Very general formation of tetrahydropterin cation radicals during reaction of iron porphyrins with tetrahydropterins: model for the corresponding NO-synthase reaction

Electron transfer from tetrahydropterins to iron porphyrins, with formation of intermediate tetrahydropterin cation radicals, is a very general reaction that was shown to occur not only with tetrahydrobiopterin, as originally found in NO-synthases, but also with another important biological cofactor, tetrahydrofolate, and various iron porphyrins, either in their ferric state, or in the Fe(II)O(2) state, as in the first model of the corresponding NO-synthase reaction described in this paper.     

Structural evolution of hexa-peri-hexabenzocoronene adlayers in heteroepitaxy on n-pentacontane template monolayers

The growth and structure of self-assembled adlayers of hexakis(n-dodecyl)-peri-hexabenzocoronene (HBC-C12) adsorbed on highly ordered pyrolitic graphite (HOPG) decorated by an n-pentacontane (n-C50H102) monolayer have been investigated by scanning tunneling microscopy (STM). Whereas on HOPG the HBC-C12 molecules readily self-assemble into a unique stable 2D structure, on the [n-C50H102 monolayer/graphite] system we observe morphological phase transitions with formation of time dependent alpha, beta, and gamma phases (alpha-->beta-->gamma). The initial alpha-phase is similar to that obtained on bare graphite, while intermediate beta- and final gamma-structures present molecular dimers and rows, respectively. The observed two-dimensional polymorphism is due to weak interaction between HBC-C12 molecules and n-C50H102-modified graphite substrate. Our results constitute an important step toward the control of the growth and structure of highly ordered monolayers of functional conjugated molecules by modifying the graphite surface with an n-alkane monolayer of appropriate chain length.     

Development of a custom high-throughput preparative liquid chromatography/mass spectrometer platform for the preparative purification and analytical analysis of compound libraries

Solution-phase parallel synthesis has had a profound impact on the speed of compound synthesis delivering relatively pure compounds (>80%) in short order. However, to develop structure activity relationships (SAR) for a compound series, each library member should preferably be >95% pure. Historically, achieving and quantifying such high-purity criteria for each library member proved to be the slow step for most lead discovery groups. To address this issue, significant modifications have been made to a commercial Agilent preparative LC/MS system to allow for the general mass-guided purification of diverse compound libraries. The custom modifications include (1) the "DMSO slug" approach for the purification of samples with poor solubility; (2) an active splitter to reduce system back-pressure, reduce the delay volume, and allow for a variable split ratio; (3) a sample loading pump for the quick purification of large, dilute samples; (4) a preparative column-selection valve to quickly change column selectivity or sample loading; and (5) an analytical injector with a separate flow path for crude reaction or fraction analyses.     

Mechanism of the aromatic hydroxylation of thiophene by acid-catalyzed peracid oxidation

The oxidation of thiophene (1) with peracids in a strongly acidic environment yielded thiophen-2-one (4) as the product of an apparent direct hydroxylation of the thiophene aromatic ring together with the anticipated thiophene-S-oxide dimers, 2a,b, as the main products. Formation of the latter dimers can be rationalized in a straightforward manner by initial oxidation at the sulfur atom of thiophene (1) to yield thiophene-S-oxide followed by subsequent dimerization in a Diels-Alder type reaction. Trapping experiments in the presence of a competing dienophile indicated that thiophen-2-one (4) did not originate from the monomeric thiophene-S-oxide but was the product of an independent reaction pathway. The extent of thiophen-2-one (4) formation correlated with the acidity of the reaction medium and was suppressed in the presence of water, the latter presumably acting as a competing base. As evidenced by the use of 2,5-dideuterated thiophene (1-D), its mechanism of formation involved a 1,2-hydride shift, a feature commonly described in the peracid-mediated epoxidation of aromatic hydrocarbons and indicative for the occurrence of cationic intermediates. In agreement with all these observations we propose a mechanism involving initial protonation of thiophene followed by nucleophilic attack of the peracid in position 2 of the thiophene ring. Intramolecular epoxidation may lead to the formation of thiophene 2,3-epoxide as a highly reactive intermediate that then undergoes heterolytic ring opening and a 1,2-hydride shift to yield thiophen-2-one (4) after a final, acid-catalyzed, isomerization of the double bond.     

Preparation and self-assembly of chiral porphyrin diads on the gold electrodes of quartz crystal microbalances: a novel potential approach to the development of enantioselective chemical sensors

Porphyrin diad 1 was synthesized by reaction of the acyl chloride of porphyrin 2 and trans-1,2-dithiane-4,5-diol. The Co complex of this diad was studied as a potential enantioselective receptor for chiral recognition in solution and in the solid state. In solution both enantiomers of limonene induce significant changes in the visible and circular dichroism (CD) spectra of [Co2(1)], while a different behavior is observed in the case of the enantiomeric pair of trans-1,2-diaminocyclohexane. A different efficiency of [Co2(1)] chiral recognition is obtained for these compounds, with a remarkable degree of enantiodiscrimination observed in the case of limonene. Self-assembled monolayers of [Co2(1)] were deposited onto the gold electrodes of quartz crystal microbalances to be used as sensing materials of nanogravimetric sensors operating in the gas phase. The enantiodiscrimination properties of these sensors towards the enantiomeric pairs of chiral analytes have been studied. While in the case of analytes bearing donor ligand atoms we did not observe a remarkable enantioselectivity, a significant degree of chiral discrimination was observed in the case of limonene; this result is particularly encouraging for the potential development of enantioselective chemical sensors for use in an array configuration.     

Application of main component fraction collection method for purification of compound libraries

In order to support high-throughput library purification, a novel UV triggered fraction collection method was developed in which a maximum-seeking-algorithm-driven, six-port valve collects the largest chromatographic peak. This straightforward strategy achieves the one sample-one fraction approach, thus resulting in a simpler and less error prone workup procedure. The effectiveness of this main component fraction collection method will be illustrated here by the results of the purification of compound libraries (altogether 6086 compounds, having an averaged success rate of 79.4%). Advanced applications, where the desired component differs from the main component, will also be discussed.     

Development and method validation for testosterone and epitestosterone in human urine samples by liquid chromatography applications

An isocratic high-performance liquid chromatographic method for the determination of testosterone (T) and epitestosterone (ET) in human urine using liquid-liquid or solid-phase extraction (SPE) is developed and validated. The optimum separation is achieved using a Hypersil C(18) column, water-acetonitrile (57:43, v/v) as the mobile phase and UV-absorbance detection at 245 nm. The recoveries obtained for T and ET in liquid-liquid and SPE demonstrate that these procedures are interchangeable. Quantitation limits for T and ET are 8.6 and 5.4 ng/mL using solvent extraction and 7.3 and 5.7 ng/mL using SPE, respectively. The proposed method is used to evaluate the urinary T, ET, and the T/ET ratio for a healthy male population using liquid-liquid extraction, and the T and ET excretion profile for nine healthy men using SPE.     

Iron-catalyzed cross-coupling reactions. A scalable synthesis of the immunosuppressive agent FTY720

A chemo- and regioselective cross-coupling reaction of the functionalized aryl triflate 5 with octylmagnesium bromide catalyzed by cheap, nontoxic, and environmentally benign Fe(acac)(3) sets the basis for a practical and scaleable synthesis of the octylbenzene derivative 6, which serves as a key building block for the preparation of FTY720 (1). This 2-amino-1,3-propanediol derivative shows highly promising immunosuppressive properties and is currently in human clinical phase III trials.     

Detection of peonidin and pelargonidin glycosides in black carrots (Daucus carota ssp. sativus var. atrorubens Alef.) by high-performance liquid chromatography/electrospray ionization mass spectrometry

Anthocyanins of black carrots (Daucus carota ssp. sativus var. atrorubens Alef.) were investigated by high-performance liquid chromatography/electrospray ionization mass spectrometry. Besides the main compounds which have already been characterized in detail as cyanidin glycosides, six anthocyanins with m/z 757, 565, 595, 903, 933 and 903 were detected and unambiguously characterized as peonidin and pelargonidin glycosides, based on their fragmentation patterns. Peonidin and pelargonidin were identified after acid hydrolysis by comparison of the fragmentation patterns and retention times of the released aglycones with those of standard compounds. To the best of our knowledge this is the first report on anthocyanins in black carrots consisting of aglycones other than cyanidin.     

Comparative studies on the analysis of glycosylation heterogeneity of sialic acid-containing glycoproteins using capillary electrophoresis

Frontal affinity chromatography is a method for quantitative analysis of biomolecular interactions. We reinforced it by incorporating various merits of a contemporary liquid chromatography system. As a model study, the interaction between an immobilized Caenorhabditis elegans galectin (LEC-6) and fluorescently labeled oligosaccharides (pyridylaminated sugars) was analyzed. LEC-6 was coupled to N-hydroxysuccinimide-activated Sepharose 4 Fast Flow (100 μm diameter), and packed into a miniature column (e.g., 10×4.0 mm, 0.126 ml). Twelve pyridylaminated oligosaccharides were applied to the column through a 2-ml sample loop, and their elution patterns were monitored by fluorescence. The volume of the elution front (V) determined graphically for each sample was compared with that obtained in the presence of an excess amount of hapten saccharide, lactose (V₀); and the dissociation constant, K_d, was calculated according to the literature [K. Kasai, Y. Oda, M. Nishikawa, S. Ishii, J. Chromatogr. 376 (1986) 33]. This system also proved to be useful for an inverse confirmation; that is, application of galectins to an immobilized glycan column (in the present case, asialofetuin was immobilized on Sepharose 4 Fast Flow), and the elution profiles were monitored by fluorescence based on tryptophan. The relative affinity of various galectins for asialofetuin could be easily compared in terms of the extent of retardation. The newly constructed system proved to be extremely versatile. It enabled rapid (analysis time 12 min/cycle) and sensitive (20 nM for pyridylaminated derivatives, and 1 μg/ml for protein) analyses of lectin–carbohydrate interactions. It should become a powerful tool for elucidation of biomolecular interactions, in particular for functional analysis of a large number of proteins that should be the essential issues of post-genome projects.