Polymer binding to carbon nanotubes in aqueous dispersions: residence time on the nanotube surface as obtained by NMR diffusometry.

The binding of block copolymer Pluronic F-127 in aqueous dispersions of single- (SWCNT) and multiwalled (MWCNT) carbon nanotubes has been studied by pulsed-field-gradient (PFG) (1)H NMR spectroscopy. We show that a major fraction of polymers exist as a free species while a minor fraction is bound to the carbon nanotubes (CNT). The polymers exchange between these two states with residence times on the nanotube surface of 24 ± 5 ms for SWCNT and of 54 ± 11 ms for MWCNT. The CNT concentration in the solution was determined by improved thermal gravimetric analysis (TGA) indicating that the concentration of SWCNT dispersed by F-127 was significantly higher than that for MWCNT. For SWCNT, the area per adsorbed Pluronic F-127 molecule is estimated to be about 40 nm(2).

Ion channels and anisotropic ion mobility in a liquid-crystalline columnar phase as observed by multinuclear NMR diffusometry.

The anisotropic diffusion of anions and cations in the columnar and isotropic phases of a fan-shaped imidazolium hexafluorophosphate salt is measured by (1)H and (19)F diffusion NMR experiments. The macroscopic orientation of the columnar phase is investigated by (2)H NMR spectroscopy. We find that the anions, confined by the cations, diffuse faster than the cations along the columns but slowly across them, which exemplifies the ion channel model of these materials. The cations and anions are dissociated in the columnar phase but are paired or clustered in the isotropic phase.

Ion conductive behaviour in a confined nanostructure: NMR observation of self-diffusion in a liquid-crystalline bicontinuous cubic phase.

The diffusion of ions in an ionic liquid crystal exhibiting a bicontinuous cubic liquid-crystalline phase has been investigated by NMR spectroscopy in order to examine the behaviour of ions in an ordered nanostructure.

Direct epoxidation in Candida antarctica lipase B studied by experiment and theory.

Candida antarctica lipase B (CALB) is a promiscuous serine hydrolase that, besides its native function, catalyzes different side reactions, such as direct epoxidation. A single-point mutant of CALB demonstrated a direct epoxidation reaction mechanism for the epoxidation of alpha,beta-unsaturated aldehydes by hydrogen peroxide in aqueous and organic solution. Mutation of the catalytically active Ser105 to alanine made the previously assumed indirect epoxidation reaction mechanism impossible. Gibbs free energies, activation parameters, and substrate selectivities were determined both computationally and experimentally. The energetics and mechanism for the direct epoxidation in CALB Ser105Ala were investigated by density functional theory calculations, and it was demonstrated that the reaction proceeds through a two step-mechanism with formation of an oxyanionic intermediate. The active-site residue His224 functions as a general acid-base catalyst with support from Asp187. Oxyanion stabilization is facilitated by two hydrogen bonds from Thr40.