(3ay) Solution Phase Self-Assembly of Soft Materials

Self-assembly of surfactants, phospholipids, block copolymers and biopolymers in solution to form diverse equilibrium and non-equilibrium microstructures have great significance from basic evolution of life to development of engineered advanced nanomaterials. Understanding the mechanism of microstructure formation at different length scales necessitates appropriate model systems and analytical tools. In this context, I describe (i) the self assembly behavior of a simple dodecylsulphate surfactant influenced by cesium counterions, (ii) the lamellar phase immiscibilty of the swelling amphiphile polyethoxysorbitan trioleate (Tween85) to form concentric cylinders, and (iii) the structural reorganization of clathrin protein triskelia to form 3D cage-like assemblies, as studied using Optical Microscopy, Dynamic Light Scattering (DLS), Cryo Transmission Electron Microscopy (cryo-TEM), cryo Electron Tomography (cryo-ET), and Freeze Fracture Transmission Electron Microscopy (FFTEM).

Charged surfactants self-assemble to form elongated structures such as worm-like micelles and crystalline precipitates depending on the extent of electrostatic screening of head group repulsion by electrolytes, oppositely charged surfactants, or hydrotropes. Here, the effect of screening the electrostatic repulsion between anionic headgroups of dodecylsulphate by hydrated cesium counterions to form worm-like protrusions and tubular structures of complex hexagonal phase was explored using Cryo-TEM in correlation with earlier scattering studies. Compared to sodium dodecylsulphate, the concentration of cesium dodecylsulphate required to form extended microstructures has significantly decreased. These data demonstrate that the nature of counterion influences self-assembly and phase behavior of charged surfactant systems.

To understand the lamellar phase immiscibility exhibited by swelling amphiphiles, the formation of tubular microstructures made of concentric lamellar cylinders via dissolution of polyethoxy sorbitan trioleate (Tween 85) in water was studied using optical microscopy and FFTEM. The dissolution characteristic of lamellar phase indicated that the gel phase coexists with fluid lamellar bilayers. Further investigation at elevated temperatures revealed the existence of a monotectic phase transition leading to fluid phase immiscibility. FFTEM identification of the focal eddies evolved during diffusion of water into the concentrated surfactant, the lamellar mesophase formation, and the isotropic fluid phase nucleation helped in understanding the complex monotectic phase behavior of the Tween85/water system.

Clathrin is a ~630 kDa protein that resembles a three-legged pinwheel, i.e., a triskelion, with each leg having an extended length of ~50 nm and width of ~3 nm. Due to molecular recognition interactions between legs and the non-planar geometry of the basic building block, clathrin self-assembles to form 3D assemblies resembling fullerene structures. Here we use cryo-TEM, cryo-ET and DLS to explore the influence of pH on the kinetics and morphology of clathrin self-assembly and to discover potential new nanostructures that clathrin may form. Our results demonstrate that clathrin has strikingly different assembly kinetics at pH 5 (below the isoelectric point) where discrete spherical cages (~60-100 nm) form over the course of several days via the rearrangement of large disordered aggregates versus at pH 6 (above the isoelectric point) where stable spherical cages (~60-100 nm) form within 30 minutes. This corroborates DLS data that indicate clathrin self-assembly is a function of both pH and time, with large length-scale structural rearrangements occurring over long time-scales (i.e., days).