Noteworthy Chemistry
August 17, 2009
- Aliphatic biodegradable photoluminescent polymers
- Oxygen-doped nanodiamonds
- Antimicrobial surfaces for hospital use
- Polymer-free layer-by-layer films for electrochemical conversion
- Polycation transformation of T4 DNA morphology
- Cross-coupling reactions with very low catalyst loadings
- Indenedione luminescence behavior altered by packing arrangement
Aliphatic biodegradable photoluminescent polymers
are potentially useful in medical devices. Biodegradable photoluminescent polymers (BPLPs) and cross-linked networks (CBPLPs) derived from them would be useful biomedical materials, but they have not been made until now. J. Yang and coauthors at the University of Texas, Arlington, and the University of Texas Southwestern Medical Center (Dallas) designed and prepared a series of aliphatic fluorescent polymers and tested their in vitro biodegradability.
The synthesis begins with difunctional 1,8-octanediol (1) and trifunctional citric acid (2), which react to produce long polymer chains (3) with unreacted carboxylate and hydroxyl groups. Intermediate polymer 3 is then treated with L-cysteine to give linear polymer 4. The amino acid reacts with a vicinal hydroxyl group to form a six-membered ring, which acts as a fluorophore for the whole molecule (BPLP, 5). Finally, additional intermolecular condensation reactions create an elastomeric cross-linked network (CBPLP, 6).
The authors show that BPLP and CBPLP are biodegradable and non-cytotoxic, with only minimal chronic inflammatory responses in vivo. Their physical properties are suitable for processing into micro- and nanodevices. (Proc. Natl. Acad. Sci. USA 2009, 106, 10086–10091; Sally Peng Li)
Oxygen-doped nanodiamonds are easily produced. A unique property of the natural p-type hydrogen-terminated diamond structure is its pronounced negative electron affinity, which makes the structure potentially usable in electronic devices (e.g., field-effect transistors and electron emitters). Unlike the p-type structure, n-doping with donor atoms (e.g., O, S, N, and P) should enhance the electron-emitting properties of the diamond structure.
Theoretical studies by A. A. Fokin, P. R. Schreiner, and coauthors at Kiev Polytechnic Institute (Ukraine), Justus-Liebig University (Giessen, Germany), and Chevron Technology Ventures (Richmond, CA) suggest that replacing CH2 segments of the diamondoid structure with heteroatoms (internal doping) promotes substantial changes in the electronic properties of these nanoparticles. This prompted the authors to develop effective synthetic methods for oxygen doping the diamondoid structures, illustrated by 1 in the figure.
The steps leading to 1 proceeded in remarkably high yields, given the possible reaction sites on the starting diamondoid. This study was extended to more complex oxygen-doped diamondoids, some of which are represented by 2–5.
Brominating oxadiamondoid 1 yielded the 6-bromo derivative as the only product; it was hydrolyzed to give the 6-hydroxy derivative (6) quantitatively, demonstrating that only one of six possible tertiary C–H positions was functionalized. The authors suggest that the position of the dopant in the diamondoid cage is decisive in determining selectivities of the substitution reactions. They are currently extending their study to measure the electronic properties of n-doped diamondoids. (Org. Lett. 2009, 11, 3068–3071; W. Jerry Patterson)
Here’s a review of antimicrobial surfaces for hospital use. K. Page, M. Wilson, and I. P. Parkin* of University College London have written a comprehensive review of antimicrobial surfaces, with an emphasis on protecting patients from hospital-acquired infections. They first discuss the importance of antimicrobial surfaces in the hospital environment and how hospital-acquired infections, such as methicillin-resistant Staphylococcus aureus (MRSA), transfer from one person to another. They then discuss various antiadhesion coatings, such as poly(ethylene glycol), diamond-like carbon films, easy-clean surfaces, and zwitterionic polymer biomimetic surfaces. These coatings can prevent microbes, proteins, and mammalian cells from adhering to the surfaces; but they do not contain active antimicrobials.
The authors’ discussion of antimicrobial coatings emphasizes microbicide-releasing surfaces, polycationic antimicrobial surfaces, and light-activated antimicrobial agents. The microbicide-releasing surfaces include silver and copper surfaces and bacteriophage-modified surfaces. Silver has a wide range of antibacterial effects and is used in several commercial applications, but it can be toxic to mammalian cells. Although copper surfaces have better antimicrobial properties than stainless steel, the authors do not recommend them for hospital use because of their poor mechanical and chemical properties. Copper alloys (e.g., brass), however, may be a good choice. Titania-based photocatalysts for antibacterial applications and their proposed mechanisms are also discussed. (J. Mater. Chem. 2009, 19, 3819–3831; George Xiu Song Zhao)
Use polymer-free layer-by-layer films for electrochemical conversion. A research team at MIT (Cambridge, MA) led P. T. Hammond and Y. Shao-Horn used layer-by-layer (LBL) assembly to construct thin films that contain gold nanoparticles (AuNPs) and multiwall carbon nanotubes (MWNTs) for electrochemical conversion. They assembled LBL films of varying thicknesses (2–16 bilayers) under appropriate pH conditions on silane coated indium tin oxide glass via electrostatic interactions between positively charged 2-aminoethanethiol-functionalized gold nanoparticles (3.4 ± 0.2 nm) and negatively charged carboxylic acid–functionalized MWNTs (1–5 μm long, 15 ± 5 nm o.d.). The assembly was heated at 150 °C under vacuum for 12 h to induce amide bond formation. The figure shows a 7- (a) and a 13-bilayer (b) assembly.
The authors note that the AuNP density at constant diameter on the MWNTs in the LBL increases as the number of bilayers increases from 2 to 13. They show that the random arrangement of MWNTs can still be observed after AuNP deposition in the seven-bilayer film. Surface coverage, surface roughness, and film uniformity improve between 10 and 13 bilayers, indicating that, upon complete surface coverage, the AuNPs mask the surfaces of the MWNTs via a close-packed arrangement. With more than 13 bilayers, aggregation of AuNPs contributes to an increase in surface roughness.
The linear growth rate of the LBL films varies as a function of surface coverage; once AuNP surface coverage (~10 bilayers) is achieved, the growth rate increases. The authors also report a bathochromic shift with increasing number of bilayers caused by surface plasmon resonance associated with AuNP interactions. Below 10 bilayers, coupling of the surface plasmon resonance between adjacent AuNPs results in the bathochromic shift, whereas aggregation of the AuNPs most likely contributes to the shift between 10 and 13 bilayers.
The presence of AuNPs is crucial to diffusion-controlled MeOH oxidation in the AuNP–MWNT LBL films, and repeated cycling does not affect the electrochemical signal of the electrode device. The microstructure of the thicker LBL films (≥13 bilayers) limits electrocatalytic oxidation of MeOH because of hindered diffusion through the closely packed AuNPs and the lower AuNP surface activity in the aggregated state. (Chem. Mater. 2009, 21, 2993–3001; LaShanda Korley)
Polycations can transform the morphology of T4 DNA. DNA molecules are polymeric, and the polymer chains can self-assemble into random coil or spherical morphology upon environmental stimuli. A. A. C. C. Pais and co-workers at the University of Coimbra (Portugal) explored the morphology transition of DNA molecules under the influence of polycations.
The test molecule was T4 DNA, and the ionic reagent was protamine, a polycationic protein. The experiments were run in two sets of three solutions; within each set, the concentrations of DNA (0.25 μM) and protamine (1.0 μM or 0.05 μM) were the same, but the solutions used to make up these mixtures were different: small volume of concentrated DNA with large volume of dilute protamine (mixture A); equal volumes of intermediate concentrations of both components (mixture B); and large volume of dilute DNA with small volume of concentrated protamine (mixture C).
The authors found that the number of globules in solution can be controlled by changing the DNA concentration before mixing it with low-concentration protamine solutions (mixtures A–C at 0.05 μM). Bundles are produced when concentrated DNA solutions are mixed with high-concentration protamine solutions (mixture A, 1.0 μM). Overcharged globules result from relatively dilute DNA solutions combined with concentrated protamine solutions (mixture C, 1.0 μM). Finally, an even distribution occurs when DNA and protamine are dilute (mixture C, 0.05 μM) or when DNA and protamine, at relatively high concentrations (mixture B, 1.0 μM), are mixed in comparatively large volumes. (Biomacromolecules 2009, 10, 1319–1323; Sally Peng Li)
Cross-coupling reactions proceed with very low catalyst loadings. The useful Ullmann coupling reaction is attractive because it often uses low-cost starting materials and is catalyzed by readily available copper salts. Despite these advantages, the catalytic Ullmann process typically has fairly high copper salt loadings of 5–10 mol%. Decreasing the loading leads to extended reaction times and lower product yields.
P.-O. Norrby, C. Bolm, and coauthors at the University of Gothenburg (Göteborg, Sweden) and RWTH Aachen University (Germany) report an unexpected solution to this problem. They examined a test coupling between
pyrazole (1) and PhI with CuCl2 catalyst loadings as low as 0.01 mol%. Reaction ingredients included N,N’-dimethylethylenediamine (DMEDA) as the ligand and K3PO4 as the required base. They were surprised to obtain yields of coupling product 2 as high as 88% at the 0.01-mol% catalyst level; an optimized catalyst loading of 0.08 mol% resulted in almost quantitative product yields.
The coupling reaction with PhI was expanded to include several N-, O-, and S-nucleophiles by using CuO (rather than CuCl2) catalyst loadings as low as 0.001 mol%. Although the results were variable, product yields as high as 86–87% were obtained in at least two cases. In each case, the authors varied the base to obtain optimum yields. In this process, a crucial feature was a high ligand/metal ratio; the DMEDA level was always 20 mol%.
The authors could not explain the results from ultralow catalyst loadings, although they identified ligand concentration and reaction temperature as dominant variables. They suspect that the ligand shifts the equilibria away from low-coordinated copper species that would otherwise be deactivated by aggregation. (Angew. Chem., Int. Ed. 2009, 48, 5691–5693; W. Jerry Patterson)
Indenedione luminescence behavior is altered by packing arrangement in the solid state. Molecular packing in the condensed phase is controlled by a delicate balance of various weak intermolecular forces involving π-conjugated cores and nonconjugated tails. How changes in the π-conjugation affect luminescence has been the subject of extensive studies, but how variations in the remote group influence light emission has been investigated much less. S. Das and coauthors at the National Institute for Interdisciplinary Science and Technology (Kerala, India) and the University of Missouri–St. Louis examined how the lengths of alkyl tails in indenedione derivatives 1–4 alter their molecular packing arrangements and their solid-state luminescence properties.
The emissions of these luminogens are very weak in solution, but they are enhanced and red-shifted in the solid state. The extent of the shift and enhancement varies with the alkyl tail length. The emission of 1 is greatly shifted but barely enhanced because of the cofacial arrangement of its molecules that leads to the formation of stacked H-dimers. Molecules 2 and 3 form J-aggregates and their emissions are shifted less but greatly enhanced. There is no π-overlap between the molecules of 4, so its solid-state emission differs little from that of its solutions. The structural rigidification, however, intensifies its light emission. (J. Phys. Chem. C 2009, 113, 11927–11935; Ben Zhong Tang)