Enhanced Solid-State Fluorescence of Flavin Derivatives by Incorporation in the Metal-Organic Frameworks MIL-53(Al) and MOF-5.

The flavin derivatives 10-methyl-isoalloxazine (MIA) and 6-fluoro-10-methyl-isoalloxazine (6F-MIA) were incorporated in two alternative metal-organic frameworks, (MOFs) MIL-53(Al) and MOF-5. We used a post-synthetic, diffusion-based incorporation into microcrystalline MIL-53 powders with one-dimensional (1D) pores and an in-situ approach during the synthesis of MOF-5 with its 3D channel network. The maximum amount of flavin dye incorporation is 3.9 wt% for MIA@MIL-53(Al) and 1.5 wt% for 6F-MIA@MIL-53(Al), 0.85 wt% for MIA@MOF-5 and 5.2 wt% for 6F-MIA@MOF-5. For the high incorporation yields the probability to have more than one dye molecule in a pore volume is significant. As compared to the flavins in solution, the fluorescence spectrum of these flavin@MOF composites is broadened at the bathocromic side especially for MIA. Time-resolved spectroscopy showed that multi-exponential fluorescence lifetimes were needed to describe the decays. The fluorescence-weighted lifetime of flavin@MOF of 4 ± 1 ns also corresponds to those in solution but is significantly prolonged compared to the solid flavin dyes with less than 1 ns, thereby confirming the concept of "solid solutions" for dye@MOF composites. The fluorescence quantum yield (ΦF) of the flavin@MOF composites is about half of the solution but is significantly higher compared to the solid flavin dyes. Both the fluorescence lifetime and quantum yield of flavin@MOF decrease with the flavin loading in MIL-53 due to the formation of various J-aggregates. Theoretical calculations using plane-wave and QM/MM methods are in good correspondence with the experimental results and explain the electronic structures as well as the photophysical properties of crystalline MIA and the flavin@MOF composites. In the solid flavins, π-stacking interactions of the molecules lead to a charge transfer state with low oscillator strength resulting in aggregation-caused quenching (ACQ) with low lifetimes and quantum yields. In the MOF pores, single flavin molecules represent a major population and the computed MIA@MOF structures do not find π-stacking interactions with the pore walls but only weak van-der-Waals contacts which reasons the enhanced fluorescence lifetime and quantum yield of the flavins in the composites compared to their neat solid state. To analyze the orientation of flavins in MOFs, we measured fluorescence anisotropy images of single flavin@MOF-5 crystals and a static ensemble flavin@MIL53 microcrystals, respectively. Based on image information, anisotropy distributions and overall curve of the time-resolved anisotropy curves combined with theoretical calculations, we can prove that all fluorescent flavins species have a defined and rather homogeneous orientation in the MOF framework. In MIL-53, the transition dipole moments of flavins are orientated along the 1D channel axis, whereas in MOF-5 we resolved an average orientation that is tilted with respect to the cubic crystal lattice. Notably, the more hydrophobic 6F-MIA exhibits a higher degree order than MIA. The flexible MOF MIL-53(Al) was optimized essentially to the experimental large-pore form in the guest-free state with QuantumEspresso (QE) and with MIA molecules in the pores the structure contracted to close to the experimental narrow-pore form which was also confirmed by PXRD. In summary, the incorporation of flavins in MOFs yields solid-state materials with enhanced rigidity, stabilized conformation, defined orientation and reduced aggregations of the flavins, leading to increased fluorescence lifetime and quantum yield as controllable photo-luminescent and photo-physical properties.

Synthesis and Characterization of Bis[(R or S)-N-1-(X-C6H4)ethyl-2-oxo-1-naphthaldiminato-κ2N,O]-Λ/Δ-cobalt(II) (X = H, p-CH3O, p-Br) with Symmetry- and Distance-Dependent Vibrational Circular Dichroism Enhancement and Sign Inversion.

The enantiopure Schiff bases (R or S)-N-1-(X-C6H4)ethyl-2-hydroxy-1-naphthaldimine {X = H [(R or S)-HL1], p-CH3O [(R or S)-HL2], and p-Br [(R- or S)-HL3]} react with cobalt(II) acetate to give bis[(R or S)-N-1-(X-C6H4)ethyl-2-oxo-1-naphthaldiminato-κ2N,O]-Λ/Δ-cobalt(II) {X = H [Λ/Δ-Co-(R or S)-L1], p-CH3O [Λ/Δ-Co-(R or S)-L2], and p-Br [Λ/Δ-Co-(R or S)-L3]} (1-3), respectively. Induced Λ and Δ chirality originates at the metal center of the C2-symmetric molecule in pseudotetrahedral geometry. Differential scanning calorimetry analyses explored the thermal stability of the complexes, which undergo reversible phase transformation from crystalline solid to isotropic liquid phase for 1 and 3 but irreversible phase transformation for 2. Like other cobalt(II) complexes, compounds 1-3 exhibit a continuous ensemble of absorption and circular dichroism bands, which span from the UV to IR region and can be collected into a superspectrum. Infrared vibrational circular dichroism (IR-VCD) spectra witness the coupling between Co2+-centered low-lying electronic states and ligand-centered vibrations. The coupling produces enhanced and almost monosignate VCD spectra, with both effects being mode-dependent in terms of the A or B symmetry (in the C2 point group) and distance from the Co2+ core.

Increase of network hydrophilicity from sql to lvt supramolecular isomers of Cu-MOFs with the bifunctional 4-(3,5-dimethyl-1H-pyrazol-4-yl)benzoate linker.

The bifunctional linker 4-(3,5-dimethyl-1H-pyrazol-4-yl)benzoic acid (H2mpba) was used for the synthesis of new (square lattice) sql 2D metal-organic frameworks (MOFs) [Cu(Hmpba)2]·L (L = DMF or ACN) in a solvent-mixture of dimethylformamide/water and acetonitrile/water. These sql 2D MOFs are supramolecular isomers of the lvt 3D network [Cu(Hmpba)2]·4MeOH·1H2O (lvt-MeOH) that was synthesized previously by Richardson and co-workers. All these frameworks are potentially porous structures with solvent molecules included in the channels of the as synthesized materials. After activation all three materials showed good CO2 adsorption capacity, demonstrated here for lvt-MeOH for the first time, with a saturation uptake of 113 cm3 g-1 (lvt-MeOH-act.), 111 cm3 g-1 (sql-DMF-act.) and 90 cm3 g-1 (sql-ACN-act.) at 195 K. The flexibility of the lvt-MeOH-act. network is evidenced by a gate-opening effect seen in the CO2 measurement at 195 K and under gravimetric high-pressure CO2 adsorption. According to the water and ethanol sorption measurements the new sql frameworks can be categorized as hydrophobic materials in contrast to the hydrophilic lvt framework. In the lvt-MeOH structure the crystal solvent can be replaced with water to yield the structurally authenticated water-only network lvt-H2O containing 3D arrays of S4-symmetric (H2O)20 clusters.

New eremophilane-type sesquiterpenes and maleimide-bearing compounds from Carpesium abrotanoides L.

Two new eremophilane-type sesquiterpenes, carperemophilanes A and B (1-2), three new maleimide-bearing compounds, carpesiumaleimides A-C (3-5), along with a known sesquiterpene, carabrol (6), were isolated from the ethanol extract of Carpesium abrotanoides L. Their structures were elucidated by analysis of their NMR and MS data as well as by comparison with the literature. The absolute configuration of carperemophilane A (1) was determined by single-crystal X-ray diffraction analysis. All isolated compounds (1-6) were evaluated in vitro for cytotoxicity against two human cancer cell lines MDA-MB-231 and HGC-27 using the MTT method. Compounds 1, 2 and 6 showed cytotoxic activities with IC50 values ranging from 7.45 to 37.35 µM.

Azaphilone pigments and macrodiolides from the coprophilous fungus Coniella fragariae.

Two azaphilone pigments (1 and 2), two dihydrobenzofurans (3 and 4), two macrodiolides (5 and 6), and a dimeric alkyl aromatic constituent (7) were isolated from the goose dung-derived fungus Coniella fragariae. Compounds 1-3 proved to be new natural products. Coniellins H and I (1 and 2) feature a tetracyclic core and an aldehyde group at C-5, which is unusual for azaphilone derivatives. The X-ray structure of pyrenophorin (5) is reported for the first time. Pyrenophorin (5) showed strong cytotoxicity against several cancer cell lines with IC50 values ranging from 0.07 to 7.8 µM.

Metal-Organic Gels Based on a Bisamide Tetracarboxyl Ligand for Carbon Dioxide, Sulfur Dioxide, and Selective Dye Uptake.

A metal-organic gel (metallogel) based on the new tetracarboxyl ligand N1, N4-(diterephthalic acid)terephthalamide in combination with chromium(III) has been converted into its xero- and aerogel and demonstrated to have excellent specific sorption properties for dyes in its metallogel state, where fuchsine is adsorbed faster than the two other dyes, calcein and disulfine blue, and for water, sulfur dioxide and carbon dioxide in its xero- and aerogel state. The metallogel showed very good shape retention and could be extruded from molds in designed shapes. In a rheology experiment, the storage modulus was determined to be 1440 Pa, and the metallogel is elastic up to 3 Hz, breaking at strains higher than 0.3%. Additional metallogels utilizing the same ligand with a wide range of metal ions (Al(III), Fe(III), Co(III), In(III), and Hg(II)) have also been synthesized, and the aluminum and mixed aluminum-chromium derivative were also converted into its aerogel. The highly porous Cr, Al, and AlCr metal-organic aerogels proved stable against water vapor in a physisorption experiment and were used to model breakthrough curves for SO2/CO2 gas mixtures with the idealized adsorbed solution theory from their physisorption isotherms. The breakthrough simulation utilized SO2/CO2 equivalencies from a real world application and showed effective retention of SO2 from the gas mixture. Furthermore, the materials in this work exhibit the highest SO2 uptake values for metal-organic aerogels so far (up to 116.8 cm3 g-1, or 23.4 wt %).

Metabolites from the endophytic fungus Cylindrocarpon sp. isolated from tropical plant Sapium ellipticum.

Three new polyketides, cylindrocarpones A-C (1-3), two new pyridone alkaloids, cylindrocarpyridones A-B (5-6), a new pyrone cylindropyrone (7), together with seven know compounds were isolated from the endophytic fungus, Cylindrocarpon sp., obtained from the tropical plant Sapium ellipticum. The structures of the new compounds were elucidated by extensive analysis of their spectroscopic data (1D and 2D NMR, HRESIMS). The absolute configuration of 19-O-methyl-pyrrocidine B (13) was confirmed by X-ray analysis. All isolated compounds were screened for their cytotoxic and antibacterial activities. Pyrrocidine A (12) exhibited potent cytotoxicity against the human ovarian cancer cell line A2780 with an IC50 value of 1.7 µM. 19-O-Methyl-pyrrocidine B (13) showed moderate antibacterial activity against S. aureus ATCC25923 and ATCC700699 with MIC values of 50 and 25 µM, respectively.

Substituted l-tryptophan-l-phenyllactic acid conjugates produced by an endophytic fungus Aspergillus aculeatus using an OSMAC approach.

The endophytic fungus Aspergillus aculeatus isolated from leaves of the papaya plant Carica papaya was fermented on solid rice medium, yielding a new l-tryptophan-l-phenyllactic acid conjugate (1) and thirteen known compounds (11, 14-25). In addition, an OSMAC approach was employed by adding eight different sodium or ammonium salts to the rice medium. Addition of 3.5% NaNO3 caused a significant change of the metabolite pattern of the fungus as indicated by HPLC analysis. Subsequent isolation yielded several new substituted l-tryptophan-l-phenyllactic acid conjugates (1-10) in addition to three known compounds (11-13), among which compounds 2-10, 12-13 were not detected in the rice control culture. All structures were unambiguously elucidated by one and two dimensional NMR spectroscopy and by mass spectrometry. The absolute configuration of the new compounds was determined by Marfey's reaction and X-ray single crystal diffraction. Compounds 19-22 showed cytotoxicity against the L5178Y mouse lymphoma cell line with IC50 values of 3.4, 1.4, 7.3 and 23.7 µM, respectively.