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