Molecular Lipid Films on Microengineering Materials.

In this study, we have systematically investigated the formation of molecular phospholipid films on a variety of solid substrates fabricated from typical surface engineering materials and the fluidic properties of the lipid membranes formed on these substrates. The surface materials comprise of borosilicate glass, mica, SiO2, Al (native oxide), Al2O3, TiO2, ITO, SiC, Au, Teflon AF, SU-8, and graphene. We deposited the lipid films from small unilamellar vesicles (SUVs) by means of an open-space microfluidic device, observed the formation and development of the films by laser scanning confocal microscopy, and evaluated the mode and degree of coverage, fluidity, and integrity. In addition to previously established mechanisms of lipid membrane-surface interaction upon bulk addition of SUVs on solid supports, we observed nontrivial lipid adhesion phenomena, including reverse rolling of spreading bilayers, spontaneous nucleation and growth of multilamellar vesicles, and the formation of intact circular patches of double lipid bilayer membranes. Our findings allow for accurate prediction of membrane-surface interactions in microfabricated devices and experimental environments where model membranes are used as functional biomimetic coatings.

NMR Determination of the Binding Constant of Ionic Species: A Caveat.

Determination of the dissociation constant of ionic complexes with the standard NMR titration and NMR dilution techniques may yield a severely compromised result, due to the typically unconsidered chemical shift alteration induced by the gradual change of the ionic strength during the experiment. We show that the reliability of an NMR titration experiment is markedly improved upon keeping the overall ionic strength constant.

The Interaction Modes of Haloimidazolium Salts in Solution.

We performed a comparative study on the interaction modes of 2-haloimidazolium salts with anions in solution, particularly with regard to halogen bonding, hydrogen bonding and anion-π interactions. The syntheses and solid-state analyses of a series of sterically and electronically modified 2-haloimidazolium structures are presented. Detailed isothermal titration calorimetry (ITC) measurements, quantum mechanics/molecular mechanics (QM/MM), classical molecular dynamics simulations (MD) and free-energy calculations together with NMR spectroscopy were used to elucidate the binding modes in solution. Our work reveals the absence of a potential anion-π interaction between the cationic imidazolium ring and the Lewis basic counteranion, and corroborates a formation of halogen bonding via the Lewis acidic iodine moiety and hydrogen bonding via the backbone hydrogen atoms, with repercussions in the field of organocatalysis.

Focused electron beam based direct-write fabrication of graphene and amorphous carbon from oxo-functionalized graphene on silicon dioxide.

Controlled patterning of graphene is an important task towards device fabrication and thus is the focus of current research activities. Graphene oxide (GO) is a solution-processible precursor of graphene. It can be patterned by thermal processing. However, thermal processing of GO leads to decomposition and CO2 formation. Alternatively, focused electron beam induced processing (FEBIP) techniques can be used to pattern graphene with high spatial resolution. Based on this approach, we explore FEBIP of GO deposited on SiO2. Using oxo-functionalized graphene (oxo-G) with an in-plane lattice defect density of 1% we are able to image the electron beam-induced effects by scanning Raman microscopy for the first time. Depending on electron energy (2-30 keV) and doses (50-800 mC m-2) either reduction of GO or formation of permanent lattice defects occurs. This result reflects a step towards controlled FEBIP processing of oxo-G.

Multiple Multidentate Halogen Bonding in Solution, in the Solid State, and in the (Calculated) Gas Phase.

The binding properties of neutral halogen-bond donors (XB donors) bearing two multidentate Lewis acidic motifs toward halides were investigated. Employing polyfluorinated and polyiodinated terphenyl and quaterphenyl derivatives as anion receptors, we obtained X-ray crystallographic data of the adducts of three structurally related XB donors with tetraalkylammonium chloride, bromide, and iodide. The stability of these XB complexes in solution was determined by isothermal titration calorimetry (ITC), and the results were compared to X-ray analyses as well as to calculated binding patterns in the gas phase. Density functional theory (DFT) calculations on the gas-phase complexes indicated that the experimentally observed distortion of the XB donors during multiple multidentate binding can be reproduced in 1:1 complexes with halides, whereas adducts with two halides show a symmetric binding pattern in the gas phase that is markedly different from the solid state structures. Overall, this study demonstrates the limitations in the transferability of binding data between solid state, solution, and gas phase in the study of complex multidentate XB donors.

Activation of glycosyl halides by halogen bonding.

Halogen bonding is the formation of a non-covalent interaction between an electrophilic halogen substituent and a Lewis base, for instance, a halide. These kinds of relatively weak interactions have found applications in crystal engineering and initial applications in solution-phase chemistry are starting to appear. We report on the exploration of bis(iodoimidazolium) compounds as halogen-based Lewis acids in the activation of glycosyl halides. We show that these dicationic halogen-bond donors can be used to activate glycosyl halides if the carbohydrate core is sufficiently reactive enough. Furthermore, we provide comparison experiments which indicate that the mode of activation is indeed based on halogen bonding. This represents the first glycosylation reaction mediated by a (carbon-backbone-based) halogen-bond donor.

Activation of a carbonyl compound by halogen bonding.

Using a prototypical Diels-Alder reaction as benchmark, we show that dicationic halogen-bond donors are capable of activating a neutral organic substrate. By various comparison experiments, the action of traces of acid or of other structural features of the halogen-bond donor not related to halogen bonding are excluded with high certainty.