A Pd4Br4 macrocycle trapped by cocrystallization from a highly dynamic equilibrium of η3‐cycloheptatrienide complexes

In the coordination chemistry of palladium, dimers bridged via halides are a common motif. Higher oligomers, however, are still rare. We report the structure of an alternating eight‐membered [Pd₄Br₄]⁴⁻ ring framed by cycloheptatrienide ligands, which was obtained by cocrystallization of dimers and tetramers of the complex salt bromido{η³‐[3‐(2,6‐diisopropylphenyl)imidazolium‐1‐yl]cycloheptatrienido}palladium(II) tetrafluoroborate, namely bis[di‐μ‐bromido‐bis({η³‐[3‐(2,6‐diisopropylphenyl)imidazolium‐1‐yl]cycloheptatrienido}palladium(II))] cyclo‐tetra‐μ‐bromido‐tetrakis({η³‐[3‐(2,6‐diisopropylphenyl)imidazolium‐1‐yl]cycloheptatrienido}palladium(II)) octakis(tetrafluoroborate) dichloromethane octasolvate, [Pd₄Br₄(C₂₂H₂₆N₂)₄][Pd₂Br₂(C₂₂H₂₆N₂)₂]₂(BF₄)₈·8CH₂Cl₂. These dimers and tetramers form a highly dynamic equilibrium in solution which was studied by low‐temperature NMR spectroscopy. In the light of the presented results, tetrameric Pd^II species can be assumed to co‐exist as a second species in many cases where by current knowledge only a dimeric compound would be expected.     

Stabilization of the Triplet Biradical Intermediate of 5-Methylcytosine Enhances Cyclobutane Pyrimidine Dimer (CPD) Formation in DNA

Cyclobutane pyrimidine dimer (CPD) is a photoproduct formed by two stacked pyrimidine bases through a cycloaddition reaction upon irradiation. Owing to its close association with skin cancer, the mechanism of CPD formation has been studied thoroughly. Among many aspects of CPD, its formation involving 5-methylcytosine (5mC) has been of special interest because the CPD yield is known to increase with C5-methylation of cytosine. In this work, high-level quantum mechanics/molecular mechanics (QM/MM) calculations are used to examine a previously experimentally detected pathway for CPD formation in hetero (thymine-cytosine and thymine-5mC) dipyrimidines, which is facilitated through intersystem crossing in thymine and formation of a triplet biradical intermediate. A DNA duplex model system containing a core sequence TmCG or TCG is used. The stabilization of a radical center in the biradical intermediate by the methyl group of 5mC can lead to increased CPD yield in TmCG compared with its non-methylated counterpart, TCG, thereby suggesting the existence of a new pathway of CPD formation enhanced by 5mC.     

6,6′-Thiobis(methylene)-β-cyclodextrin dimer as a dimeric host and reusable promoter for synthesis of chromenones in water

Herein, two unit of β-cyclodextrin linked by thiomethylene bridge was synthesized and employed as a novel efficient supramolecular host for the synthesis of biologically active chromenones templates via three-component single-pot reaction. A possible reaction mechanism through molecular complexation is suggested based on 2D ROESY NMR spectroscopic analysis. Moreover, the catalyst could be easily recycled, while a 94% yield and 89% rate of catalyst recovery could be achieved after four cycles of catalyst recycling. Environmentally benign reaction conditions, excellent yields, and avoidance of organic solvent and conventional isolation as well as purification are the noteworthy credits of this developed protocol.     

Stepwise Self-assembly of a Tripeptide from Molecular Dimers to Supramolecular β-sheets in Crystals and Amyloid-like Fibrils in the Solid State

The terminally protected tripeptide Boc–Ala(1)–Leu(2)–Ala(3)–OMe 1 forms antiparallel hydrogen-bonded dimers of two different conformers in the asymmetric unit and the individual dimers then self-associate to form supramolecular β-sheet structures in crystals and amyloid-like fibrils in the solid state.     

Molecule-binding dependent assembly of split aptamer and γ-cyclodextrin: A sensitive excimer signaling approach for aptamer biosensors

A highly sensitive and selective fluorescence aptamer biosensors for the determination of adenosine triphosphate (ATP) was developed. Binding of a target with splitting aptamers labeled with pyrene molecules form stable pyrene dimer in the γ-cyclodextrin (γ-CD) cavity, yielding a strong excimer emission. We have found that inclusion of pyrene dimer in γ-cyclodextrin cavity not only exhibits additive increases in quantum yield and emission lifetime of the excimer, but also facilitates target-induced fusion of the splitting aptamers to form the aptamer/target complex. As proof-of-principle, the approach was applied to fluorescence detection of adenosine triphosphate. With an anti-ATP aptamer, the approach exhibits excimer fluorescence response toward ATP with a maximum signal-to-background ratio of 32.1 and remarkably low detection limit of 80 nM ATP in buffer solution. Moreover, due to the additive fluorescence lifetime of excimer induced by γ-cyclodextrin, time-resolved measurements could be conveniently used to detect as low as 0.5 μM ATP in blood serum quantitatively.     

A cytotoxic dimeric furanoheliangolide from Piptocoma rufescens

A new sesquiterpene lactone, rufescenolide C (1), the first furanoheliangolide dimer, was isolated from the leaves of Piptocoma rufescens, collected in the Dominican Republic. Its structure was determined by interpretation of its spectroscopic data, with the absolute configuration being established by analysis of the CD spectrum. A plausible biogenesis of this dimer is proposed. This compound showed potent cytotoxicity with an IC₅₀ value of 150 nM, when tested against HT-29 human colon cancer cells.