A Ditopic Ion‐Pair Receptor Based on Stacked Nucleobase Quartets

Pass the salt, please! State‐of‐the‐art computations indicate that the stacking complex of a guanine quartet and an adenine quartet (G₄A₄) can function as a potent ditopic receptor for NaCl in aqueous solution (see picture; Na⁺, Cl^? yellow, O red, N blue, C black, H white).

     

Self-assembly of semifluorinated dendrons attached to electron-donor groups mediates their pi-stacking via a helical pyramidal column

Semifluorinated first-generation self-assembling dendrons attached via a flexible spacer to electron-donor molecules induce pi-stacking of the donors in the center of a supramolecular helical pyramidal column. These helical pyramidal columns self-organize in various columnar liquid crystal phases that mediate self-processing of large single crystal liquid crystal domains of columns and self-repair their intracolumnar structural defects. In addition, all supramolecular columns exhibit a columnar phase at lower temperatures that maintains the helical pyramidal columnar supramolecular structure and displays higher intracolumnar order than that in the liquid crystals phases. The results described here demonstrate the universality of this concept, the power of the fluorous phase or the fluorophobic effect in self-assembly and the unexpected generality of pyramidal liquid crystals.     

Supramolecular switches based on the guanine-cytosine (GC) Watson-Crick pair: effect of neutral and ionic substituents

We have theoretically analyzed Watson-Crick guanine-cytosine (GC) base pairs in which purine-C8 and/or pyrimidine-C6 positions carry a substituent X = NH(-), NH(2), NH(3) (+) (N series), O(-), OH, or OH(2) (+) (O series), using the generalized gradient approximation (GGA) of density functional theory at the BP86/TZ2P level. The purpose is to study the effects on structure and hydrogen-bond strength if X= H is substituted by an anionic, neutral, or cationic substituent. We found that replacing X = H by a neutral substituent has relatively small effects. Introducing a charged substituent, on the other hand, led to substantial and characteristic changes in hydrogen-bond lengths, strengths, and hydrogen-bonding mechanism. In general, introducing an anionic substituent reduces the hydrogen-bond-donating and increases the hydrogen-bond-accepting capabilities of a DNA base, and vice versa for a cationic substituent. Thus, along both the N and O series of substituents, the geometric shape and bond strength of our DNA base pair can be chemically switched between three states, thus yielding a chemically controlled supramolecular switch. Interestingly, the orbital-interaction component in some of these hydrogen bonds was found to contribute to more than 49 % of the attractive interactions and is thus virtually equal in magnitude to the electrostatic component, which provides the other (somewhat less than) 51 % of the attraction.     

Ultra-performance liquid chromatography-tandem mass spectrometry for the analysis of heterocyclic amines in food

A new ultra-performance liquid chromatography-electrospray tandem mass spectrometry (UPLC-ESI-MS/MS) method was developed to perform the determination of 16 mutagenic heterocyclic amines (HAs) in complex food samples in less than 2 min. The UPLC separation was carried out using an Acquity BEH C18 column (50 mm x 2.1 mm i.d., 1.7 microm particle size) that provided high efficiency and resolution in combination with high linear velocities. The UPLC system was coupled to the triple quadrupole Waters Micromass Quattro Premier. This system permits high-speed data acquisition without peak intensity degradation which is required to monitor the narrow chromatographic peaks (1-2 s) of HAs. The determination was performed in selected reaction monitoring (SRM) mode. The quality parameters of the developed method were established, obtaining instrumental LODs lower than 0.23 pg injected and a repeatability at low concentration level lower than 9.1% CV (n = 6). To evaluate the performance of the method in high throughput analysis of complex samples, the UPLC-MS/MS method was applied to the analysis of HAs in two meat extracts.     

Synthesis, characterization, cytotoxic activity and crystal structures of tri- and di-organotin(IV) complexes constructed from the β-{[(E)-1-(2-hydroxyaryl)alkylidene]amino}propionate and β-{[(2Z)-(3-hydroxy-1-methyl-2-butenylidene)]amino}propionate skeletons

Reactions of potassium β-{[(E)-1-(2-hydroxyaryl)alkylidene]amino}propionates (L¹HK-L³HK) and potassium β-{[(2Z)-(3-hydroxy-1-methyl-2- butenylidene)]amino}propionate (L⁴HK) with R₃SnCl (R = Ph and ⁿBu) and ⁿBu₂SnCl₂ yielded complexes of composition Ph₃SnL¹H (1), Ph₃SnL²H (2), Ph₃SnL⁴H (3), ⁿBu₃SnL¹H (4), and {[ⁿBu₂Sn(L²H)]₂O}₂ (5) and {[ⁿBu₂Sn(L³H)]₂O}₂ (6), respectively. These complexes have been characterized by ¹H, ¹³C, ¹¹⁹Sn NMR, ESI-MS, IR and ^119mSn Mössbauer spectroscopic techniques in combination with elemental analyses. The crystal structures of 1, 4, 5 and 6 were determined. In the solid state, compound 1 is a one-dimensional polymer built from SnPh₃ moieties bridged by single carboxylate ligands, but two alternating modes of bridging are present along the polymeric chain. Compound 4 is also a one-dimensional polymer built from SnBu₃ moieties bridged by the two carboxylate O-atoms of a single ligand, but only one mode of bridging is present. Di-n-butyltin compounds 5 and 6 are centrosymmetric tetranuclear bis(dicarboxylatotetrabutyldistannoxane) complexes containing a planar Sn₄O₂ core in which two μ₃-oxo O-atoms connect an Sn₂O₂ ring to two exocyclic Sn-atoms. The four carboxylate ligands display two different modes of coordination where both modes involve bridging of two Sn-atoms. The solution structures were predicted by ¹¹⁹Sn NMR spectroscopy. The in vitro cytotoxic activity of compound 5 against WIDR, M19 MEL, A498, IGROV, H226, MCF7 and EVSA-T human tumor cell lines is reported.     

Tetrakis(μ2‐4‐aminobenzoato)di‐μ3‐oxido‐tetrakis[dibutyltin(IV)]

The molecule of the title compound, [Sn₄(C₄H₉)₈(C₇H₆NO₂)₄O₂], lies about an inversion centre and is a tetranuclear bis(tetrabutyldicarboxylatodistannoxane) complex containing a planar Sn₄O₂ core in which two μ₃‐oxide O atoms connect an Sn₂O₂ ring to two exocyclic Sn atoms. Each Sn atom has a highly distorted octahedral coordination. In the molecule, the carboxylate groups of two aminobenzoate ligands bridge the central and exocyclic Sn atoms, while two further aminobenzoate ligands have highly asymmetric bidentate chelation to the exocyclic Sn atoms plus long O...Sn interactions with the central Sn atoms. Each Sn atom is also coordinated by two pendant n‐butyl ligands, which extend roughly perpendicular to the plane of the Sn₄O₁₀ core. Only one of the four unique hydrogen‐bond donor sites is involved in a classic N—H...O hydrogen bond, and the resulting supramolecular hydrogen‐bonded structure is an extended two‐dimensional network which lies parallel to the (100) plane and consists of a checkerboard pattern of four‐connected molecular cores acting as nodes. The amine groups not involved in the hydrogen‐bonding interactions have significant N—H...π interactions with neighbouring aminobenzene rings.     

A microsystem for sensing and patterning oxidative microgradients during cell culture

We present the design, modeling, fabrication and testing of a microsystem for the electrolytic patterning and sensing of oxidative microgradients within 1 x 1 mm2 area during cell culture. The system employs an array of microfabricated electrodes (3-40 microm in width) embedded in gas-permeable microchannels to generate precise doses of dissolved oxygen (ranging from 10 fmol O2 mm(-2) s(-1) to 100 nmol O2 mm(-2) s(-1)) via electrolysis. The microgradients generated by different microelectrodes in the array can be superimposed to pattern multi-dimensional oxygen profiles not possible with other methods. We demonstrate the patterning, sensing and quantification of dissolved oxygen microgradients in the 0 to 40% dO2 range using this microsystem. Reactive oxygen species generation and dosing is also quantified. Lastly, we demonstrate how the microtechnology enables new types of experiments in three different cell culture models: localized hyperoxia-induced apoptosis in C2C12 myoblasts, dynamic aerotaxis assays of Bacillus subtilis, and studies of calcium release in an ischemia/re-oxygenation myoblast model.     

Characterization of the tryptophan residues of human placental ribonuclease inhibitor and its complex with bovine pancreatic ribonuclease A by steady-state and time-resolved emission spectroscopy

Human placental ribonuclease inhibitor (hRI) containing six tryptophan (Trp) residues located at positions 19, 261, 263, 318, 375, and 438 and its complex with RNase A have been studied using steady-state and time-resolved fluorescence (298 K) as well as low-temperature phosphorescence (77 K). Two Trp residues in wild-type hRI and also in the protein-protein complex with RNase A are resolved optically. The accessible surface area values of Trp residues in the wild-type hRI and its complex and consideration of inter-Trp energy transfer in the wild-type hRI reveal that one of the Trp residues is Trp19, which is located in a hydrophobic buried region. The other Trp residue is tentatively assigned as Trp375 based on experimental results on wild-type hRI and its complex. This residue in the wild-type hRI is more or less solvent exposed. Both the Trp residues are perturbed slightly on complex formation. Trp19 moves slightly toward a more hydrophobic region, and the environment of Trp375 becomes less solvent exposed. The complex formation also results in a more heterogeneous environment for both the optically resolved Trp residues.     

Ti(III)-mediated radical cyclization of β-aminoacrylate containing epoxy alcohol moieties: synthesis of highly substituted azacycles

Ti(III)-mediated radical cyclization of β-aminoacrylate containing 2,3-epoxy alcohol moieties led to the formation of highly substituted piperidine and pyrrolidine rings. The pyrrolidine ring system was then transformed into an indolizidine framework present in many natural products.