Understanding the contrast mechanism in scanning tunneling microscopy (STM) images of an intermixed tetraphenylporphyrin layer on Ag(111)

The appearance of tetraphenylporphyrins in scanning tunneling micrographs depends strongly on the applied bias voltage. Here, we report the observation and identification of certain features in scanning tunneling microscopy (STM) images of intermixed layers of tetraphenylporphyrin (2HTPP) and cobalt-tetraphenylporphyrin (CoTPP) on Ag(111). A significant fraction of an ordered monolayer of commercially available CoTPP appears as "pits" at negative bias voltages around -1 V. The obvious possibility that these pits are missing molecules within the ordered layer could be ruled out by imaging the molecules at reduced bias voltages, at which the contrast of the pits fades, and at positive bias voltages around +1 V, at which the image contrast is inverted. With the investigation of the electronic structure, in particular the density of states (DOS) close to the Fermi level, of CoTPP and 2HTPP layers by means of ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS), the contrast mechanism was clarified. The correlation of the bias dependent contrast with the UPS data enabled us to interpret the "pits" as individual 2HTPP molecules. Additional evidence could be provided by imaging layers of different mixtures of 2HTPP and CoTPP and by high-resolution STM imaging of the features in CoTPP.     

Size Effects at Corners of Anisotropic Material Discontinuities

Reinforcement patches of composite laminates often possess corners due to design and manufacturing necessities. Hence, the patches reconstitute the demanded effective strength or stiffness in the region considered but at their boundaries also constitute a source for stress localizations. The complex potential method is a means for the investigation of such stress localizations. With the help of appropriate complex potentials the mechanical in-plane fields around the reinforcement corner can be expressed as series representations. A first analysis step is to obtain the exponents and corresponding modes which cause singular behavior of the membrane forces. Then, the determination of appropriately defined generalized membrane force intensity factors is used to show whether and how the singularity exponents are in effect. On this basis it is possible to deduce what impact a specific loading condition or the reinforcement corner's material combination and geometry have on the character of the singularity. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Universal state-selective corrections to multi-reference coupled-cluster theories with single and double excitations

The recently proposed universal state-selective (USS) corrections [K. Kowalski, J. Chem. Phys. 134, 194107 (2011)] to approximate multi-reference coupled-cluster (MRCC) energies can be commonly applied to any type of MRCC theory based on the Jeziorski-Monkhorst [B. Jeziorski and H. J. Monkhorst, Phys. Rev. A 24, 1668 (1981)] exponential ansatz. In this paper we report on the performance of a simple USS correction to the Brillouin-Wigner and Mukherjee's MRCC approaches employing single and double excitations (USS-BW-MRCCSD and USS-Mk-MRCCSD). It is shown that the USS-BW-MRCCSD correction, which employs the manifold of single and double excitations, can be related to a posteriori corrections utilized in routine BW-MRCCSD calculations. In several benchmark calculations we compare the USS-BW-MRCCSD and USS-Mk-MRCCSD results with the results obtained with the full configuration interaction method.     

Direct Dynamic Protein-Affinity Selection Mass-Spectrometry

Mobile phase chromatographic conditions for the selective separation of metoprolol from related amino alcohols have been evaluated using Hypercarb as support and carbon dioxide with addition of methanol as mobile phase. The objective for the presented study was to show the unique ability of the porous graphitized carbon surface to separate closely structurally related substances. Experimental results, using Hypercarb and “chromatographic normal phase conditions” (SFC), are presented with focus on how to control retention and to improve peak performance. A high concentration of basic aliphatic amine additive was required in order to elute the amine analytes as symmetrical peaks. N,N-Dimethyloctylamine was preferred over triethylamine since the retention was markedly shorter though the selectivity and resolution were virtually the same. The selectivity can be optimized by altering the temperature of the column. A high selectivity was demonstrated between metoprolol and two homologues with one and two extra methylene groups inserted between the secondary nitrogen atom and the carbon atom with a hydroxyl group attached. The mobile phase flow rate effect the column efficiency only to a minor extent as the steepness of the van Deemter curve for metoprolol was virtually flat in the range studied: 0.5–3.0 mL min^?1.     

Identification of the Biotransformation Products of 2-Ethylhexyl 4-(N,N-Dimethylamino)benzoate

Nowadays, 2-ethylhexyl 4-(N,N-dimethylamino)benzoate (EDP) is one of the most widely used UV filters in sunscreen cosmetics and other cosmetic products. However, undesirable processes such as percutaneous absorption and biological activity have been attributed to this compound. The in vitro metabolism of EDP was elucidated in the present work. First of all, the phase I biotransformation was studied in rat liver microsomes and two metabolites, N,N-dimethyl-p-aminobenzoic acid (DMP) and N-monomethyl-p-aminobenzoic acid (MMP), were identified by GC-MS analysis. Secondly, the phase II metabolism was investigated by means of LC-MS. The investigated reactions were acetylation and glucuronidation working with rat liver cytosol and with both human and rat liver microsomes, respectively. Analogue studies with p-aminobenzoic acid (PABA) were carried out in order to compare the well established metabolic pathway of PABA with the unknown biotransformation of EDP. In addition, a method for the determination of EDP and its two phase I metabolites in human urine was developed. The methodology requires a solid-phase extraction prior to LC-MS analysis. The method is based on standard addition quantification and has been fully validated. The repeatability of the method, expressed as relative standard deviation, was in the range 3.4–7.4% and the limit of detection for all quantified analytes was in the low ng mL^?1 range.     

On-line electrochemistry–bioaffinity screening with parallel HR-LC-MS for the generation and characterization of modified p38α kinase inhibitors

In this study, an integrated approach is developed for the formation, identification and biological characterization of electrochemical conversion products of p38α mitogen-activated protein kinase inhibitors. This work demonstrates the hyphenation of an electrochemical reaction cell with a continuous-flow bioaffinity assay and parallel LC-HR-MS. Competition of the formed products with a tracer (SKF-86002) that shows fluorescence enhancement in the orthosteric binding site of the p38α kinase is the readout for bioaffinity. Parallel HR-MSⁿ experiments provided information on the identity of binders and non-binders. Finally, the data produced with this on-line system were compared to electrochemical conversion products generated off-line. The electrochemical conversion of 1-{6-chloro-5-[(2R,5S)-4-(4-fluorobenzyl)-2,5-dimethylpiperazine-1-carbonyl]-3aH-indol-3-yl}-2-morpholinoethane-1,2-dione resulted in eight products, three of which showed bioaffinity in the continuous-flow p38α bioaffinity assay used. Electrochemical conversion of BIRB796 resulted, amongst others, in the formation of the reactive quinoneimine structure and its corresponding hydroquinone. Both products were detected in the p38α bioaffinity assay, which indicates binding to the p38α kinase.     

On the closure of the feasible set in generalized semi-infinite programming

In generalized semi-infinite programming the feasible set is known to be not closed in general. In this paper, under natural and generic assumptions, the closure of the feasible set is described in explicit terms. Oliver Stein gratefully acknowledges support through a Heisenberg grant of the Deutsche Forschungsgemeinschaft.     

Conformation Controls Mobility: 2H-Tetranaphthylporphyrins on Cu(111)

The adsorption behavior and the mobility of 2H-Tetranaphthylporphyrin (2HTNP) on Cu(111) was investigated by scanning tunneling microscopy (STM) at room temperature (RT). The molecules adsorb, like the structurally related 2HTPP, in the “inverted” structure with the naphthyl plane restricted to an orientation parallel to the Cu surface. The orientation of the four naphthyl groups yields altogether 16 possible conformations. Due to the existence of rotamer pairs, 10 different appearances are expected on the surface, and all of them are identified by STM at RT. Most interestingly, the orientation of the naphthyl groups significantly influences the diffusion behavior of the molecules on Cu(111). We identify three different groups of conformers, which are either immobile, medium or fast diffusing at RT. The mobility seems to decrease with increasing size of the footprint of the conformers on the surface.