Selective Removal of Denatured Proteins Using MOF Nanopores.

Here we present, for the first time, the selective adsorption of denatured proteins using a metal-organic framework (MOF), demonstrating promising potential for protein purification. Typical proteins, such as lysozyme and carbonic anhydrase B, enter the pores of MIL-101 through their narrow apertures when they are denatured to an unfolded state. Selective adsorption is achieved by finely tuning two key features: the sizes of the aperture and cage of the MOF nanopores, which are responsible for sorting unfolded polypeptide chains and inhibiting the translocation of the native form into the pores, respectively. By leveraging this selective adsorption, we successfully purified a mixture of native and denatured proteins by adding MOF to the mixture, achieving a native purity of over 99%.

Metal-Organic Frameworks as Versatile Media for Polymer Adsorption and Separation.

Molecular recognition is of paramount importance for modern chemical processes and has now been achieved for small molecules using well-established host-guest chemistry and adsorption-science principles. In contrast, technologies for recognizing polymer structure are relatively undeveloped. Conventional polymer separation methods, which are mostly limited in practice to size-exclusion chromatography and reprecipitation, find it difficult to recognize minute structural differences in polymer structures as such small structural alterations barely influence the polymer characteristics, including molecular size, polarity, and solubility. Therefore, most of the polymeric products being used today contain mixtures of polymers with different structures as it is challenging to completely control polymer structures during synthesis even with state-of-the-art substitution and polymerization techniques. In this context, development of novel techniques that can resolve the challenges of polymer recognition and separation is in great demand, as these techniques hold the promise of a new paradigm in polymer synthesis, impacting not only materials chemistry but also analytical and biological chemistry.In biological systems, precise recognition and translation of base monomer sequences of mRNA are achieved by threading them through small ribosome tunnels. This principle of introducing polymers into nanosized channels can possibly help us design powerful polymer recognition and separation technologies using metal-organic frameworks (MOFs) as ideal and highly designable recognition media. MOFs are porous materials comprising organic ligands and metal ions and have been extensively studied as porous beds for gas separation and storage. Recently, we found that MOFs can accommodate large polymeric chains in their nanopores. Polymer chains can spontaneously infiltrate MOFs from neat molten and solution phases by threading their terminals into MOF nanochannels. Polymer structures can be recognized and differentiated due to such insertion processes, resulting in the selective adsorption of polymers on MOFs. This enables the precise recognition of the polymer terminus structure, resulting in the perfect separation of a variety of terminal-functionalized polymers that are otherwise difficult to separate by conventional polymer separation methods. Furthermore, the MOFs can recognize polymer shapes, thus enabling the large-scale separation of high purity cyclic polymers from the complex crude mixtures of linear polymers, which are used as precursor materials in common cyclization reactions. In solution-phase adsorption, many factors, including molecular weight, terminal groups, polymer shape, polymer-MOF interaction, and coexisting solvent molecules, influence the selective adsorption behavior; this yields a new liquid chromatography-based polymer separation technology using an MOF as the stationary phase. MOF-packed columns, in which a novel separation mode based on polymer insertion into the MOF operates under a dynamic insertion/rejection equilibrium at the liquid/solid interface, exhibited excellent polymer separation capability. The polymer recognition principle described in this study thus has a high probability for realizing previously unfeasible polymer separations based on monomer composition and sequences, stereoregularity, regioregularity, helicity, and block sequences in synthetic polymers and biomacromolecules.

Crystal Flexibility Design through Local and Global Motility Cooperation.

Incorporating local mobility into a flexible framework promises to create cooperative properties unattainable in a conventional soft porous crystal. In this study, we propose a design strategy that integrates substituent moieties and a flexible porous crystal framework via intra-framework π-π interactions. This integration not only facilitates framework structural transitions but also gives the porous coordination polymers (PCPs) different guest-free structures that depend on the activation conditions. The incorporated flexibility gives the material the ability to discriminate C6 alkane isomers based on different gate-opening behaviors. Thus, the PCP has potential applications in C6 isomer separation, a critical step in the petroleum refining process to produce gasoline with high octane rating. This strategy, based on ligand designability, offers a new approach to flexible PCP structural and functional design.

Metal-Organic Frameworks for Practical Separation of Cyclic and Linear Polymers.

The purification step in the manufacturing of cyclic polymers is difficult as complete fractionation to eliminate linear impurities requires considerable effort. Here, we report a new polymer separation methodology that uses metal-organic frameworks (MOFs) to discriminate between linear and cyclic polyethylene glycols (PEGs) via selective polymer insertion into the MOF nanopores. Preparation of a MOF-packed column allowed analytical and preparative chromatographic separation of these topologically distinct pairs. In addition, gram-scale PEGs with only cyclic structures were successfully obtained from a crude reaction mixture by using MOF as an adsorbent.

Recognition of Polymer Terminus by Metal-Organic Frameworks Enabling Chromatographic Separation of Polymers.

Unlike commonly used molecular recognition techniques, recognition of polymer structures requires an additional aspect of extremely high recognition ability, by which marginal structural differences can be identified in a large polymer chain. Herein we show that metal-organic frameworks (MOFs) can recognize polymer terminal structures, thus enabling the first reported chromatographic separation of polymers. End-functionalized polyethylene glycols (PEGs) are selectively inserted into the MOF channel, the insertion kinetics being dependent on the projection size of the PEG terminus. This size-selective insertion mechanism facilitates precise discrimination of end-functionalized PEGs using liquid chromatography (LC). An MOF-packed column thus provides an efficient and easily accessible method for the separation of such end-functionalized polymers using conventional LC systems.

Pseudo-Gated Adsorption with Negligible Volume Change Evoked by Halogen-Bond Interaction in the Nanospace of MOFs.

The enhancement of gas adsorption utilizing weak interactions in porous compounds is highly demanding for the design of energy-efficient storage materials. Here, we present a rational design for such an adsorption process by using synergistic functions between dynamic motion in a local module and weak but specific host-guest interactions, that is, halogen-bond (XB) interactions in metal-organic frameworks (MOFs). We designed a new porous coordination polymer (PCP), that is, Br-PCP, the pore surfaces of which are decorated with -CH2 Br groups and could be useful for interaction with CO2 molecules. In accordance with our anticipation, in-situ studies suggest that the adsorption step at approximately 54 kPa during CO2 adsorption is indeed facilitated by XB interactions with little change in the structural volume. This approach of integrating flexible XB modules in rigid PCPs is applicable for designing advanced gas storage systems.

Chemistry of Soft Porous Crystals: Structural Dynamics and Gas Adsorption Properties.

In this Minireview, we discuss the fundamental chemistry of soft porous crystals (SPCs) by characterizing their common structural features and the resulting structural softness and transitions. In particular, we focus on the recently emerging properties based on metastable transitions and those arising from local dynamics. By comparing the resulting adsorption properties to those of commonly applied rigid adsorbents, we highlight the potential of SPCs to revolutionize adsorption-based technologies, considering our current understanding of the thermodynamic and kinetic aspects. We provide brief outlines for the experimental and computational characterization of such phenomena and offer an outlook toward next-generation SPCs likely to be discovered in the next decade.

Structural-Deformation-Energy-Modulation Strategy in a Soft Porous Coordination Polymer with an Interpenetrated Framework.

To achieve unique molecular-recognition patterns, a rational control of the flexibility of porous coordination polymers (PCPs) is highly sought, but it remains elusive. From a thermodynamic perspective, the competitive relationship between the structural deformation energy (Edef ) of soft PCPs and the guest interaction is key for selective a guest-triggered structural-transformation behavior. Therefore, it is vital to investigate and control Edef to regulate this competition for flexibility control. Driven by these theoretical insights, we demonstrate an Edef -modulation strategy via encoding inter-framework hydrogen bonds into a soft PCP with an interpenetrated structure. As a proof of this concept, the enhanced Edef of PCP enables a selective gate-opening behavior toward CHCl3 over CH2 Cl2 by changing the adsorption-energy landscape of the compounds. This study provides a new direction for the design of functional soft porous materials.

Modular Design of Porous Soft Materials via Self-Organization of Metal-Organic Cages.

Metal-organic frameworks (MOFs) and porous coordination polymers (PCPs) have been well-recognized as emerging porous materials that afford highly tailorable and well-defined nanoporous structures with three-dimensional lattices. Because of their microporous nature, MOFs can accommodate small molecules in their lattice structure, thus discriminating them on the basis of their size and physical properties and enabling their separation even in the gas phase. Such characteristics of MOFs have attracted significant attention in recent years for diverse applications and have ignited a worldwide race toward their development in both academic and industrial fields. Most recently, new challenges in porous materials science demand processable liquid, melt, and amorphous forms of MOFs. This trend will provide a new fundamental class of microporous materials for further widespread applications in many fields. In particular, the application of flexible membranes for gas separation is expected as an efficient solution to tackle current energy-intensive issues. To date, amorphous MOFs have been prepared in a top-down approach by the introduction of disorder into the parent frameworks. However, this new paradigm is still in its infancy with respect to the rational design principles that need to be developed for any approach that may include bottom-up synthesis of porous soft materials. Herein we describe recent progress in bottom-up "modular" approaches for the synthesis of porous, processable MOF-based materials, wherein metal-organic cages (MOCs), alternatively called metal-organic polyhedra (MOPs), are used as "modular cavities" to build porous soft materials. The outer periphery of a MOP is decorated with polymeric and dendritic side chains to obtain a polymer-grafted MOP, imparting both solution and thermal processability to the MOP cages, which have an inherent nanocavity along with high tailorability analogous to MOFs. Well-ordered MOP assemblies can be designed to obtain phases ranging from crystals to liquid crystals, allowing the fabrication of flexible free-standing sheets with preservation of the long-range ordering of MOPs. Furthermore, future prospects of the modular design for porous soft materials are provided with the anticipation that the bottom-up design will combine porous materials and soft matter sciences, leading to the discovery and development of many unexplored new materials and devices such as MOF-based self-healing membranes possessing well-defined nanochannels. The macroscopic alignment of channels can be controlled by external factors, including electric and magnetic fields, external forces, and modified surfaces (templating and patterning), which are conventionally used for engineering of soft materials.

Finely Controlled Stepwise Engineering of Pore Environments and Mechanistic Elucidation of Water-Stable, Flexible 2D Porous Coordination Polymers.

Two porous coordination polymers (PCPs) with different topologies (NTU-19: sql and NTU-20: dia) underwent finely controlled, stepwise crystal conversions to yield a common water-stable, flexible 2D framework (NTU-22: kgm). The crystal conversions occurred directly at higher temperature via the 3D intermediate (NTU-21: nbo), which could be observed at lower temperature. The successful isolation of the intermediate product of NTU-21, characterization with in situ PXRD and UV/Vis spectra were combined with DFT calculations to allow an understanding of the dynamic processes at the atomic level. Remarkably, breakthrough experiments demonstrate NTU-22 with integral structural properties allowed significant CO2 /CH4 mixture separation.

Atomic Force Microscopy Study of the Influence of the Synthesis Conditions on the Single-Crystal Surface of Interdigitated Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) containing two or more types of metal ions exhibit promising new functionalities. To precisely control the distribution of metal ions, the relationship between the crystal-growth mechanism and the metal-ion type is important. Therefore, we investigated this relationship by using atomic force microscopy (AFM), which provided significant information about crystal-growth processes. We focus on interdigitated MOFs of the type: {[M(ip)(bpy)]⋅solvent}n (M=Zn or Cd, ip=isophthalate, bpy=4,4'-bipyridine, solvent=water or N,N-dimethylformamide). AFM images show two surface condition types that strongly depend on the solvent or synthetic conditions. We also demonstrate that the adsorption properties between different crystal surfaces are affected by the surface conditions. Finally, we synthesize solid-solution MOFs and observe their surface conditions. Such mechanistic insights will aid in the design of the adsorption properties of multi-metal MOFs.

Efficient CO2 Removal for Ultra-Pure CO Production by Two Hybrid Ultramicroporous Materials.

Removal of CO2 from CO gas mixtures is a necessary but challenging step during production of ultra-pure CO as processed from either steam reforming of hydrocarbons or CO2 reduction. Herein, two hybrid ultramicroporous materials (HUMs), SIFSIX-3-Ni and TIFSIX-2-Cu-i, which are known to exhibit strong affinity for CO2 , were examined with respect to their performance for this separation. The single-gas CO sorption isotherms of these HUMs were measured for the first time and are indicative of weak affinity for CO and benchmark CO2 /CO selectivity (>4000 for SIFSIX-3-Ni). This prompted us to conduct dynamic breakthrough experiments and compare performance with other porous materials. Ultra-pure CO (99.99 %) was thereby obtained from CO gas mixtures containing both trace (1 %) and bulk (50 %) levels of CO2 in a one-step physisorption-based separation process.

Reversible Switching between Highly Porous and Nonporous Phases of an Interpenetrated Diamondoid Coordination Network That Exhibits Gate-Opening at Methane Storage Pressures.

Herein, we report that a new flexible coordination network, NiL2 (L=4-(4-pyridyl)-biphenyl-4-carboxylic acid), with diamondoid topology switches between non-porous (closed) and several porous (open) phases at specific CO2 and CH4 pressures. These phases are manifested by multi-step low-pressure isotherms for CO2 or a single-step high-pressure isotherm for CH4 . The potential methane working capacity of NiL2 approaches that of compressed natural gas but at much lower pressures. The guest-induced phase transitions of NiL2 were studied by single-crystal XRD, in situ variable pressure powder XRD, synchrotron powder XRD, pressure-gradient differential scanning calorimetry (P-DSC), and molecular modeling. The detailed structural information provides insight into the extreme flexibility of NiL2 . Specifically, the extended linker ligand, L, undergoes ligand contortion and interactions between interpenetrated networks or sorbate-sorbent interactions enable the observed switching.

Cooperative Bond Scission in a Soft Porous Crystal Enables Discriminatory Gate Opening for Ethylene over Ethane.

Here we report a soft porous crystal possessing hemilabile cross-links in its framework that exhibits exclusive gate opening for ethylene, enabling the discriminatory adsorption of ethylene over ethane. A Co-based porous coordination polymer (PCP) bearing vinylogous tetrathiafulvalene (VTTF) ligands, [Co(VTTF)], forms Co-S bonds as intermolecular cross-links in its framework in the evacuated closed state. The PCP recognizes ethylene via d-π complexation on the accessible metal site that displaces and cleaves the Co-S bond to "unlock" the closed structure. This ethylene-triggered unlocking event facilitates remarkable nonporous-to-porous transformations that open up accessible void space. This structural transformation follows a two-step gate-opening process. Each phase, including the intermediate structure, was successfully characterized by single-crystal X-ray diffraction analysis, which revealed an intriguing "half-open" structure suggestive of a disproportionate gate-opening phenomenon. The gate-opening mechanism was also investigated theoretically; density functional theory and Monte Carlo calculations revealed that the unique "half-open" phase corresponds to a substantially stable intermediate over the possible transformation trajectories. While ethylene opens the gate, ethane does not because it is unable to coordinate to the Co center. This feature is maintained even at pressures above 1 MPa and at a temperature of 303 K, demonstrating the potential of the "gate-locking/unlocking" mechanism that exploits the hemilabile cross-linking in soft porous crystals.

Metal-Organic Polyhedral Core as a Versatile Scaffold for Divergent and Convergent Star Polymer Synthesis.

We herein report the divergent and convergent synthesis of coordination star polymers (CSP) by using metal-organic polyhedrons (MOPs) as a multifunctional core. For the divergent route, copper-based great rhombicuboctahedral MOPs decorated with dithiobenzoate or trithioester chain transfer groups at the periphery were designed. Subsequent reversible addition-fragmentation chain transfer (RAFT) polymerization of monomers mediated by the MOPs gave star polymers, in which 24 polymeric arms were grafted from the MOP core. On the other hand, the convergent route provided identical CSP architectures by simple mixing of a macroligand and copper ions. Isophthalic acid-terminated polymers (so-called macroligands) immediately formed the corresponding CSPs through a coordination reaction with copper(II) ions. This convergent route enabled us to obtain miktoarm CSPs with tunable chain compositions through ligand mixing alone. This powerful method allows instant access to a wide variety of multicomponent star polymers that conventionally have required highly skilled and multistep syntheses. MOP-core CSPs are a new class of star polymer that can offer a design strategy for highly processable porous soft materials by using coordination nanocages as a building component.

Forced unfolding of single-chain polymeric nanoparticles.

Atomic force microscopy (AFM)-based single-molecule force spectroscopy (SMFS) is applied to single-chain polymeric nanoparticles (SCPNs) to acquire information about the internal folding structure of SCPNs and inherent kinetic parameters of supramolecular self-assembling motifs embedded into the SCPNs. The SCPNs used here are polyacrylate-based polymers carrying 2-ureido-4-[1H]-pyrimidinone (UPy) or benzene-1,3,5-tricarboxamide (BTA) pendants that induce an intramolecular chain collapse into nanoparticles consisting of one polymer chain only via internal supramolecular cross-linking. The SCPN is stretched by an AFM cantilever to unfold mechanically, which allows measuring of force-extension profiles of the SCPNs. Consecutive peaks observed in the force profiles are attributed to rupture events of self-assembled UPy/BTA units in the SCPNs. The force profiles have been analyzed statistically for a series of polymers with different UPy/BTA incorporation densities. The results provide insights into the internal conformation of SCPNs, where the folding structure can be changed with the incorporation density of UPy/BTA. In addition, dynamic loading rate analysis allows the determination of kinetic parameters of BTA self-assembly, which has not been accessible by any other method. This study offers a rational tool for understanding the folding structure, kinetics, and pathway of two series of SCPNs.

Orthogonal self-assembly in folding block copolymers.

We herein report the synthesis and characterization of ABA triblock copolymers that contain two complementary association motifs and fold into single-chain polymeric nanoparticles (SCPNs) via orthogonal self-assembly. The copolymers were prepared using atom-transfer radical polymerization (ATRP) and possess different pendant functional groups in the A and B blocks (alcohols in the A block and acetylenes in the B block). After postfunctionalization, the A block contains o-nitrobenzyl-protected 2-ureidopyrimidinone (UPy) moieties and the B block benzene-1,3,5-tricarboxamide (BTA) moieties. While the protected UPy groups dimerize after photoinduced deprotection of the o-nitrobenzyl group, the BTA moieties self-assemble into helical aggregates when temperature is reduced. In a two-step thermal/photoirradiation treatment under dilute conditions, the ABA block copolymer forms both BTA-based helical aggregates and UPy dimers intramolecularly. The sequential association of the two self-assembling motifs results in single-chain folding of the polymer, affording nanometer-sized particles with a compartmentalized interior. Variable-temperature NMR studies showed that the BTA and UPy self-assembly steps take place orthogonally (i.e., without mutual interference) in dilute solution. In addition, monitoring of the intramolecular self-assembly of BTA moieties into helical aggregates by circular dichroism spectroscopy showed that the stability of the aggregates is almost independent of UPy dimerization. Size-exclusion chromatography (SEC) and small-angle X-ray scattering analysis provided evidence of significant reductions in the hydrodynamic volume and radius of gyration, respectively, after photoinduced deprotection of the UPy groups; a 30-60% reduction in the size of the polymer chains was observed using SEC in CHCl(3). Molecular imaging by atomic force microscopy (AFM) corroborated significant contraction of individual polymer chains due to intramolecular association of the BTA and UPy groups. The stepwise folding process resulting from orthogonal self-assembly-induced supramolecular interactions yields compartmentalized SCPNs comprised of distinct microdomains that mimick two secondary-structuring elements in proteins.

Topological entrapment of macromolecules during the formation of metal-organic framework.

Here we present our preliminary results on a novel approach to encapsulate large guest molecules in nanoporous materials, metal-organic frameworks (MOFs), via a newly discovered in situ crystal formation. This method has exciting prospects not only in the design of new organic/inorganic hybrids but also in capturing and separating molecules that are significantly larger than the actual pore size of the host MOF.

An approach to MOFaxanes by threading ultralong polymers through metal-organic framework microcrystals.

Mechanically interlocked architecture has inspired the fabrication of numerous molecular systems, such as rotaxanes, catenanes, molecular knots, and their polymeric analogues. However, to date, the studies in this field have only focused on the molecular-scale integrity and topology of its unique penetrating structure. Thus, the topological material design of such architectures has not been fully explored from the nano- to the macroscopic scale. Here, we propose a supramolecular interlocked system, MOFaxane, comprised of long chain molecules penetrating a microcrystal of metal-organic framework (MOF). In this study, we describe the synthesis of polypseudoMOFaxane that is one of the MOFaxane family. This has a polythreaded structure in which multiple polymer chains thread a single MOF microcrystal, forming a topological network in the bulk state. The topological crosslinking architecture is obtained by simply mixing polymers and MOFs, and displays characteristics distinct from those of conventional polyrotaxane materials, including suppression of unthreading reactions.

Toughening and stabilizing MOF crystals via polymeric guest inclusion.

Improving the mechanical properties and stability of metal-organic frameworks (MOFs) is of significant interest due to their practical applications. Herein, we tune the mechanical properties of MOFs by filling the MOF pores with polymer chains. The mechanical properties reflect the filling rate, molecular weight, and inherent flexibility of the polymeric guests, imparting MOFs with improved resilience and toughness against mechanical pressures.