Supramolecular Hydrogel Formation in a Series of Self-Assembling Lipopeptides with Varying Lipid Chain Length.

The self-assembly in aqueous solution of three lipopeptides comprising a bioactive motif conjugated at the N terminus to dodecyl, tetradecyl or hexadecyl lipid chains has been examined. The bioactive motif is the peptide block YEALRVANEVTLN; a C-terminal fragment of the lumican proteoglycan. This study was motivated by our previous studies on the hexadecyl homologue C16-YEALRVANEVTLN, which showed aggregation into ß-sheet structures above a critical aggregation concentration (cac), but most remarkably, we found that these aggregates were stable to dilution below the cac.1 Here we find that the C12- and C14-homologues also self-assemble above a cac into ß-sheet nanotapes based on bilayer packing. The cac decreases with increasing lipopeptide hydrophobicity. Unexpectedly, the ß-sheet secondary structure is present upon dilution and the aggregates are thermally stable. These results indicate that the dilution trapping of ß-sheet secondary structure is not associated with lipid chain melting behavior. Instead, we associate it with pH-dependent favorable intermolecular electrostatic interactions. Investigation of the pH-dependence of aggregation led to the discovery of conditions for formation of lipopeptide hydrogels (initial sample preparation at pH 10 in NaOH solution, followed by reduction to pH ∼ 1 by addition of HCl). The lipopeptide hydrogels comprise networks of bilayer-based peptide nanotape bundles and to our knowledge this type of hydrogel is unprecedented. These hydrogels may have future applications based on processes such as encapsulation and release that involve fast switches between solution and hydrogel nanostructures.

Radicals as probes of ferroelectric phase transitions in EPR investigations of pyridinium tetrafluoroborate.

This paper reports CW-EPR studies of pyridinium tetrafluoroborate undergoing solid-solid phase transitions at 240 and 205 K, of which the one occurring at the higher temperature was found to be paraelectric-ferroelectric. EPR spectra of gamma-irradiated polycrystalline samples were taken at room temperature, at the liquid nitrogen temperature, and at temperatures ranging from 163 to 273 K in the cycles of cooling and heating. The radical appearing under the effect of gamma irradiation was established to be formed by attachment of a hydrogen atom to the carbon atom C(2) in the pyridine ring. The unpaired electron was found to delocalize onto the pyridine ring. The g factor was established as 2.0021(4) and the values of the hyperfine splitting constants testify to the occurrence of an alpha-type interaction, Aalpha = 0.98(5) mT, of the unpaired electron with protons at the C(3), C(5), and N(1) positions, two beta1- and beta2-type interactions with the hydrogen nuclei at the C(2) position, Abeta1 = 6.68(5) mT and Abeta2 = 4.85(5) mT, and the interaction with the nitrogen nucleus at N(1), AN = 0.86(5) mT. The complex character of the spectra obtained prompted us to employ computer analysis of the signals to enhance their resolution. Analysis of the temperature dependence of the linewidth parameter of individual components of the pyridine radical spectrum shows that EPR together with the computer resolution enhancement method can be a useful method for studying the nature of phase transitions in pyridinium tetrafluoroborate.