Magnetically Directed Two-Dimensional Crystallization of OmpF Membrane Proteins in Block Copolymers

Homogeneous hybrid droplet interface bilayers assembled from binary mixtures of DPhPC phospholipids and PB-b-PEO diblock copolymers

Hybrid membranes built from phospholipids and amphiphilic block copolymers seek to capitalize on the benefits of both constituents for constructing biomimetic interfaces with improved performance. However, hybrid membranes have not been formed or studied using the droplet interface bilayer (DIB) method, an approach that offers advantages for revealing nanoscale changes in membrane structure and mechanics and offers a path toward assembling higher-order tissues. We report on hybrid droplet interface bilayers (hDIBs) formed in hexadecane from binary mixtures of synthetic diphytanoyl phosphatidylcholine (DPhPC) lipids and low molecular weight 1,2 polybutadiene-b-polyethylene oxide (PBPEO) amphiphilic block copolymers and use electrophysiology measurements and imaging to assess the effects of PBPEO in the membrane. This work reveals that hDIBs containing up to 15 mol% PBPEO plus DPhPC are homogeneously mixtures of lipids and polymers, remain highly resistive to ion transport, and are stable-including under applied voltage. Moreover, they exhibit hydrophobic thicknesses similar to DPhPC-only bilayers, but also have significantly lower values of membrane tension. These characteristics coincide with reduced energy of adhesion between droplets and the formation of alamethicin ion channels at significantly lower threshold voltages, demonstrating that even moderate amounts of amphiphilic block copolymers in a lipid bilayer provide a route for tuning the physical properties of a biomimetic membrane.

Protein Nanopore Membranes Prepared by a Simple Langmuir-Schaefer Approach

Filtration through membranes with nanopores is typically associated with high transmembrane pressures and high energy consumption. This problem can be addressed by reducing the respective membrane thickness. Here, a simple procedure is described to prepare ultrathin membranes based on protein nanopores, which exhibit excellent water permeance, two orders of magnitude superior to comparable, industrially applied membranes. Furthermore, incorporation of either closed or open protein nanopores allows tailoring the membrane's ion permeability. To form such membranes, the transmembrane protein ferric hydroxamate uptake protein component A (FhuA) or its open-pore variant are assembled at the air-water interface of a Langmuir trough, compressed to a dense film, crosslinked by glutaraldehyde, and transferred to various support materials. This approach allows to prepare monolayer or multilayer membranes with a very high density of protein nanopores. Freestanding membranes covering holes up to 5 μm in diameter are visualized by atomic force microscopy (AFM), helium ion microscopy, and transmission electron microscopy. AFM PeakForce quantitative nanomechanical property mapping (PeakForce QNM)  demonstrates remarkable mechanical stability and elastic properties of freestanding monolayer membranes with a thickness of only 5 nm. The new protein membrane can pave the way to energy-efficient nanofiltration.

Magnetic Control over the Fractal Dimension of Supramolecular Rod Networks

Controlling supramolecular polymerization is of fundamental importance to create advanced materials and devices. Here we show that the thermodynamic equilibrium of Gd³⁺-bearing supramolecular rod networks is shifted reversibly at room temperature in a static magnetic field of up to 2 T. Our approach opens opportunities to control the structure formation of other supramolecular or coordination polymers that contain paramagnetic ions.

Protein secondary structure signatures from energy loss spectra recorded in the electron microscope

Infrared spectroscopy is a powerful technique for characterising protein structure. It is now possible to record energy losses corresponding to the infrared region in the electron microscope and to avoid damage by positioning the probe in the region adjacent to the structure being studied. Spectra from bacteriorhodopsin, a protein that is predominately a α helix, and OmpF porin, a protein that is mainly β sheet show significant differences over a spectral range from ∼0.1 to 0.25 eV (∼1000 to 1800 cm^-1 ). Although the energy resolution equivalent to 60 cm^-1 is inferior to Fourier Transform InfraRed Spectroscopy (FTIR) the spectra are very sensitive to molecular orientation. Polar bonds aligned parallel to the specimen grid make particularly strong contributions to the energy loss spectra. Ultra-high-resolution energy loss spectroscopy in the electron microscope can potentially add useful information to imaging and diffraction for determining the secondary structure misfolding believed to be responsible for dementia diseases such as Alzheimer's.

Kinetic Control of Length and Morphology of Segmented Polymeric Nanofibers in Microfluidic Chips

In this work, we report an approach to prepare segmented polymer nanofibers (SPNFs) composed of rodlike subunits by kinetically controlled self-assembly of polystyrene-b-poly(4-vinylpyridine)-based supramolecules in microfluidic chips. The length and morphology of the SPNFs could be effectively adjusted by changing the total flow rate (V_total) and the molar ratio (x) of 4-vinylpyridine (4VP) unit to a hydrogen-bonding molecule, 3-n-pentadecyphenol. Moreover, the subunits of SPNFs could transform from short rods to spheres when the interfacial tension between PS core and solvent increased. On the contrary, the SPNFs elongated along the major axis when the interfacial tension decreased. This work not only offers mechanism insights into the hierarchical self-assembly of block copolymer-based supramolecules but also provides a versatile and effective method for kinetically controlling the hierarchical structures of assemblies.

Magnetic Field-Induced Alignment of Nanofibrous Supramolecular Membranes: A Molecular Design Approach to Create Tissue-like Biomaterials

A molecular design approach to fabricate nanofibrous membranes by self-assembly of aromatic cationic peptides with hyaluronic acid (HA) and nanofiber alignment under a magnetic field is reported. Peptides are designed to contain a block composed of four phenylalanine residues at the C-terminus, to drive their self-assembly by hydrophobic association and aromatic stacking, and have a positively charged domain of lysine residues for electrostatic interaction with HA. These two blocks are connected by a linker with a variable number of amino acids and the ability to adopt distinct conformations. Zeta potential measurements and circular dichroism confirm their positive charge and variable conformation (random coil, β-sheet, or α-helix), which depend on the pH and sequence. Their self-assembly, examined by fluorescence spectroscopy, small-angle X-ray scattering, and transmission electron microscopy, show the formation of fiberlike nanostructures in the micromolar range. When the peptides are combined with HA, hydrogels or flat membranes are formed. The molecular structure tunes the mechanical behavior of the membranes and the nanofibers align in the direction of magnetic field due to the high diamagnetic anisotropy of phenylalanine residues. Mesenchymal stem cells cultured on magnetically aligned membranes elongate in direction of the nanofibers supporting their application for soft tissue engineering.

Rapid fabrication of precise high-throughput filters from membrane protein nanosheets

Biological membranes are ideal for separations as they provide high permeability while maintaining high solute selectivity due to the presence of specialized membrane protein (MP) channels. However, successful integration of MPs into manufactured membranes has remained a significant challenge. Here, we demonstrate a two-hour organic solvent method to develop 2D crystals and nanosheets of highly packed pore-forming MPs in block copolymers (BCPs). We then integrate these hybrid materials into scalable MP-BCP biomimetic membranes. These MP-BCP nanosheet membranes maintain the molecular selectivity of the three types of β-barrel MP channels used, with pore sizes of 0.8 nm, 1.3 nm, and 1.5 nm. These biomimetic membranes demonstrate water permeability that is 20-1,000 times greater than that of commercial membranes and 1.5-45 times greater than that of the latest research membranes with comparable molecular exclusion ratings. This approach could provide high performance alternatives in the challenging sub-nanometre to few-nanometre size range.

Biomimetic Separation of Transport and Matrix Functions in Lamellar Block Copolymer Channel-Based Membranes

Cell membranes control mass, energy, and information flow to and from the cell. In the cell membrane a lipid bilayer serves as the barrier layer, with highly efficient molecular machines, membrane proteins, serving as the transport elements. In this way, highly specialized transport properties are achieved by these composite materials by segregating the matrix function from the transport function using different components. For example, cell membranes containing aquaporin proteins can transport ∼4 billion water molecules per second per aquaporin while rejecting all other molecules including salts, a feat unmatched by any synthetic system, while the impermeable lipid bilayer provides the barrier and matrix properties. True separation of functions between the matrix and the transport elements has been difficult to achieve in conventional solute separation synthetic membranes. In this study, we created membranes with distinct matrix and transport elements through designed coassembly of solvent-stable artificial (peptide-appended pillar[5]arene, PAP5) or natural (gramicidin A) model channels with block copolymers into lamellar multilayered membranes. Self-assembly of a lamellar structure from cross-linkable triblock copolymers was used as a scalable replacement for lipid bilayers, offering better stability and mechanical properties. By coassembly of channel molecules with block copolymers, we were able to synthesize nanofiltration membranes with sharp selectivity profiles as well as uncharged ion exchange membranes exhibiting ion selectivity. The developed method can be used for incorporation of different artificial and biological ion and water channels into synthetic polymer membranes. The strategy reported here could promote the construction of a range of channel-based membranes and sensors with desired properties, such as ion separations, stimuli responsiveness, and high sensitivity.

Infinite Assembly of Folded Proteins in Evolution, Disease, and Engineering

Mutations and changes in a protein's environment are well known for their potential to induce misfolding and aggregation, including amyloid formation. Alternatively, such perturbations can trigger new interactions that lead to the polymerization of folded proteins. In contrast to aggregation, this process does not require misfolding and, to highlight this difference, we refer to it as agglomeration. This term encompasses the amorphous assembly of folded proteins as well as the polymerization in one, two, or three dimensions. We stress the remarkable potential of symmetric homo‐oligomers to agglomerate even by single surface point mutations, and we review the double‐edged nature of this potential: how aberrant assemblies resulting from agglomeration can lead to disease, but also how agglomeration can serve in cellular adaptation and be exploited for the rational design of novel biomaterials.

Directed Self-Assembly of Templatable Block Copolymers by Easily Accessible Magnetic Control

Magnetic control has been a prosperous and powerful contactless approach in arraying materials into high-order nanostructures. However, it is tremendously difficult to control organic polymers in this way on account of the weak magnetic response. The preparation of block copolymers (BCPs) with high magnetostatic energy is reported here, relying on an effective electrostatic coupling between paramagnetic ions and polymer side chains. As a result, the BCPs undergo a magnetically directed self-assembly to form microphase-segregated nanostructures with long-range order. It is emphasized that such a precisely controlled alignment of the BCPs is performed upon a single commercial magnet with low-intensity field (0.35 Tesla). This strategy is profoundly easy-to-handle in contrast to routine electromagnetic methods with high-intensity field (5-10 Tesla). More significantly, the paramagnetic metal component in the BCP samples can be smartly removed, providing a template effect with a preservation of the directed self-assembled nanofeatures for patterning follow-up functionalized species through the original binding site.

Hierarchical Optimization of High-Performance Biomimetic and Bioinspired Membranes

Biomimetic and bioinspired membranes have emerged as an innovative platform for water purification and aqueous separations. They are inspired by the exceptional water permeability (∼10⁹ water molecules per second per channel) and perfect selectivity of biological water channels, aquaporins. However, only few successes have been reported for channel-based membrane fabrication due to inherent challenges of realizing coherence between channel design at the angstrom level and development of scalable membranes that maintain these molecular properties at practice-relevant scales. In this article, we feature recent progress toward practical biomimetic membranes, with the review organized along a hierarchical structural perspective that biomimetic membranes commonly share. These structures range from unitary pore shapes and tubular hydrophobic channel geometries to self-assembled bilayer structures and finally to macroscale membranes covering a size range from the angstrom, to the micrometer scale, and finally to the centimeter and larger scales. To maximize the advantage of water channel implementation into membranes, each feature needs to be optimized in an appropriate manner that provides a path to successful scale-up to achieve high performance in practical biomimetic and bioinspired membranes.

A periodic magnetic field as a special environment for scientific research created by rotating permanent magnet pairs

A magnetic field is an often-encountered physical environment that can affect many processes, including chemical, physical, and biochemical processes. Utilization of magnetic fields is thus very helpful in a wide variety of applications, such as scientific research in various disciplines, materials processing (e.g., crystal growth and separation) in industry, and nuclear fusion. There are many different types of magnetic fields generated by different magnets, such as superconducting magnets, electromagnets, hybrid magnets, pulsed magnets, and permanent magnets. In this paper, we introduce a newly designed periodic magnetic field generated by rotating permanent magnet pairs. Preliminary tests showed that the periodic magnetic field is valuable in water evaporation, silver deposition, and protein crystallization. Apparently, in such a new environment that can generate a periodic magnetic field, a periodic force field will also be simultaneously generated on the sample. Further work shall be carried out to explore the potential applications of this magnetic field.

PoreDesigner for tuning solute selectivity in a robust and highly permeable outer membrane pore

Monodispersed angstrom-size pores embedded in a suitable matrix are promising for highly selective membrane-based separations. They can provide substantial energy savings in water treatment and small molecule bioseparations. Such pores present as membrane proteins (chiefly aquaporin-based) are commonplace in biological membranes but difficult to implement in synthetic industrial membranes and have modest selectivity without tunable selectivity. Here we present PoreDesigner, a design workflow to redesign the robust beta-barrel Outer Membrane Protein F as a scaffold to access three specific pore designs that exclude solutes larger than sucrose (>360 Da), glucose (>180 Da), and salt (>58 Da) respectively. PoreDesigner also enables us to design any specified pore size (spanning 3-10 Å), engineer its pore profile, and chemistry. These redesigned pores may be ideal for conducting sub-nm aqueous separations with permeabilities exceeding those of classical biological water channels, aquaporins, by more than an order of magnitude at over 10 billion water molecules per channel per second.

Achieving high permeability and enhanced selectivity for Angstrom-scale separations using artificial water channel membranes

Synthetic polymer membranes, critical to diverse energy-efficient separations, are subject to permeability-selectivity trade-offs that decrease their overall efficacy. These trade-offs are due to structural variations (e.g., broad pore size distributions) in both nonporous membranes used for Angstrom-scale separations and porous membranes used for nano to micron-scale separations. Biological membranes utilize well-defined Angstrom-scale pores to provide exceptional transport properties and can be used as inspiration to overcome this trade-off. Here, we present a comprehensive demonstration of such a bioinspired approach based on pillar[5]arene artificial water channels, resulting in artificial water channel-based block copolymer membranes. These membranes have a sharp selectivity profile with a molecular weight cutoff of ~ 500 Da, a size range challenging to achieve with current membranes, while achieving a large improvement in permeability (~65 L m-2 h-1 bar-1 compared with 4-7 L m-2 h-1 bar-1) over similarly rated commercial membranes.

Layer-by-layer Assembled Membranes with Immobilized Porins

With the synthesis and functionalization of membranes for selective separations, reactivity, and stimuli responsive behavior arises new and advanced opportunities. The integration of bio-based channels is one of these advancements in membrane technologies. By a layer-by-layer (LbL) assembly of polyelectrolytes, outer membrane protein F trimers (OmpF) or "porins" from Escherichia coli with a central pore of ~2 nm diameter at its opening and ~0.7 × 1.1 nm at its constricted region are immobilized within the pores of poly(vinylidene fluoride) microfiltration membranes, as opposed to traditional ruptured lipid bilayer or vesicles processes. These OmpF-membranes demonstrate selective rejections of non-charged organics over ionic solutes, allowing the passage of salts up to 2 times higher than traditional nanofiltration membranes starting with rejections of 84% for 0.4-1.0 kDa organics. The presence of charged groups in OmpF membranes also leads to pH-dependent salt rejection through Donnan exclusion. These OmpF-membranes also show exceptional durability and stability, delivering consistent and constant permeability and recovery for over 160 h of operation. Characterization of solutions containing OmpF, and membranes were conducted during each stage of the process, including detection by fluorescence labelling (FITC), zeta potential, pH responsiveness, flux changes, and rejections of organic-inorganic solutions.

Toward Artificial Mitochondrion: Mimicking Oxidative Phosphorylation in Polymer and Hybrid Membranes

For energy supply to biomimetic constructs, a complex chemical energy-driven ATP-generating artificial system was built. The system was assembled with bottom-up detergent-mediated reconstitution of an ATP synthase and a terminal oxidase into two types of novel nanocontainers, built from either graft copolymer membranes or from hybrid graft copolymer/lipid membranes. The versatility and biocompatibility of the proposed nanocontainers was demonstrated through convenient system assembly and through high retained activity of both membrane-embedded enzymes. In the future, the nanocontainers might be used as a platform for the functional reconstitution of other complex membrane proteins and could considerably expedite the design of nanoreactors, biosensors, and artificial organelles.

Stoichiometry determination of macromolecular membrane protein complexes

Gaining knowledge of the structural makeup of protein complexes is critical to advance our understanding of their formation and functions. This task is particularly challenging for transmembrane protein complexes, and grows ever more imposing with increasing size of these large macromolecular structures. The last 10 years have seen a steep increase in solved high-resolution membrane protein structures due to both new and improved methods in the field, but still most structures of large transmembrane complexes remain elusive. An important first step towards the structure elucidation of these difficult complexes is the determination of their stoichiometry, which we discuss in this review. Knowing the stoichiometry of complex components not only answers unresolved structural questions and is relevant for understanding the molecular mechanisms of macromolecular machines but also supports further attempts to obtain high-resolution structures by providing constraints for structure calculations.

Computing the Diamagnetic Susceptibility and Diamagnetic Anisotropy of Membrane Proteins from Structural Subunits

The behavior of large, complex molecules in the presence of magnetic fields is experimentally challenging to measure and computationally intensive to predict. This work proposes a novel, mixed-methods approach for efficiently computing the principal magnetic susceptibilities and diamagnetic anisotropy of membrane proteins. The hierarchical primary (amino acid), secondary (α helical and β sheet), and tertiary (α helix and β barrel) structure of transmembrane proteins enables analysis of a complex molecule using discrete subunits of varying size and resolution. The proposed method converts the magnetic susceptibility tensor for all protein subunits to a unit coordinate system and sums them to build the magnetic susceptibility tensor for the membrane protein. Using this approach, we calculate the diamagnetic anisotropy for all transmembrane proteins of known structure and investigate the effect of different subunit resolutions on the resulting predictions of diamagnetic anisotropy. We demonstrate that amino acid residues with aromatic side groups exhibit higher diamagnetic anisotropies. On average, high percentages of aromatic amino acid subunits, a β barrel tertiary structure, and a small volume are correlated with high volumetric diamagnetic anisotropy. Finally, we demonstrate that accounting for the spatial position of the residues with respect to one another is critical to accurately computing the magnetic properties of the complex protein molecule.

Durable vesicles for reconstitution of membrane proteins in biotechnology

The application of membrane proteins in biotechnology requires robust, durable reconstitution systems that enhance their stability and support their functionality in a range of working environments. Vesicular architectures are highly desirable to provide the compartmentalisation to utilise the functional transmembrane transport and signalling properties of membrane proteins. Proteoliposomes provide a native-like membrane environment to support membrane protein function, but can lack the required chemical and physical stability. Amphiphilic block copolymers can also self-assemble into polymersomes: tough vesicles with improved stability compared with liposomes. This review discusses the reconstitution of membrane proteins into polymersomes and the more recent development of hybrid vesicles, which blend the robust nature of block copolymers with the biofunctionality of lipids. These novel synthetic vesicles hold great promise for enabling membrane proteins within biotechnologies by supporting their enhanced in vitro performance and could also contribute to fundamental biochemical and biophysical research by improving the stability of membrane proteins that are challenging to work with.