Dynamics of water confined in mesoporous magnesium carbonate.

We have measured the dynamics of water confined in a porous magnesium carbonate material, Upsalite®, using the high-resolution neutron backscattering spectrometer SPHERES. We found quasielastic scattering that does not flatten out up to 360 K, which means that the dynamics of water are much slower than in other matrix materials. Specifically, a single Lorentzian line could be fitted to the quasielastic part of the acquired spectra between 220 and 360 K. This, accompanied by an elastic line from dynamically frozen water present at all experimental temperatures, even above the melting point, signaled a significant amount of bound or slow water.

Crossover from localized to diffusive water dynamics in carbon nanohorns: A comprehensive quasielastic neutron-scattering analysis.

Incoherent neutron scattering by water confined in carbon nanohorns was measured with the backscattering spectrometer SPHERES and analyzed in exemplary breadth and depth. Quasielastic spectra admit δ-plus-Kohlrausch fits over a wide q and T range. From the q and T dependence of fitted amplitudes and relaxation times, however, it becomes clear that the fits do not represent a uniform physical process, but that there is a crossover from localized motion at low T to diffusive α relaxation at high T. The crossover temperature of about 210 to 230 K increases with decreasing wave number, which is incompatible with a thermodynamic strong-fragile transition. Extrapolated diffusion coefficients D(T) indicate that water motion is at room temperature about 2.5 times slower than in the bulk; in the supercooled state this factor becomes smaller. At even higher temperatures, where the α spectrum is essentially flat, a few percentages of the total scattering go into a Lorentzian with a width of about 1.6µeV, probably due to functional groups on the surface of the nanohorns.

Oblique self-assemblies and order-order transitions in polypeptide complexes with PEGylated triple-tail lipids.

We report on highly ordered oblique self-assemblies in ionic complexes of PEGylated triple-tail lipids and cationic polypeptides, as directed by side-chain crystallization, demonstrating also reversible oblique-to-hexagonal order-order transitions upon melting of the side chains. This is achieved in bulk by complexing cationic homopolypeptides, poly-l-lysine (PLys), poly-l-arginine (PArg), and poly-l-histidine (PHis), in stoichiometric amounts with anionic lipids incorporating two hydrophobic alkyl tails and one hydrophilic polyethylene glycol (PEG) tail in a star-shaped A(2)B geometry. Based on Fourier transform infrared spectroscopy (FTIR), the PLys and PArg complexes fold into α-helical conformation. Aiming to periodicities at different length scales, that is, hierarchies, the PEG tails were selected to control the separation of the polypeptide helices in one direction while the alkyl tails determine the distance between the hydrophilic polypeptide/PEG layers, resulting in an oblique arrangement of the helices. We expect that the high overall order, where the self-assembled domains are in 2D registry, is an outcome of a favorable interplay of plasticization due to the hydrophobic and hydrophilic lipid tails combined with the shape persistency of the peptide helices and the crystallization of the lipid alkyl chains. Upon heating the complexes over the melting temperature of the alkyl tails, an order-order transition from oblique to hexagonal columnar morphology was observed. This transition is reversible, that is, the oblique structure with 2D correlation of the helices is fully returned upon cooling, implying that the alkyl tail crystallization guides the structure formation. Also PHis complex forms an oblique self-assembly. However, instead of α-helices, FTIR suggests formation of helical structures lacking intramolecular hydrogen bonds, stabilized by steric crowding of the lipid. The current study exploits competition between the soft and harder domains, which teaches on concepts toward well-defined polypeptide-based materials.

Self-assembly and induced circular dichroism in dendritic supramolecules with cholesteric pendant groups.

We report on the solid-state structural features of self-assembled chiral supramolecules based on ionic complexation of chiral cholesteric pendant groups with achiral dendritic macromolecules and show that their optical activity exhibits a systematic change in the ultraviolet/visible light (UV-vis) absorption and enhancement in the circular dichroism (CD) signal, indicating the occurrence of supramolecular chirality, also referred to as induced circular dichroism (ICD). We construct a homologous series of complexes by varying systematically from 1 to 3 the generation of dendritic units contained in dendrons, dendrimers, and dendronized polymers. The structural properties of the complexes are investigated by means of small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Depending on the class of dendritic molecule and the generation, lamellar, columnar hexagonal, oblique columnar, and rectangular columnar phases can be found, with a direct correlation among the degrees of freedom of the dendritic macromolecules used and the level of order achieved in the self-assembled solid-state structures. The enhancement of the optical signals of these mesoscopic structures appears to be correlated with their order in the solid state. Complexes with the longest lattice correlation lengths also show the most enhanced CD signals. These results show the unique versatility of dendritic macromolecules as supramolecular templates capable of organizing low molecular weight chiral pendant units into a variety of solid-state structures with amplified optical properties.