Vesicles were made from amphiphilic diblock copolymers and characterized by micromanipulation. The average molecular weight of the specific polymer studied, polyethyleneoxide-polyethylethylene (EO40-EE37), is several times greater than that of typical phospholipids in natural membranes. Both the membrane bending and area expansion moduli of electroformed polymersomes (polymer-based liposomes) fell within the range of lipid membrane measurements, but the giant polymersomes proved to be almost an order of magnitude tougher and sustained far greater areal strain before rupture. The polymersome membrane was also at least 10 times less permeable to water than common phospholipid bilayers. The results suggest a new class of synthetic thin-shelled capsules based on block copolymer chemistry.

No abstract

Vesicles made completely from diblock copolymers-polymersomes-can be stably prepared by a wide range of techniques common to liposomes. Processes such as film rehydration, sonication, and extrusion can generate many-micron giants as well as monodisperse, approximately 100 nm vesicles of PEO-PEE (polyethyleneoxide-polyethylethylene) or PEO-PBD (polyethyleneoxide-polybutadiene). These thick-walled vesicles of polymer can encapsulate macromolecules just as liposomes can but, unlike many pure liposome systems, these polymersomes exhibit no in-surface thermal transitions and a subpopulation even survive autoclaving. Suspension in blood plasma has no immediate ill-effect on vesicle stability, and neither adhesion nor stimulation of phagocytes are apparent when giant polymersomes are held in direct, protracted contact. Proliferating cells, in addition, are unaffected when cultured for an extended time with an excess of polymersomes. The effects are consistent with the steric stabilization that PEG-lipid can impart to liposomes, but the present single-component polymersomes are far more stable mechanically and are not limited by PEG-driven micellization. The results potentiate a broad new class of technologically useful, polymer-based vesicles.

Massively cross-linked and property-tunable membranes have been fabricated by free radical polymerization of self-assembled, block copolymer vesiclespolymersomes. Similar efforts with cross-linkable lipids would appear frustrated in the past due to at least two factors: limited reactivity and membrane fragility under local stresses of nano-confined cross-linking. We describe here a diblock copolymer of poly(ethylene oxide)polybutadiene that has a hydrophilic weight fraction like that of lipids and forms robust fluid phase membranes in water. The polymersomes sustain free radical polymerization of the hydrophobic butadiene, thereby generating a semipermeable nano-shell. Cross-linked giant vesicles prove stable in chloroform and can also be dehydrated and re-hydrated without rendering the ∼9 nm thick membrane core; the results imply defectfree membranes many microns-squared in area. Surface elastic moduli as well as sustainable wall stresses up to 10 3 Atm, orders of magnitude greater than any natural lipid membrane, appear consistent with strong tethering between close-packed neighbors. The enormous stability of the giant vesicles can be tuned down for application: blending in the hydrogenated analogue poly(ethylene oxide)-polyethylethylene modulates the effective elastic constants as well as the rupture strength by orders of magnitude. The results appear consistent with rigidity percolation through a finite-layer stack of two-dimensional lattices. Moreover, below the percolation limit, a regime of hyper-instability emerges, reflecting perhaps nanoscale demixing and suggestive of the limitations encountered with low reactivity lipids. The results provide general insights into covalent cross-linking within self-assembled nanostructures.

Emulating functions of natural enzymes in man-made constructs has proven challenging. Here we describe a man-made protein platform that reproduces many of the diverse functions of natural oxidoreductases without importing the complex and obscure interactions common to natural proteins. Our design is founded on an elementary, structurally stable 4-α-helix protein monomer with a minimalist interior malleable enough to accommodate various light- and redox-active cofactors and with an exterior tolerating extensive charge patterning for modulation of redox cofactor potentials and environmental interactions. Despite its modest size, the construct offers several independent domains for functional engineering that targets diverse natural activities, including dioxygen binding and superoxide and peroxide generation, interprotein electron transfer to natural cytochrome c and light-activated intraprotein energy transfer and charge separation approximating the core reactions of photosynthesis, cryptochrome and photolyase. The highly stable, readily expressible and biocompatible characteristics of these open-ended designs promise development of practical in vitro and in vivo applications.

No abstract

We have designed and implemented a practical nanoelectronic interface to G-protein coupled receptors (GPCRs), a large family of membrane proteins whose roles in the detection of molecules outside eukaryotic cells make them important pharmaceutical targets. Specifically, we have coupled olfactory receptor proteins (ORs) with carbon nanotube transistors. The resulting devices transduce signals associated with odorant binding to ORs in the gas phase under ambient conditions and show responses that are in excellent agreement with results from established assays for OR–ligand binding. The work represents significant progress on a path toward a bioelectronic nose that can be directly compared to biological olfactory systems as well as a general method for the study of GPCR function in multiple domains using electronic readout.

To determine if lateral phase separation occurs in films of pulmonary surfactant, we used epifluorescence microscopy and Brewster angle microscopy (BAM) to study spread films of calf lung surfactant extract (CLSE). Both microscopic methods demonstrated that compression produced domains of liquid-condensed lipids surrounded by a liquid-expanded film. The temperature dependence of the pressure at which domains first emerged for CLSE paralleled the behavior of its most prevalent component, dipalmitoyl phosphatidylcholine (DPPC), although the domains appeared at pressures 8-10 mN/m higher than for DPPC over the range of 20-37 degrees C. The total area occupied by the domains at room temperature increased to a maximum value at 35 mN/m during compression. The area of domains reached 25 +/- 5% of the interface, which corresponds to the predicted area of DPPC in the monolayer. At pressures above 35 mN/m, however, both epifluorescence and BAM showed that the area of the domains decreased dramatically. These studies therefore demonstrate a pressure-dependent gap in the miscibility of surfactant constituents. The monolayers separate into two phases during compression but remain largely miscible at higher and lower surface pressures.