Experimental determination of conformational isomerization energy thresholds in serotonin

Stimulated emission pumping-population transfer (SEP-PT) and hole-filling (SEP-HF) spectroscopies were used to determine the energy thresholds to isomerization between thirteen reactant-product conformer pairs in the biomolecule serotonin (SERO). Serotonin is a close structural analog of tryptamine (TRA), differing in having a hydroxyl group in the 5 position of the indole ring. A previous spectroscopic study (LeGreve; et al. J. Am. Chem. Soc. 2007, 129 (13), 4028) identified eight conformational isomers of SERO, whose interconversion involves motion of the 3-ethylamine side chain, the 5-OH group, or both. In the cases in which only an ethylamine side chain reorientation occurred, the barriers were found to be similar to, but systematically somewhat smaller than, those in TRA, which has been studied by similar methods (Dian; et al. Science 2004, 303 (5661), 1169; Clarkson; et al. J. Chem. Phys. 2005, 122 (21), Art. No. 214311). In most cases, the experimental thresholds are well reproduced by calculated transition states separating the conformational wells; however, tunneling effects may artificially reduce the thresholds observed for isomerization of SERO(A,Gpy(out)) and SERO(B,Gpy(up)) into SERO(C,Gph(out)). The A --> A' isomerization involving only the OH rotation from anti to syn was found to be 721-761 cm-1, in accordance with the calculated classical barrier. For isomerizations in which the ethylamine side chain reorients as does the OH group, the barriers to isomerization were consistent with sequential rather than concerted motion of both groups. Finally, some evidence for mode-specific effects in the product quantum yields near threshold is presented.     

Separability of tight and roaming pathways to molecular decomposition

Recent studies have questioned the separability of the tight and roaming mechanisms to molecular decomposition. We explore this issue for a variety of reactions including MgH(2) → Mg + H(2), NCN → CNN, H(2)CO → H(2) + CO, CH(3)CHO → CH(4) + CO, and HNNOH → N(2) + H(2)O. Our analysis focuses on the role of second-order saddle points in defining global dividing surfaces that encompass both tight and roaming first-order saddle points. The second-order saddle points define an energetic criterion for separability of the two mechanisms. Furthermore, plots of the differential contribution to the reactive flux along paths connecting the first- and second-order saddle points provide a dynamic criterion for separability. The minimum in the differential reactive flux in the neighborhood of the second-order saddle point plays the role of a mechanism divider, with the presence of a strong minimum indicating that the roaming and tight mechanisms are dynamically distinct. We show that the mechanism divider is often, but not always, associated with a second-order saddle point. For the formaldehyde and acetaldehyde reactions, we find that the minimum energy geometry on a conical intersection is associated with the mechanism divider for the tight and roaming processes. For HNNOH, we again find that the roaming and tight processes are dynamically separable but we find no intrinsic feature of the potential energy surface associated with the mechanism divider. Overall, our calculations suggest that roaming and tight mechanisms are generally separable over broad ranges of energy covering most kinetically relevant regimes.     

Total synthesis of cyclic diterpene tonantzitlolone based on a highly stereoselective substrate-controlled aldol reaction and ring-closing metathesis

The total synthesis of the cyclic diterpene ent-tonantzitlolone (ent-1) is presented. Key steps for assembling the macrocyclic core structure of 1 are a highly selective aldol reaction and an E selective ring-closing metathesis reaction. A detailed investigation of these two steps and the final transformations towards the completion of the synthesis is disclosed.     

Stereocontrolled synthesis of beta-D-mannuronic acid esters: synthesis of an alginate trisaccharide

A facile synthesis route toward beta-linked mannuronic acid oligomers using the corresponding 1-thiomannuronic acid esters in combination with the Ph2SO/Tf2O or NIS/TMSOTf reagent combinations is presented. The presence of the remotely attached carboxylic ester sufficiently influences the electronic environment to allow good to excellent beta-selectivities.     

A transferable electrostatic map for solvation effects on amide I vibrations and its application to linear and two-dimensional spectroscopy

A method for modeling infrared solvent shifts using the electrostatic field generated by the solvent is presented. The method is applied to the amide I vibration of N-methyl acetamide. Using ab initio calculations the fundamental frequency, anharmonicity, and the transition dipoles between the three lowest vibrational states are parametrized in terms of the electrostatic field. The generated map, which takes into account the electric field and its gradients at four molecular positions, is tested in a number of common solvents. Agreement of solvent shift and linewidths with experimental Fourier transform infrared (FTIR) data is found to within seven and four wave numbers, respectively, for polar solvents. This shows that in these solvents electrostatic contributions dominate solvation effects and the map is transferable between these types of solvents. The effect of motional narrowing arising from the fast solvent fluctuations is found to be substantial for the FTIR spectra. Also the two-dimensional infrared (2DIR) spectra, simulated using the constructed map, reproduce experimental results very well. The effect of anharmonicity fluctuations on the 2DIR spectra was found to be negligible.     

High internal phase CO2-in-water emulsions stabilized with a branched nonionic hydrocarbon surfactant

A nonionic-methylated branched hydrocarbon surfactant, octa(ethylene glycol) 2,6,8-trimethyl-4-nonyl ether (5b-C12E8) emulsifies up to 90% CO2 in water with polyhedral cells smaller than 10 microm, as characterized by optical microscopy. The stability of these concentrated CO2/water (C/W) emulsions increases with pressure and in some cases exceeds 24 h. An increase in pressure weakens the attractive van der Waals interactions between the CO2 cells across water and raises the disjoining pressure. It also enhances the solution of the surfactant tail and drives the surfactant from water towards the water-CO2 interface, as characterized by the change in emulsion phase behavior and the decrease in interfacial tension (gamma) to 2.1 mN/m. As the surfactant adsorption increases, the greater tendency for ion adsorption is likely to increase the electrostatic repulsion in the thin lamellae and raise the disjoining pressure. As pressure increases, the increase in disjoining pressure and decrease in the capillary pressure (due to the decrease in gamma) each favor greater stability of the lamellae against rupture. The electrical conductivity is predicted successfully as a function of Bruggeman's model for concentrated emulsions. Significant differences in the stability are observed for concentrated C/W emulsions at elevated pressure versus air/W or C/W foams at atmospheric pressure.     

Prerequisites for chiral charge order

The chiral charge density wave state which was recently discovered in TiSe₂ can be understood as a combination of orbital and charge order. Here, we discuss the prerequisite material properties for this type of chiral charge order to emerge. We find that although both the lattice and orbital structure constrain the set of candidate materials, there remains a class of materials in which chiral charge order is expected to emerge.     

High-throughput dielectrophoretic separator based on printed circuit boards

The separation of particles with respect to their intrinsic properties is an ongoing task in various fields such as biotechnology and recycling of electronic waste. Especially for small particles in the lower micrometer or nanometer range, separation techniques are a field of current research since many existing approaches lack either throughput or selectivity. Dielectrophoresis (DEP) is a technique that can address multiple particle properties, making it a potential candidate to solve challenging separation tasks. Currently, DEP is mostly used in microfluidic separators and thus limited in throughput. Additionally, DEP setups often require expensive components, such as electrode arrays fabricated in the clean room. Here, we present and characterize a separator based on two inexpensive custom-designed printed circuit boards (80 × 120 mm board size). The boards consist of interdigitated electrode arrays with $250\mu$m electrode width and spacing. We demonstrate the separation capabilities using polystyrene particles ranging from 500 nm to $6\mu$m in monodisperse experiments. Further, we demonstrate selective trapping at flow rates up to 240 ml/h in the presented device for a binary mixture. Our experiments demonstrate an affordable way to increase throughput in electrode-based DEP separators.