A Stimulus-Responsive Shape-Persistent Micelle Bearing a Calix[4]arene Building Block: Reversible pH-Dependent Transition between Spherical and Cylindrical Forms

A series of cationic calix[4]­arene-based lipids with alkyl chains of varying length were newly synthesized, and the ones with propyl and hexyl tails, denoted by CaL[4]­C3 and C6, respectively, were found to form spherical micelles at low pH (protonated state of the amine headgroup). Upon deprotonation with increasing pH, CaL[4]­C3 showed a sphere-to-cylinder transition, while CaL[4]­C6 changed from sphere, to cylinder, to monolayer vesicle. Synchrotron small-angle X-ray scattering (SAXS) patterns from both spherical and cylindrical CaL[4]­C3 micelles exhibited a sharp intensity minimum, indicating shape monodispersity. The monodispersity of the CaL[4]­C3 spherical micelles was further confirmed by analytical ultracentrifugation (AUC). SAXS, AUC, and static light scattering agreeingly indicated an aggregation number of 6. In contrast, CaL[4]­C6 exhibited polydispersity with an average aggregation number of 12. When the number of carbons of the alkyl chain was increased to 9 (CaL[4]­C9), cylinder formed at low pH, while at high pH, no clear morphology could be observed. The present results indicate that a very precise combination of tail length, head volume, and rigidity of the building block is required to produce shape-persistent micelles and that the shape-persistence can be maintained upon a structural transition. An attempt to reconstruct a molecular model for the spherical CaL[4]­C3 micelle was made with an ab initio shape determining program

Hydrophobic Molecules Infiltrating into the Poly(ethylene glycol) Domain of the Core/Shell Interface of a Polymeric Micelle: Evidence Obtained with Anomalous Small-Angle X‑ray Scattering

Polymeric micelles have been extensively studied as nanoscale drug carriers. Knowing the inner structure of polymeric micelles that encapsulate hydrophobic drugs is important to design effective carriers. In our study, the hydrophobic compound tetrabromocathecol (TBC) was chosen as a drug-equivalent model molecule. The bromine atoms in TBC act as probes in anomalous small-angle X-ray scattering (ASAXS) allowing for its localization in the polymeric micelles whose shape and size were determined by normal small-angle X-ray scattering (SAXS). Light scattering measurements coupled with field flow fractionation were also carried out to determine the aggregation number of micelles. A core–corona spherical model was used to explain the shape of the micelles, while the distribution of bromine atoms was explained with a hard-sphere model. Interestingly, the radius of the spherical region populated with bromine atoms was larger than the one of the sphere corresponding to the hydrophobic core of the micelle. This result suggests that the TBC molecules infiltrate the PEG hydrophilic domain in the vicinity of the core/shell interface. The results of light scattering and SAXS indicate that the PEG chains at the shell region are densely packed, and thus the PEG domain close to the interface has enough hydrophobicity to tolerate the presence of hydrophobic compounds

X‑ray Scattering from Immunostimulatory Tetrapod-Shaped DNA in Aqueous Solution To Explore Its Biological Activity–Conformation Relationship

We carried out synchrotron X-ray scattering experiments from four DNA supermolecules designed to form tetrapod shapes; these supermolecules had different sequences but identical numbers of total base pairs, and each contained an immunostimulatory CpG motif. We confirmed that the supermolecules did indeed form the expected tetrapod shape. The sample that had the largest radius of gyration (<i>R</i>_g) induced the most cytokine secretion from cultured immune cells. Structural analysis in combination with a rigid tetrapod model and an atomic scale DNA model revealed that the larger <i>R</i>_g can be ascribed to dissociation of the DNA double strands in the central connecting portion of the DNA tetrapod. This finding suggests that the biological activity is related to the ease with which single DNA strands can be formed