Structural Dynamics of Bilayer-Folded Lamellae via Bending Rigidity Manipulation

Lipid molecules with hydrophilic heads and hydrophobic tails naturally form bilayer membranes that separate cells from their environment. A balance between interfacial tension and elastic energy drives this formation. Although bilayers are usually rigid due to noncovalent interactions, certain cellular structures like the Golgi apparatus and mitochondria have highly curved membranes due to external proteins. [1] Creating such folded structures in amphiphilic self-assembly has been difficult. We previously showed that specific amphiphilic random copolymers can form bilayer folded lamellar (L_f) mesophases due to local curvature mismatch, thus avoiding entropic loss from chain stretching. [2] This study introduces a method to control the folding height of the L_f phase formed by amphiphilic random copolymers in water. We synthesized copolymers with different alkyl chain lengths by combining various alkyl acrylates and oligo(ethylene glycol) acrylates using reversible addition-fragmentation chain transfer (RAFT) polymerization. We systematically analyzed the folding behavior of the copolymers based on alkyl chain length, focusing on the relationship between folding height and alkyl chain length. The results showed that as the alkyl chain length increased, the bilayer’s bending rigidity also increased, leading to a higher folding height. Small-angle X-ray scattering (SAXS) confirmed the presence of the L_f phase, and quantitative analysis revealed phase changes according to alkyl chain length. The scaling exponent in the relationship between the carbon number in the alkyl chain and the folding height matches that found in bilayer membranes. This study enables the manipulation of folding frequency over a wider range and will be valuable for creating soft materials with hierarchical structures that can be controlled at different length scales.