title: A stimulus-responsive shape-persistent micelle bearing a calix[4]arene building block: reversible pH-dependent transition between spherical and cylindrical forms.
abstract: 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.
authors.affiliation: Department of Chemistry and Biochemistry, University of Kitakyushu, Hibikino, Kitakyushu 808-0135, Japan.
authors.forename: Shota
authors.initials: S
authors.lastname: Fujii
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Sanada
authors.forename: Tomoki
authors.initials: T
authors.lastname: Nishimura
authors.forename: Isamu
authors.initials: I
authors.lastname: Akiba
authors.forename: Kazuo
authors.initials: K
authors.lastname: Sakurai
authors.forename: Naoto
authors.initials: N
authors.lastname: Yagi
authors.forename: Efstratios
authors.initials: E
authors.lastname: Mylonas