Thierry Muller

Cellular uptake of platinum nanoparticles in human colon carcinoma cells and their impact on cellular redox systems and DNA integrity.

Supercritical fluid reactive deposition was used for the deposition of highly dispersed platinum nanoparticles with controllable metal content and particle size distribution on beta-cyclodextrin. The average particle size and size distribution were steered by the precursor reduction conditions, resulting in particle preparations <20, <100, and >100 nm as characterized by transmission electron microscopy and scanning electron microscopy (SEM). These particle preparations of different size distributions were used to address the question as to whether metallic platinum particles are able to invade cells of the gastrointestinal tract as exemplified for the human colon carcinoma cell line HT29 and thus affect the cellular redox status and DNA integrity. Combined focused ion beam and SEM demonstrated that platinum nanoparticles were taken up into HT29 cells in their particulate form. The chemical composition of the particles within the cells was confirmed by energy-dispersive X-ray spectroscopy. The potential influence of platinum nanoparticles on cellular redoxsystems was determined in the DCF assay, on the translocation of Nrf-2 and by monitoring the intracellular glutathione (GSH) levels. The impact on DNA integrity was investigated by single cell gel electrophoresis (comet assay) including the formation of sites sensitive to formamidopyrimidine-DNA-glycosylase. Platinum nanoparticles were found to decrease the cellular GSH level and to impair DNA integrity with a maximal effect at 1 ng/cm(2). These effects were correlated with the particle size in an inverse manner and were enhanced with increasing incubation time but appeared not to be based on the formation of reactive oxygen species.

Click chemistry finds its way into covalent porous organic materials.

Porous organic materials are becoming increasingly important owing to their applications in catalysis and optoelectronics. They are, however, best known for their gas adsorption/storage capacities. Here, they clearly outperform their inorganic and metal–organic counterparts. As they are composed entirely of light elements, they have exceptionally low densities which are particularly appealing for the automotive sector where weight reduction is essential. The development of permanently porous organic materials is challenging as nature tends to minimize pore volume because of the higher surface energies of porous materials. Upon solidification these materials either achieve efficient packing or existing voids collapse to give denser structures. Permanent porosity in wholly organic structures is obtained only by preventing efficient packing in the solid state. Hence, rigid, sterically demanding, and/or contorted organic building units have to be used to generate permanently porous organic materials. Two different concepts are generally applied. The first relies on covalently bonded organic cages with intrinsic porosity which are prefabricated and then assembled to give crystalline porous materials. The Cooper and Mastalerz groups independently used reversible imine condensation to produce cages that assemble to give microporous materials with surface areas comparable to those of classic polymeric or crystalline porous materials. The second approach involves the self-assembly of rigid but essentially nonporous building blocks, through either hydrogen bonding or covalent bonding. Examples of covalent materials reported by Yaghi et al. rely on reversible boronate or imine formation to generate threedimensional crystalline networks. Since crystallinity is not a prerequisite for molecular control over pore size in rigid frameworks, narrow pore size distributions have been obtained in three-dimensional porous organic polymers, showing that long-range order is not necessary for obtaining uniform pore sizes. The essence is that thermodynamically and kinetically controlled processes can both be used to generate microporous organic frameworks. Thus, irreversible but high-yielding reactions such as organometallic crosscouplings have been widely used to generate amorphous covalent porous networks. These reactions are a rather obvious choice, as the monomers, which are typically composed of aromatic rings and alkyne units, concurrently fulfill the stiffness requirement to allow for inefficient packing and subsequent permanent porosity. However, neither boronate and imine formation nor organometallic cross-coupling reactions meet all the criteria outlined by Sharpless in 2001 for click reactions. These reactions—mostly 1,3-dipolar cycloaddition reactions of organic azides and terminal alkynes—have had a strong impact on material sciences, especially polymer science. Thus, it is all the more astonishing that click chemistry has until very recently not served in the preparation of porous organic materials. This has now been achieved: within a year s time two independent groups reported on “clicked” porous organic material. The two studies are based on identical tetrahedral monomers. Cooper et al. prepared a conjugated microporous polymer (CMP) by reacting two complementary azido and alkyne tetrakisphenylmethanes (Scheme 1). The resulting CMP has a respectable Brunauer–Emmett–Teller (BET) surface area of 1128 m g . TGA analysis indicated the decomposition of residual azide and alkyne groups at 125 and 275 8C, respectively. Shortly afterwards, Nguyen and colleagues reported the first in-depth study of the same “clicked” network—termed this time porous organic polymer (POP)—using slightly modified reaction conditions (Scheme 1). They found that the surface area drastically increased at higher reaction temperature (440 m g 1 at 25 8C versus 1260 m g 1 at 100 8C), and that it decreased with the addition of sodium ascorbate. This finding was attributed to higher concentrations of Cu, the active catalyst, which led to more cross-linking and thus a lower surface area. When 10 mol% of sodium ascorbate was used, an essentially microporous material having a narrow pore size distribution with a primary pore width of 9.2 was suggested by nonlocal density functional theory (NLDFT) pore size distribution analysis. Since the pore width for a perfect diamond network lies around 21 , this finding constitutes strong evidence for interpenetrating networks. The optimized reaction protocol delivered a POP with a slightly higher surface area (1440 m g ) than that of Cooper s CMP. [*] Dr. T. Muller, Prof. S. Br se Institute of Organic Chemistry and DFG Center for Functional Nanostructures (CFN), Karlsruhe Institute of Technology (KIT) Fritz-Haber-Weg 6, 76131 Karlsruhe (Germany) E-mail: thierry.muller@kit.edu braese@kit.edu Homepage: http://www.ioc.kit.edu/braese/

Branched DNA That Forms a Solid at 95 °C

Control over the structure of materials may be achieved by using predictable interactions, such as base pairing. Base pairing between DNA strands is emerging as one of the most versatile design principles of nanoconstruction. A range of hybridization and folding motifs of linear and circular DNA have been reported. The flexibility of the design has been further expanded by linking oligonucleotides to synthetic branching elements or “cores”. 5] The resulting construct can have properties not found in natural DNA. This includes DNA-coated gold nanoparticles that assemble into three-dimensional aggregates, the melting transitions of which are exceptionally sharp. Nanoparticle size and linker structure affect the association behavior, and crystallization may be induced in favorable cases. For DNA hybrids with organic cores, the effect of linking the DNA to a branching element can be more dramatic still. Four-arm hybrid 1 (Scheme 1) with its tetrahedral core was recently shown to assemble into a macroscopic material, even though its oligonucleotide arms are just dimers. The assembly process is sequence specific, as demonstrated by mismatch controls, but the UV-melting transitions are broad, not sharp as in the case of gold nanoparticles. Shortly after the publication of the unusually stable assemblies of 1, the first designed DNA crystals were reported. The fact that the association of the rigid triangle motifs that serve as rigid “cores” in these crystals is also driven by no more than dimer “sticky ends” again suggests that the rules for 3D construction of periodic assemblies are quite different from those of linear DNA.

Two Base Pair Duplexes Suffice to Build a Novel Material

Tetrahedral DNA hybrids with tetrakis(p‐hydroxyphenyl)methane cores hybridize in a sequence‐specific fashion at much higher temperatures than isolated linear duplexes. Dinucleotide DNA arms suffice to induce the formation of a solid at room temperature; this demonstrates the strength of multivalent binding. The graphic shows a view of a modeled assembly.

Click chemistry produces hyper-cross-linked polymers with tetrahedral cores

Methane and adamantane based hyper-cross-linked polymers have been prepared by click chemistry reacting the corresponding tetraalkynes with 1,4-diazidobenzene. The adamantane based HCP proved to be very efficient for CO2 capture at low pressures.

Well-defined star shaped polymer-fullerene hybrids via click chemistry†

The use of a hexakisazido macrocyclic methanofullerene proves to be highly efficient in the preparation of 6-arm star polymers via copper(I) catalyzed azide-alkyne cycloaddition.

Di‐ and Dodeca‐Mitsunobu Reactions on C60 Derivatives: Post‐Functionalization of Fullerene Mono‐ and Hexakis‐Adducts

The diverse addition patterns of fullerene adducts make them attractive molecules, generating widespread application. Firstly, in terms of the latter, mono-adducts are extensively used in opto-electronic devices, owing to the fullerene core s capacity to accept up to six electrons. A second important application involves the utilization of fullerene adducts as structural components in material sciences. With their octahedral topology, Th-symmetric hexakis-adducts, are particularly intriguing. Amongst others, the latter have been used in C60-based star polymers. [5] In the aforementioned contexts, the relatively stable methanofullerene adducts obtained through Bingel cyclopropanation reactions are of particular interest. However, this functionalization approach—primarily developed by A. Hirsch and later optimized by Y.-P. Sun—suffers from two critical weaknesses. One is its effectively exclusive limitation to malonates; only a few exceptions are known. The other is a structural concern. In fact, quite often only the use of fairly simple malonates leads to the desired fullerene adducts in reasonable yields without tedious purifications. In order to overcome these problems, several groups have developed post-functionalizations of methanofullerene adducts. For instance, J.F. Nierengarten adapted the copper-mediated Huisgen 1,3dipolar cycloaddition reaction and applied it to the preparation of complex hexasubstituted fullerenes. A. Hirsch developed a selective deprotection–functionalization sequence of fullerene e,e,e-trisadducts. Our own group derivatized hexakis methanofullerenes by using several organometallic cross-coupling reactions. This article outlines the use of the Mitsunobu reaction as an efficient tool for the post-functionalization of fullerene monoand hexakis-adducts. The mild, virtually neutral conditions under which this dehydrative coupling of an alcohol with a pronucleophile (generally an acid) proceeds, prompted our group to use this reaction for the derivatization of fullerene adducts. To this end, we have prepared a C60 mono-adduct bearing 2 hydroxyl groups and a hexakisadduct with 12 hydroxyl functions. The preparation of hexakis compound 4 is depicted in Scheme 1. Ethylene glycol was mono-protected as tert-butyldimethylsilyl ether according to a previously reported procedure. Subsequent diesterification of malonic acid was performed in the presence of DCC in 97% yield. Hexakis-adduct 3 was then readily synthesized by using the optimized conditions

Combining modular ligation and supramolecular self-assembly for the construction of star-shaped macromolecules

A well-defined random copolymer of styrene (S) and chloromethylstyrene (CMS) featuring lateral chlorine moieties with an alkyne terminal group is prepared (P(S-co-CMS), M(n) = 5500 Da, PDI = 1.13). The chloromethyl groups are converted into Hamilton wedge (HW) entities (P(S-co-HWS), M(n) = 6200 Da, PDI = 1.13). The P(S-co-HWS) polymer is subsequently ligated with tetrakis(4-azidophenyl)methane to give HW-functional star-shaped macromolecules (P(S-co-HWS))(4), M(n) = 25,100 Da, PDI = 1.08). Supramolecular star-shaped copolymers are then prepared via self-assembly between the HW-functionalized four-arm star-shaped macromolecules (P(S-co-HW))(4) and cyanuric acid (CA) end-functionalized PS (PS-CA, M(n) = 3700 Da, PDI = 1.04), CA end-functionalized poly(methyl methacrylate) (PMMA-CA, M(n) = 8500 Da, PDI = 1.13) and CA end-functionalized polyethylene glycol (PEG-CA, M(n) = 1700 Da, PDI = 1.05). The self-assembly is monitored by (1)H NMR spectroscopy and light scattering analyses.

Influence of Perfluorinated End Groups on the SFRD of [Pt(cod)Me(CnF2n+1)] onto Porous Al2O3 in CO2 under Reductive Conditions

The optimized synthesis of a range of cyclooctadiene-stabilized Pt complexes that contained different perfluoro-alkane chains, [Pt(cod)Me(Cn F2n+1 )], is presented. These metal-organic compounds were employed in the so-called supercritical fluid reactive deposition (SFRD) in CO2 under reductive conditions to generate metallic nanoparticles on aluminum oxide as a porous support. Thus, Al2 O3 -supported Pt nanoparticles with a narrow particle-size distribution were obtained. At a reduction pressure of 15.5 MPa and a temperature of 353 K, particle diameters of d50 =2.3-2.8 nm were generated. Decreasing the pressure during the reduction reaction led to slightly larger particles whilst decreasing the amount of organometallic precursor in CO2 yielded a decrease in the particle size from x50 =3.2 nm to 2.6 nm and a particle-size distribution of 2.2 nm. Furthermore, substitution of the CH3 end group by the Cn F2n+1 end groups led to a significant drop in Pt loading of about 50 %. Within the series of perfluorinated end groups that were considered, the Pt complex that contained a branched perfluoro-isopropyl group showed the most-interesting results when compared to the control precursor, [Pt(cod)Me2 ] (1).

Enantioselective total synthesis of plakotenin, a cytotoxic metabolite from Plakortis sp

The first total enantioselective synthesis of plakotenin is described. This marine natural product was isolated from an Okinawan sponge of the genus Plakortis and shows potent biological activity against several cancerous cell lines. A biomimetic intramolecular Diels-Alder reaction served as a key step in the total synthesis. The synthesis proves the relative and absolute stereochemistry of natural plakotenin.