Structural analysis of a calix[4]arene-based Platonic Micelle

Efstratios Mylonas,Kazuo Sakurai,Naoto Yagi,Hideaki Moriyama,Tomoya Ueda,Tadashi Okobira,Kazuya Uezu,Kodai Ikesue,Shota Fujii,Yusuke Sanada

doi:10.1038/s41598-018-38280-1

Abstract

We have recently introduced the concept of “Platonic micelles”, the preference of spherical micelles to specific aggregation numbers mostly coinciding with the number of faces of platonic solids. This effect was observed on bulky, mostly calix[4]arene-based surfactant systems with small aggregation numbers. The preferred aggregation numbers result in better sphere coverage, highliting the packing and the “protection” of hydrophobic cores from the aqueous solvent as the most important factor for this preference. In the present study we further explore the interactions that drive the packing of the highly charged PACaL3 surfactant into highly symmetrical hexameric micelles. We performed a series of molecular dynamics simulations that yielded a large set of structures and an ensemble in good agreement with the experimental Small Angle X-ray Scattering data was selected. The geometry and the rigidity of the calix[4]arene group with proper tail length and headgroup volume are the driving forces for the high symmetry and monodispersity of the micelle. The charge of the headgroups is mainly responsible for inhibiting the formation of higher order structures. Sodium, shown to be important for the stability of the micelle, is not directly interacting with the micelle implying that the calix[4]arene ring is a C2ν symmetry conformation.

Highlights

Micelles formed by amphiphilic molecules, i.e. lipids or surfactants, have always attracted scientific interest because of their ability to self assemble and form interesting three-dimensional structures
Molecular dynamics (MD) simulations can prove useful in identifying the interactions that lead to the formation and the properties of these micelles
From the above results it is evident that MD simulations in combination with Small Angle X-ray Scattering (SAXS) can be very valuable in determining the structure of molecules or supramolecular assemblies

Summary

Introduction

Micelles formed by amphiphilic molecules, i.e. lipids or surfactants, have always attracted scientific interest because of their ability to self assemble and form interesting three-dimensional structures. A variety of different surfactants can form completely uniform and structurally precise monodisperse micelles, depending on the tail length, hydrophilic head volume, granted that they have sufficiently small aggregation numbers. A variety of different surfactants can form completely uniform and structurally precise monodisperse micelles, depending on the tail length, hydrophilic head volume, granted that they have sufficiently small aggregation numbers4 These numbers are good solutions to the Tammes problem, i.e. provide good coverage of a sphere with specific numbers of spherical caps. With the structural information provided by the SAXS data, we employ these findings to the analysis of structural models produced by MD simulations and explain them on the basis of the interactions that facilitate the formation and monodispersity of these micelles

Methods

Results

Conclusion