Über die Darstellung und den Schmelzpunkt von Stickstoffpentoxyd

Monatshefte für Chemie - Chemical Monthly -     

Elektrometrie

Ohne Zusammenfassung     

Adsorption and hybridization of oligonucleotides on mercaptoacetic acid-capped CdSe/ZnS quantum dots and quantum dot-oligonucleotide conjugates

Interest in the unique optical properties of quantum dots (QDs) has resulted in the development QD-bioconjugates for imaging and diagnostics. Although these applications are numerous, considerably less is known about the interactions between QDs and biomolecules. In this work, we describe hydrogen-bonding interactions between oligonucleotides and CdSe/ZnS quantum dots capped with mercaptoacetic acid ligands. The strength of the interactions can be modulated by changes in the pH and ionic strength, the addition of formamide, and differences between ssDNA and dsDNA. Fluorescence resonance energy transfer experiments have shown that conjugated oligonucleotides adopt a conformation that lies across the surface of the QD. The hydrogen-bonding interactions also affect the kinetics of hybridization with QD-DNA conjugates and the thermal stability of QD-conjugated dsDNA. The former is analogous to conventional solid-phase hybridization, where stronger oligonucleotide adsorption leads to faster kinetics. With respect to the latter, interactions with the QD surface can sharpen the melt transition and alter the melt temperature of dsDNA. These effects are largely absent when adsorptive interactions are minimized.     

An efficient synthesis of a potent PPARpan agonist

An efficient synthesis of 2-{4-[({4-{[4-(4-methoxyphenyl)piperazin-1-yl]methyl}-2-[4-(trifluoromethyl)phenyl]-1,3-thiazol-5-yl}methyl)thio]phenoxy}-2-methylpropanoic acid (1), a potent PPARpan agonist, is described. The seven-step synthesis, which afforded 1 in 30% overall yield, includes a highly regioselective carbon-sulfur bond formation via coupling of a bishydroxymethylthiazole (3) with 4-hydroxythiophenol, displacement of the remaining alcohol through a three-step telescoped sequence involving an efficient cleavage of an aryl mesylate, and an efficient and practical method of introducing an isobutyric acid fragment.     

The active site of arsenite oxidase from Alcaligenes faecalis

Arsenite oxidase, a member of the DMSO reductase family of molybdenum enzymes, has two molecules of guanosine dinucleotide molybdenum cofactor coordinating the molybdenum at the active site. X-ray absorption spectroscopy indicates that the Mo-S bonds shorten from 2.47 to 2.37 A upon reduction with the physiological substrate. It also indicates the presence of an oxo ligand at 1.70 A in both oxidized and reduced forms of the enzyme, together with a short, 1.83 A, Mo-O bond in the oxidized form that is lost upon reduction. Resonance Raman spectroscopy indicates that the two pterin dithiolene moieties have different aromaticities, with one, the Q-pterin, having a more discrete dithiolate structure while the other, the P-pterin, has considerable pi-delocalization. Our results indicate that the structure of arsenite oxidase is intermediate between that seen in other molybdenum enzymes, in which one ligand to the metal is provided by the polypeptide (serine, cysteine, or selenocysteine), and tungsten enzymes that lack a peptide ligand.     

Real versus artifactual symmetry-breaking effects in Hartree-Fock, density-functional, and coupled-cluster methods

We have examined the relative abilities of Hartree-Fock, density-functional theory (DFT), and coupled-cluster theory in describing second-order (pseudo) Jahn-Teller (SOJT) effects, perhaps the most commonly encountered form of symmetry breaking in polyatomic molecules. As test cases, we have considered two prototypical systems: the 2Sigmau+ states of D( infinity h) BNB and C3+ for which interaction with a low-lying 2Sigmag+ excited state leads to symmetry breaking of the nuclear framework. We find that the Hartree-Fock and B3LYP methods correctly reproduce the pole structure of quadratic force constants expected from exact SOJT theory, but that both methods appear to underestimate the strength of the coupling between the electronic states. Although the Tamm-Dancoff (CIS) approximation gives excitation energies with no relationship to the SOJT interaction, the random-phase-approximation (RPA) approach to Hartree-Fock and time-dependent DFT excitation energies predicts state crossings coinciding nearly perfectly with the positions of the force constant poles. On the other hand, the RPA excited-state energies exhibit unphysical curvature near their crossings with the ground (reference) state, a problem arising directly from the mathematical structure of the RPA equations. Coupled-cluster methods appear to accurately predict the strength of the SOJT interactions between the 2Sigmau+ and 2Sigmag+ states, assuming that the inclusion of full triple excitations provides a suitable approximation to the exact wave function, and are the only methods examined here which predict symmetry breaking in BNB. However, coupled-cluster methods are plagued by artifactual force constant poles arising from the response of the underlying reference molecular orbitals to the geometric perturbation. Furthermore, the structure of the "true" SOJT force constant poles predicted by coupled-cluster methods, although correctly positioned, has the wrong structure.     

Limited sample recovery in coupled methods of high-performance liquid chromatography of synthetic polymers

Complex polymer systems, which exhibit multiple distributions in their molecular parameters can be characterized by coupled liquid chromatographic methods. The latter combine entropic (exclusion) and enthalpic (interaction) retention mechanisms. However, recent experimental results suggest that some coupled liquid chromatographic methods may suffer from incomplete sample recovery. This refers, for example, to liquid chromatography under critical conditions of enthalpic interactions and to eluent gradient liquid chromatography. Sample recovery in both latter methods was investigated for selected model systems applying adsorption retention mechanism. Reduced sample recovery was confirmed for both methods. It was revealed that even very high final strength of mobile phase may be insufficient for complete elution of polymer samples in eluent gradient polymer liquid chromatography.     

Franck--Condon factors and r-Centroids for the A 1∑+u-X1∑+g Band System of 107,109Ag2: Comparison of the Observed and Calculated Absorption Band Strengths

Franck-Condon factors and r-centrolds for the, A^1 ∑^＋ u-X^1∑^＋ 9 band system of ^107,109Ag2 are computed using Morse and Rydberg-Klein-Rees potentials for both lower and upper electronic states. The differences between the two sets of results are typically in the third decimal place for transitions involving vibrational levels with ν＇ and ν＂ up to about 15. Somewhat larger deviations appear for higher vibrational levels, but both sets of results follow the same pattern, which is to match well with the relative absorption band strength distribution in our experimental spectrum. The relative absorption band strengths are calculated by assuming that the electronic transition moment has only a weak dependence on the internuclear distance r. Good agreement between our measured and calculated absorption band strength ratios is found, which provides an excellent test of the calculated Franck- Condon factors and relative absorption band strengths. The r-centrold value for the （ν＇ = 0, ν＂ = 0） band is found to be approximately equal to the average value of r＇ and r＇＇e , indicating that the potentials of both states are not significantly aaharmonic around their minimum regions.