisthiazole). Because of their low compressive strengths, poly(benzobisthiazole) (PBZT) fibers have limited use in high-performance applications. X. Hu, M. B. Polk, and S. Kumar* have developed a thermal cross-linking approach that substantially improves compressive properties of these fibers. They used standard procedures to form a tetra methyl-substituted PBZT liquid crystalline polymer (1), in which the methyl groups serve as sites for thermal cross-linking. Upon heating at 500 °C, the polymer eliminates methane by linking polymer chains through methylene bridges (2).

This chemical cross-linking is an alternative to chain-to-chain hydrogen bonding, which has also been used to enhance transverse strength in liquid crystalline polymer fibers. The authors report that the thermal cross-linking process improves modulus and recoil compressive strength by 20–30%. However, these improvements come at the expense of tensile strength—it is reduced by ~40%. (Macromolecules 2000, 33, 3342–3348; DAS)

Use sugar-coated dendrimers as reactive surfaces for noble metal particle fabrication. K. Esumi and co-workers report making gold and silver nanoparticles by reducing noble metal salts in aqueous solution in the presence of sugar-persubstituted poly(amidoamine) (PAMAM) dendrimers. In contrast to earlier efforts by these authors and others, no external reductant was required in this system. The sugar–PAMAM dendrimers serve simultaneously as protective colloids and as reducing agents. The authors formed the metal particles by dissolving the sugar dendrimers and the metal salt (e.g., HAuCl₄) in aqueous solution, allowing reduction to metal nanoparticles. The authors report that the average particle size depends on the dendrimer generation number and concentration and is typically 2–3 nm for gold and 4–5 nm for silver particles. Both particle types were characterized by a relatively wide size distribution. (J. Colloid Interface Sci. 2000, 226, 346–352; GAB)

This catalytic process improves selectivity in chromatography. Catalysis is certainly the most stringent criterion for the selection of different enzymes—more so than substrate affinity. L. A. Jurado, J. T. Drummond, and H. W. Jarrett found that this criterion can be used also in chromatography. The authors reasoned that by binding enzymes to an affinity column containing immobilized substrate and subsequently releasing the enzyme by adding either a cosubstrate or a cofactor, they could achieve a more selective elution than by conventional means. Compared with traditional affinity chromatography, their method is indeed more efficient. For example, the Eco RI restriction enzyme was bound to an agarose resin carrying DNA stretches—identical to classical affinity chroma tography. Then a divalent Mg²⁺ cation (a cofactor necessary for cleavage of the DNA) was added, and the enzyme converted substrate to product and selectively eluted from the column. The authors found that this enzyme was recovered in a single step with much greater yield and purity than that obtained using traditional KCl elution. (Anal. Biochem. 2000, 282, 39–45; OR)