Here we show that amides of bicyclic 7-azabicyclo[2.2.1]heptane are intrinsically nitrogen-pyramidal. Single-crystal X-ray diffraction structures of some relevant bicyclic amides, including the prototype N-benzoyl-7-azabicyclo[2.2.1]heptane, exhibited nitrogen-pyramidalization in the solid state. We evaluated the rotational barriers about the amide bonds of various N-benzoyl-7-azabicyclo[2.2.1]heptanes in solution. The observed reduction of the rotational barriers of the bicyclic amides, as compared with those of the monocyclic pyrrolidine amides, is consistent with a nitrogen-pyramidal structure of 7-azabicyclo[2.2.1]heptane amides in solution. A good correlation was found between the magnitudes of the rotational barrier of N-benzoyl-7-azabicyclo[2.2.1]heptanes bearing para-substituents on the benzoyl group and the Hammett's sigma(p)(+) constants, and this is consistent with the similarity of the solution structures. Calculations with the density functional theory reproduced the nitrogen-pyramidal structures of these bicyclic amides as energy minima. The calculated magnitudes of electron delocalization from the nitrogen nonbonding n(N) orbital to the carbonyl pi orbital of the amide group evaluated by application of the bond model theory correlated well with the rotational barriers of a variety of amides, including amides of 7-azabicyclo[2.2.1]heptane. The nonplanarity of the amide nitrogen of 7-azabicyclo[2.2.1]heptanes would be derived from nitrogen-pyramidalization due to the CNC angle strain and twisting of the amide bond due to the allylic strain.
Heating and then cooling down a dispersion of a peptide amphiphile in water forms hierarchical fibril structures leading to a supramolecular hydrogel. When the gel was physically broken apart by shaking, it transformed into a sol state. After aging it at room temperature for a given time, it returned to the gel state (re-gelation). To obtain a better understanding of such re-gelation processes, we have applied particle tracking to the sol obtained by disrupting the gel, as a function of aging time. The sol was more heterogeneous at the micrometer scale than the initial gel in terms of its viscoelastic properties, and the extent of the heterogeneity in the sol decreased as the re-gelation proceeded. The origin of the heterogeneity could be directly associated with a fibril network confirmed from Fourier-transform infrared spectroscopic, small-angle X-ray scattering and fluorescence microscopic measurements. The particle tracking study using different particle sizes suggested that the characteristic length scale of the heterogeneous network was not larger than 3 mm. This knowledge might be useful for understanding and controlling the gelation, thereby leading to the design and functionalization of soft materials.
There has been a considerable interest in developing new types of gels based on a network of fibrous aggregate composed of low molecular weight gelators, also known as supramolecular gels (SMGs). Unlike conventional polymer gels with chemical cross-linking, the network formation in SMGs does not involve any covalent bonds. Thus, the network in SMGs has been often regarded as homogenous or less heterogeneous in comparison with that in chemically cross-linked polymer gels. In this study, we have experimentally verified the existence of the network heterogeneity even in SMGs. The thermal motion of probe particles in SMGs, which were prepared from aqueous dispersions of gelators having a different number of peptide residues, PalGH, PalGH, and PalGH, was tracked. The gels were spatially heterogeneous in terms of the network pore size, as evidenced by the variation in the particle motion depending on the location, at which a particle existed. With varying particle size, it was found that the characteristic length scale of the heterogeneity was in the order of (sub)micrometers and was smaller in the order of the PalGH, PalGH, and PalGH gels.
We demonstrate herein the variation in viscoelastic properties of supramolecular hydrogels (SMGs) composed of two amphiphiles, N-Palmitoyl-Gly-His (PalGH) and sodium palmitate (PalNa). PalGH molecules in water form lamellar-like assemblies, which stack into sheet-shaped aggregates, resulting in the evolution of three-dimensional network structures. Once PalNa is added to PalGH, the alkyl groups of PalNa incorporate themselves into the hydrophobic cores of PalGH lamellar-like assemblies, resulting in a change in the assembly from lamellar-like to fibrous micelle-like. Consequently, sheet-shaped aggregates turn into flexible fibrils, which form bundles, resulting in network structures. Mixed hydrogel network structures differ in morphology from those in homogenous PalGH and PalNa hydrogels. Changes in the network structure eventually alter the bulk viscoelastic properties of hydrogels. These results demonstrate that the viscoelastic properties of supramolecular hydrogels can be tuned by controlling the aggregation states.
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