Bundles of taxol-stabilized microtubules (MTs) – hollow tubules comprised of assembled αβ-tubulin heterodimers – spontaneously assemble above a critical concentration of tetravalent spermine and are stable over long times at room temperature. Here we report that at concentrations of spermine several-fold higher the MT bundles (BMT) quickly become unstable and undergo a shape transformation to bundles of inverted tubulin tubules (BITT), the outside surface of which corresponds to the inner surface of the BMT tubules. Using transmission electron microscopy and synchrotron small-angle x-ray scattering, we quantitatively determined both the nature of the BMT to BITT transformation pathway, which results from a spermine-triggered conformation switch from straight to curved in the constituent taxol-stabilized tubulin oligomers, and the structure of the BITT phase, which is formed of tubules of helical tubulin oligomers. Inverted tubulin tubules provide a platform for studies requiring exposure and availability of the inside, luminal surface of MTs to MT-targeted-drugs and MT-associated-proteins.

Complex materials often achieve their remarkable functional properties by hierarchical assembly of building blocks via competing and/or synergistic interactions. Here, we describe the properties of new double-end-anchored poly (ethylene glycol)s (DEA-PEGs), macromolecules designed to impart hydrophobically mediated tethering attractions between charged lipid membranes. We synthesized DEA-PEGs (MW 2000 (2K) and 4.6K) with two double-tail (symmetric) or a double-tail and a single-tail (asymmetric) hydrophobic end anchors and characterized their equilibrium and kinetic properties using small-angle X-ray scattering. Control multilayer membranes without and with PEG-lipid (i.e. single-end-anchored PEG) swelled continuously with the interlayer spacing increasing between 30wt% and 90wt% water content, due to electrostatic as well as, in the case of PEG-lipid, steric repulsion. In contrast, interlayer spacings in lamellar membrane hydrogels containing DEA-PEGs expanded over a limited water dilution range and reached a “locked” state, which displayed a near constant membrane wall-to-wall spacing (δw) with further increases in water content. Remarkably, the locked state displays a simple relation to the PEG radius of gyration δw ≈ 1.6 RG for both 2K and 4.6K PEG. Nevertheless, δw being considerably less than the physical size of PEG (2(5/3)1/2RG) is highly unexpected and implies that, compared to free PEG, anchoring of the PEG tether at both ends leads to a considerable distortion of the PEG conformation confined between layers. Significantly, the lamellar hydrogel may be designed to reversibly transition from a locked to an unlocked (membrane unbinding) state by variations in the DEA-PEG concentration controlling the strength of the interlayer attractions due to bridging conformations. The findings with DEA-PEGs have broad implications for hydrophobic-mediated assembly of lipid- or surfactant- coated building blocks with distinct shape and size, at predictable spacing, in aqueous environments.

Reflectin proteins are widely distributed in reflective structures in cephalopods, but only in Loliginid squids are they and the sub-wavelength photonic structures they control dynamically tunable, driving changes in skin color for camouflage and communication. The reflectins are block copolymers with repeated canonical domains interspersed with cationic linkers. Neurotransmitter-activated signal transduction culminates in catalytic phosphorylation of the tunable reflectins' cationic linkers, with the resulting charge-neutralization overcoming Coulombic repulsion to progressively allow condensation and concommitant assembly to form multimeric spheres of tunable size. Structural transitions of reflectins A1 and A2 were analyzed by dynamic light scattering, transmission electron microscopy, solution small angle x-ray scattering, circular dichroism, atomic force microscopy, and fluorimetry. We analyzed the assembly behavior of phospho-mimetic, deletion, and other mutants in conjunction with pH-titration as an in vitro surrogate of phosphorylation to discover a predictive relationship between the extent of neutralization of the protein's net charge density and the size of resulting multimeric protein assemblies of narrow polydispersity. Comparison of mutants shows this sensitivity to neutralization resides in the linkers and is spatially distributed along the protein.These results are consistent with the behavior of a charge-stabilized colloidal system, while imaging of large particles, and analysis of sequence composition, suggest that assembly may proceed through a transient liquid-liquid phase separated intermediate. These results offer insights into the basis of reflectinbased tunable biophotonics and open new paths for the design of new reflectin mutants with tunable properties.

Inspired by nanotechnologies based on DNA strand displacement, herein we demonstrate that synthetic helical strand exchange can be achieved through tuning of poly(methyl methacrylate) (PMMA) triple-helix stereocomplexes. To evaluate the utility and robustness of helical strand exchange, stereoregular PMMA/polyethylene glycol (PEG) block copolymers capable of undergoing crystallization driven self-assembly via stereocomplex formation were prepared. Micelles with spherical or wormlike morphologies were formed by varying the molecular weight composition of the assembling components. Significantly, PMMA strand exchange was demonstrated and utilized to reversibly switch the micelles between different morphologies. This concept of strand exchange with PMMA-based triple-helix stereocomplexes offers new opportunities to program dynamic behaviors of polymeric materials, leading to scalable synthesis of "smart" nanosystems.