Total synthesis of (-)-CP2-disorazole C1

The total synthesis of a bis-cyclopropane analog of the antimitotic natural product (-)-disorazole C(1) was accomplished in 23 steps and 1.1% overall yield. A vinyl cyclopropane cross-metathesis reaction generated a key (E)-alkene segment of the target molecule. IC(50) determinations of (-)-CP(2)-disorazole C(1) in human colon cancer cell lines indicated low nanomolar cytotoxic properties. Accordingly, this synthetic bioisostere represents the first biologically active disorazole analog not containing a conjugated diene or polyene substructure element.     

Development, optimization, and use of an APCI source with temperature-controlled vaporization of solid and liquid samples

A trend is observed in mass spectrometry, in which solid samples without prior dissolution and chromatographic separation are brought directly into the ion source and are ionized, e.g., by corona discharge (Atmospheric Solids Analysis Probe) or plasma (Direct Analysis in Real Time). The Direct Inlet Probe-atmospheric-pressure chemical ionization (APCI) ion source presented here, which was coupled to a high-resolution quadrupole time-of-flight–mass spectrometer, differs from most of the other ion sources in having temperature-programmed heating of the sample. The resulting possibility to reduce ion suppression and ion-molecule reactions in the ion source was shown by the separation of two fatty acid methyl esters as a result of their boiling point difference. Using caffeine as sample, certain source parameters such as the auxiliary gas flow, the drying gas flow, and the position of the probe tip in the ion source were optimized. The ability to perform quantitative analyses was shown by the linear concentration response (R ²?=?0.9984) observed when analyzing different caffeine concentrations. An extract of a Chinese medicinal herb was used to examine the reproducibility (relative standard deviations of the most abundant m/z signals were ≤8.1 %). It was also possible to distinguish milled samples of Radix Angelicae sinensis and Radix Angelicae gigas from each other and to identify the coumarins they contain without sample preparation. Supplying synthetic air instead of nitrogen to the ion source makes APCI in the negative mode possible as well; this was proven by the analysis of n-nonyl-β-d-maltoside.     

High-field Mössbauer spectroscopy on dinuclear iron(II) spin crossover complexes

The dinuclear complex [{Fe(L-N₄Me₂)}₂(BzImCOO)](ClO₄)₂*0.5(CH₃)₂CO has been investigated by Mössbauer spectroscopy carried out in the temperature range from T = 5 K up to T = 190 K with externally applied magnetic fields of up to B = 5 T. By means of a consistent simulation of all experimental data sets within the Spin Hamiltonian formalism, the zero-field splitting and the rhombicity parameter of the ferrous high-spin site could be determined to be D?= 7.2 ± 0.5 cm^???1 and E/D?= 0.1 ± 0.02. The sign of the quadrupole splitting is positive which indicates that the ferrous high-spin site of the dinuclear complex [{Fe(L-N₄Me₂)}₂(BzImCOO)]-(ClO₄)₂*0.5(CH₃)₂CO has an electronic ground state with the d _xy orbital twofold occupied.     

Simultaneous determination of polyamines and acetylpolyamines in human urine by capillary electrophoresis with fluorescence detection

There has been evidence linking elevated polyamines (PAs) and acetylpolamines (AcPAs) level and cancer. So the simultaneous analysis of these compounds has become important task for cancer diagnosis and antitumor drug monitoring. A simple, fast and inexpensive CZE‐LIF method has been developed for the determination of cadaverine (CAD), putrescine (PUT), spermine (SPM), spermidine (SPD), acetylspermine (ASPM), and acetylspermidine (ASPD) in human urine using 4‐chloro‐7‐nitro‐2,1,3‐benzooxadiazole as a fluorescent reagent. Labeling reaction conditions were systematically investigated and were found to be 20 mM borate buffer at pH 7.4, labeling reaction time, and temperature were 10 min and 70°C, respectively. Under these optimized conditions the four PAs, two AcPAs and the internal standard were separated in 6 min. An Exactive‐MS with an ESI source was used for identification of the bis‐derivative of the ASPM. The method was validated in term of linearity, LODs, repeatability, intra‐ and interday assays, recovery, and selectivity. The LODs for CAD, PUT, SPM, SPD, ASPM, and ASPD were found to be 7.6, 10.0, 9.0, 8.8,7.8, and 3.3 nM, respectively. The method was successfully applied for the analysis of PAs and AcPAs in healthy human urine samples.     

Development of a multipurpose ion source for LC-MS and GC-API MS

Over the past decade, multimode ion sources operating at atmospheric pressure (i.e., more than one ionization method is operative in the ion source enclosure) have received considerable interest. Simultaneous operation of different ionization methods targeting different compound classes within one analysis run has several advantages, including enhanced sample throughput and thus significant laboratory cost reductions. Potential drawbacks are enhanced ion suppression and other undesirable effects of the simultaneous operation of ionization methods. In this contribution we present an alternative approach-the development and characterization of a widely applicable, multipurpose ion source operating at atmospheric pressure. The optimized source geometry allows rapid changing from LC-API methods (ESI, APCI, APLI) to GC-API methods (APCI, APLI, DA-APLI) along with the appropriate coupling of chromatographic equipment required. In addition, true multimode operation of the source is demonstrated for LC-ESI/APLI and LC-APCI/APLI.     

Combining chip-ESI with APLI (cESILI) as a multimode source for analysis of complex mixtures with ultrahigh-resolution mass spectrometry

Recently we have established atmospheric-pressure laser ionisation (APLI) as a method for coupling time-of-flight mass spectrometric detectors (TOF MS) with chromatographic systems (HPLC and GC) to allow two-photon ionisation of non-polar aromatic compounds. Here we demonstrate that APLI can be combined with chip-electrospray ionisation (cESI) coupled to Fourier-transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) for ultrahigh-resolution analysis of complex samples. With the laser turned off, the analytes are ionised only by ESI, whereas when the laser is switched on non-polar aromatic substances also are ionised. In combination with the extremely high mass resolution of an FT-ICR MS, simultaneous qualitative analysis of polar and non-polar analytes is possible in both positive and negative modes, as is exemplified with a crude oil sample. Nevertheless, ion suppression was observed (up to ca. 70% for D₁₀-pyrene) and thus sample preparation with chromatographic or electrophoretic pre-separation is necessary for quantitative analysis of targets. In addition, for the first time, the dopant-assisted APLI method in combination with cESI (DA-cESILI) was used for determination of 1-nitrocoronene. Philippe Schmitt-Kopplin, Klaus J. Brockmann and Oliver J. Schmitz contributed equally to the work.     

The distribution of ion acceptance in atmospheric pressure ion sources: Spatially resolved APLI measurements

It is demonstrated that spatially resolved mass selected analysis using atmospheric pressure laser ionization mass spectrometry (APLI MS) represents a new powerful tool for mechanistic studies of ion-molecule chemistry occurring within atmospheric pressure (AP) ion sources as well as for evaluation and optimization of ion source performance. A focused low-energy UV laser beam is positioned computer controlled orthogonally on a two-dimensional grid in the ion source enclosure. Resonance enhanced multiphoton ionization (REMPI) of selected analytes occurs only within the confined volume of the laser beam. Depending on the experimental conditions and the reactivity of the primary photo-generated ions, specific signal patterns become visible after data treatment, as visualized in, e.g., contour or pseudo-color plots. The resulting spatial dependence of sensitivity is defined in this context as the distribution of ion acceptance (DIA) of the source/analyzer combination. This approach provides a much more detailed analysis of the diverse processes occurring in AP ion sources compared with conventional bulk signal response measurements.     

Comprehensive characterization of Annonaceous acetogenins within a complex extract by HPLC‐ESI‐LTQ‐Orbitrap® using post‐column lithium infusion

Annonaceous acetogenins (AAGs) are a homogenous class of polyketides proposed as environmental neurotoxins. Previous dereplication studies of AAGs were limited by the use of low‐resolution mass spectrometers. Only poor information in terms of structures was provided due to the limited fragmentation of protonated or sodium cationized species. An innovative approach, using reversed‐phase high‐performance liquid chromatography coupled to a hybrid linear ion trap / orbitrap mass spectrometer (LTQ‐Orbitrap®), was therefore performed. Sensitivity was enhanced by post‐column infusion of lithium, since AAGs have a high affinity for this cation. High level of structural information was obtained from low‐energy‐collision‐induced dissociation fragmentation experiments of lithium‐cationized AAGs ([M + Li]⁺ ions) as demonstrated with purified standards. The method was then applied to a total ethyl‐acetate extract prepared from commercial soursop nectar (Annona muricata L.). The sensitivity, mass accuracy and specific fragmentation patterns proved to be particularly useful for characterization of the AAGs. Typical structural identification procedure and unexpected observations for specific structural types are illustrated, with major and minor compounds. Copyright © 2012 John Wiley & Sons, Ltd.     

Fluorophores as Optical Sensors for Local Forces

The main aim of this study is to investigate correlations between the impact of an external mechanical force on the molecular framework of fluorophores and the resultant changes in their fluorescence properties. Taking into account previous theoretical studies, we designed a suitable custom‐tailored oligoparaphenylenevinylene derivative (OPV5) with a twisted molecular backbone. Thin foils made of PVC doped with 100 nM OPV were prepared. By applying uniaxial force, the foils were stretched and three major optical effects were observed simultaneously. First, the fluorescence anisotropy increased, which indicates a reorientation of the fluorophores within the matrix. Second, the fluorescence lifetime decreased by approximately 2.5 % (25 ps). Finally, we observed an increase in the emission energy of about 0.2 % (corresponding to a blue‐shift of 1.2 nm). In addition, analogous measurements with Rhodamine 123 as an inert reference dye showed only minor effects, which can be attributed to matrix effects due to refractive index changes. To relate the observed spectroscopic changes to the underlying changes in molecular properties, quantum‐chemical calculations were also performed. Semiempirical methods had to be used because of the size of the OPV5 chromophore. Two conformers of OPV5 (C₂ and C_i symmetry) were considered and both gave very similar results. Both the observed blue‐shift of fluorescence and the reduced lifetime of OPV5 under tensile stress are consistent with the results of the semiempirical calculations. Our study proves the feasibility of fluorescence‐based local force probes for polymers under tension. Improved optical sensors of this type should in principle be able to monitor local mechanical stress in transparent samples down to the single‐molecule level, which harbors promising applications in polymer science and nanotechnology.