Step‐by‐Step Synthesis of the Endohedral Stannaspherene [Ir@Sn12]3− via the Capped Cluster Anion [Sn9Ir(cod)]3−

The endohedral stannaspherene cluster anion [Ir@Sn₁₂]^3? was synthesized in two steps. The reaction of K₄Sn₉ with [IrCl(cod)]₂ (cod: 1,5‐cyclooctadienyl) in ethylenediamine (en) solution first yielded the [K(2,2,2‐crypt)]⁺ salt (2,2,2‐crypt: 4,7,13,16,21,24‐hexaoxa‐1,10‐diazabicyclo[8.8.8]hexacosane) of the capped cluster anion [Sn₉Ir(cod)]^3?. Subsequently, crystals of this compound were dissolved in en, followed by the addition of triphenylphosphine or 1,2‐bis(diphenylphosphino)ethane and treatment at elevated temperatures. [Ir@Sn₁₂]^3? was obtained and characterized as the [K(2,2,2‐crypt)]⁺ salt. The isolation of [Sn₉Ir(cod)]^3? as an intermediate product establishes that the formation of the stannaspherene [Ir@Sn₁₂]^3? occurs through the oxidation of [Sn₉Ir(cod)]^3?. Among the structurally characterized tetrel cluster anions, [Ir@Sn₁₂]^3? is a unique example of a stannaspherene, and one of the rare spherical clusters encapsulating a metal atom that is not a member of Group 10. Single‐crystal structure determination shows that the novel Zintl ion cluster has nearly perfect icosahedral I_h point symmetry.     

Can Copper(II) Mediate Hoogsteen Base‐Pairing in a Left‐Handed DNA Duplex? A Pulse EPR Study

Pulse EPR spectroscopy is used to investigate possible structural features of the copper(II) ion coordinated to poly(dG‐dC)?poly(dG‐dC) in a frozen aqueous solution, and the structural changes of the polynucleotide induced by the presence of the metal ion. Two different copper species were identified and their geometry explained by a molecular model. According to this model, one species is exclusively coordinated to a single guanine with the N7 nitrogen atom forming a coordinative bond with the copper. In the other species, a guanine and a cytosine form a ternary complex together with the copper ion. A copper crosslink between the N7 of guanine and N3 of cytosine is proposed as the most probable coordination site. Moreover, no evidence was found for an interaction of either copper species with a phosphate group or equatorial water molecules. In addition, circular dichroism (CD) spectroscopy showed that the DNA of the Cu^II‐poly(dG‐dC)?poly(dG‐dC) adducts resembles the left‐handed Z‐form. These results suggest that metal‐mediated Hoogsteen base pairing, as previously proposed for a right‐handed DNA duplex, can also occur in a double‐stranded left‐handed DNA.     

Simultaneous quantitative determination of α-ketoglutaric acid and 5-hydroxymethylfurfural in human plasma by gas chromatography–mass spectrometry

α-Ketoglutaric acid (α-KG) and 5-hydroxymethylfurfural (5-HMF) are currently under investigation as promising cancer cell damaging agents. A method for the simultaneous quantitative determination of α-KG and 5-HMF in human plasma was established for screening these compounds in human plasma. Plasma samples were directly treated with O-(2,3,4,5,6-pentafluorobenzyl) hydroxylamine hydrochloride to form the corresponding oximes, thus facilitating subsequent liquid–liquid extraction. After formation of the trimethylsilyl ethers, samples were analyzed by gas chromatography with electron ionization mass spectrometry. Stable isotope labeled standards were used, the preparation of ¹³C₆-5-HMF is described. Limits of quantitation were set to 0.938 μg/mL for α-KG and 0.156 μg/mL for 5-HMF. Inter-day accuracy was ≤93.7% (α-KG) and ≤92.8% (5-HMF). Inter-day precision was ≤6.0% (α-KG) and ≤4.6% (5-HMF). The method has been successfully applied to pharmacokinetic profiling of the compounds after intravenous application.     

Observation of particles manipulated by ultrasound in close proximity to a cone-shaped infrared spectroscopy probe

The presented investigations aimed to enhance surface sensitive infrared spectroscopy for chemical analysis by ultrasonic particle manipulation. The combination of these techniques has the potential for new measurement concepts for use in the chemical analysis of suspensions. Local increases of particle concentration brought about by ultrasound could facilitate measurements of molecular-specific infrared spectra of the suspending phase and particles independently. By changing the frequency of an ultrasonic standing wave around 2 MHz it was possible to control the position of particles in respect to the optically sensitive region of the infrared spectroscope.Results obtained with a set-up that enabled us to explore the application of an ultrasonic standing wave to push suspended particles at or into μm distances of the sensing element of an in-line fiber optic probe and subsequently retract them from there are presented. Light micrographs suggested, that the task was successfully accomplished with polystyrene beads suspended in methanol, aggregates were manipulated to and from the cut surface of the truncated, cone-shaped fibre probe tip by changes of the ultrasonic frequency between 1.85 and 1.87 MHz. Feasibility was confirmed by infrared absorption spectra recorded when PTFE particles suspended in tetrahydrofuran were used.     

Benchmark results in vector atmospheric radiative transfer

In this paper seven vector radiative transfer codes are inter-compared for the case of underlying black surface. They include three techniques based on the discrete ordinate method (DOM), two Monte-Carlo methods, the successive orders scattering method, and a modified doubling-adding technique. It was found that all codes give very similar results. Therefore, we were able to produce benchmark results for the Stokes parameters both for reflected and transmitted light in the cases of molecular, aerosol and cloudy multiply scattering media. It was assumed that the single scattering albedo is equal to one. Benchmark results have been provided by several studies before, including Coulson et al. [22], Garcia and Siewert [7], [8], Wauben and Hovenier [10], and Natraj et al. [11] among others. However, the case of the elongated phase functions such as for a cloud and with a high angular resolution is presented here for the first time. Also in difference with other studies, we make inter-comparisons using several codes for the same input dataset, which enables us to quantify the corresponding errors more accurately.     

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).

     

A Facile System for Encoding Unnatural Amino Acids in Mammalian Cells

A shuttle system has been developed to genetically encode unnatural amino acids in mammalian cells using aminoacyl‐tRNA synthetases (aaRSs) evolved in E. coli. A pyrrolysyl‐tRNA synthetase (PylRS) mutant was evolved in E. coli that selectively aminoacylates a cognate nonsense suppressor tRNA with a photocaged lysine derivative. Transfer of this orthogonal tRNA–aaRS pair into mammalian cells made possible the selective incorporation of this unnatural amino acid into proteins.

     

Redox cycling in nanofluidic channels using interdigitated electrodes

Amperometric detection is ideally suited for integration into micro- and nanofluidic systems as it directly yields an electrical signal and does not necessitate optical components. However, the range of systems to which it can be applied is constrained by the limited sensitivity and specificity of the method. These limitations can be partially alleviated through the use of redox cycling, in which multiple electrodes are employed to repeatedly reduce and oxidize analyte molecules and thereby amplify the detected signal. We have developed an interdigitated electrode device that is encased in a nanofluidic channel to provide a hundred-fold amplification of the amperometric signal from paracetamol. Due to the nanochannel design, the sensor is resistant to interference from molecules undergoing irreversible redox reactions. We demonstrate this selectivity by detecting paracetamol in the presence of excess ascorbic acid. Figure  

Normal forms for real quadratic forms

We investigate the non-diagonal normal forms of a quadratic form on , in particular for n = 3. For this case it is shown that the set of normal forms is the closure of a 5-dimensional submanifold in the 6-dimensional Grassmannian of 2-dimensional subspaces of . Received: 27 June 2008