Chemo-, regio-, and stereoselective cyclizations of 1,3-bis(trimethylsilyloxy)-1,3-butadienes with alpha-chloroacetic acid chlorides and alpha-chloroacetic acetals

Treatment of 1,3-bis(trimethylsilyloxy)-1,3-butadienes with alpha-chlorocarboxylic acid chlorides resulted in chemo- and regioselective formation of 6-chloro3,5-dioxo esters, which were regioselectively converted into functionalised 3(2H)furanones. Chemo- and regioselective condensation of 1,3-bis(trimethylsilyloxy)-1,3-butadienes with alpha-chloroacetic dimethyl acetal afforded 6-chloro-5-methoxy-3-oxo esters, which could be regio- and stereoselectively transformed into 2-alkylidene-4-methoxytetrahydrofurans.     

Pflanzenchemie

Analytical and Bioanalytical Chemistry -     

Interpreting DNA vibrational circular dichroism spectra using a coupling model from two-dimensional infrared spectroscopy

Two-dimensional infrared spectroscopy was recently used to measure the vibrational couplings between carbonyl bonds located on DNA nucleobases (Krummel, A. T.; Mukherjee, P.; Zanni, M. T. J. Phys. Chem. B 2003, 107, 9165 and Krummel, A. T.; Zanni, M. T. J. Phys. Chem. B 2006, 110, 13991). Here, we extend the coupling model derived from these 2D IR experiments to simulate the vibrational absorption and vibrational circular dichroism (VCD) spectra of three double-stranded DNA oligomers: poly(dG)-poly(dC), poly(dG-dC), and dGGCC. Using this model, we determine that the VCD spectrum of A-form poly(dG)-poly(dC) is dominated by interactions between stacked bases, whereas the coupling between base pairs and stacked bases carries equal importance in the VCD spectrum of B-form poly(dG-dC). We also simulate the absorption and VCD spectra of dGGCC, which is a combination of A- and B-form configurations. These simulations give insight into the structural interpretation of VCD and absorption spectroscopies that have long been used to monitor DNA secondary structure and kinetics.     

A pulse sequence for directly measuring the anharmonicities of coupled vibrations: Two-quantum two-dimensional infrared spectroscopy

We have experimentally demonstrated a pulse sequence for the acquisition of heterodyned two-dimensional infrared (2D IR) spectra that correlates the overtone and combination bands to the fundamental frequencies. The spectra are generated by Fourier transforming the time domain signal that is allowed to evolve during one- and two-quantum coherence times. In this manner, the overtone and combination bands appear along the two-quantum axis, resulting in a direct determination of the diagonal and off-diagonal anharmonicities. To demonstrate this pulse sequence, we have collected two-quantum 2D IR spectra of a ruthenium dicarbonyl complex, extracted the diagonal and off-diagonal anharmonicities, and simulated the spectra using an exciton model. Several polarization conditions are presented that suppress the diagonal or cross peaks and we have used them to improve the accuracy of the measurement.     

Regularity of minimal hypersurfaces with a common free boundary

Let \(N\) be a Riemannian manifold and consider a stationary union of three or more \(C^{1,\mu }\) hypersurfaces-with-boundary \(M_k \subset N\) with a common boundary \(\Gamma \) . We show that if \(N\) is smooth, then \(\Gamma \) is smooth and each \(M_k\) is smooth up to \(\Gamma \) (real analytic in the case \(N\) is real analytic). Consequently we strengthen a result of Wickramasekera for stable codimension 1 integral varifolds regularity to conclude that under the stronger hypothesis that \(V\) is a stationary, stable, integral \(n\) -varifold in an \((n+1)\) -dimensional, smooth (real analytic) Riemannian manifold such that the support of \(\Vert V\Vert \) is nowhere locally the union of three or more smooth (real analytic) hypersurfaces-with-boundary meeting along a common boundary, the singular set of \(V\) is empty if \(n \le 6\) , is discrete if \(n = 7\) , and has Hausdorff dimension at most \(n-7\) if \(n \ge 8\) .     

Molecular orientation of asphaltenes and PAH model compounds in Langmuir-Blodgett films using sum frequency generation spectroscopy

Asphaltenes are an important class of compounds in crude oil whose surface activity is important for establishing reservoir rock wettability which impacts reservoir drainage. While many phenomenological interfacial studies with crude oils and asphaltenes have been reported, there is very little known about the molecular level interactions between asphaltenes and mineral surfaces. In this study, we analyze Langmuir-Blodgett films of asphaltenes and related model compounds with sum frequency generation (SFG) vibrational spectroscopy. In SFG, the polarization of the input (vis, IR) and output (SFG) beams can be varied, which allows the orientation of different functional groups at the interface to be determined. SFG clearly indicates that asphaltene polycyclic aromatic hydrocarbons (PAHs) are highly oriented in the plane of the interface and that the peripheral alkanes are transverse to the interface. In contrast, model compounds with oxygen functionality have PAHs oriented transverse to the interface. Computational quantum chemistry is used to support corresponding band assignments, enabling robust determination of functional group orientations.