Understanding and controlling the nucleation and growth of metal-organic frameworks.

Metal-organic frameworks offer a diverse landscape of building blocks to design high performance materials for implications in almost every major industry. With this diversity stems complex crystallization mechanisms with various pathways and intermediates. Crystallization studies have been key to the advancement of countless biological and synthetic systems, with MOFs being no exception. This review provides an overview of the current theories and fundamental chemistry used to decipher MOF crystallization. We then discuss how intrinsic and extrinsic synthetic parameters can be used as tools to modulate the crystallization pathway to produce MOF crystals with finely tuned physical and chemical properties. Experimental and computational methods are provided to guide the probing of MOF crystal formation on the molecular and bulk scale. Lastly, we summarize the recent major advances in the field and our outlook on the exciting future of MOF crystallization.

A Close Look at Molecular Self-Assembly with the Transmission Electron Microscope.

Molecular self-assembly is pervasive in the formation of living and synthetic materials. Knowledge gained from research into the principles of molecular self-assembly drives innovation in the biological, chemical, and materials sciences. Self-assembly processes span a wide range of temporal and spatial domains and are often unintuitive and complex. Studying such complex processes requires an arsenal of analytical and computational tools. Within this arsenal, the transmission electron microscope stands out for its unique ability to visualize and quantify self-assembly structures and processes. This review describes the contribution that the transmission electron microscope has made to the field of molecular self-assembly. An emphasis is placed on which TEM methods are applicable to different structures and processes and how TEM can be used in combination with other experimental or computational methods. Finally, we provide an outlook on the current challenges to, and opportunities for, increasing the impact that the transmission electron microscope can have on molecular self-assembly.

Direct Observation of Amorphous Precursor Phases in the Nucleation of Protein-Metal-Organic Frameworks.

Protein-metal-organic frameworks (p-MOFs) are a prototypical example of how synthetic biological hybrid systems can be used to develop next-generation materials. Controlling p-MOF formation enables the design of hybrid materials with enhanced biological activity and high stability. However, such control is yet to be fully realized due to an insufficient understanding of the governing nucleation and growth mechanisms in p-MOF systems. The structural evolution of p-MOFs was probed by cryo-transmission electron microscopy, revealing nonclassical pathways via dissolution-recrystallization of highly hydrated amorphous particles and solid-state transformation of a protein-rich amorphous phase. On the basis of these data, we propose a general description of p-MOF crystallization which is best characterized by particle aggregation and colloidal theory for future synthetic strategies.

Cyclodextrin metal-organic framework-based protein biocomposites.

Materials are needed to increase the stability and half-life of therapeutic proteins during delivery. These materials should be biocompatible and biodegradable. Here, we demonstrate that enzymes and immunoproteins can be encapsulated inside cyclodextrin based metal-organic frameworks using potassium as the metal node. The release profile can be controlled with the solubility of the cyclodextrin linker. The activity of the proteins after release is determined using catalytic and in vitro assays. The results show that cyclodextrin metal-organic framework-based protein biocomposites are a promising class of materials to deliver therapeutic proteins.

Role of Molecular Modification and Protein Folding in the Nucleation and Growth of Protein-Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) are a class of porous nanomaterials that have been extensively studied as enzyme immobilization substrates. During in situ immobilization, MOF nucleation is driven by biomolecules with low isoelectric points. Investigation of how biomolecules control MOF self-assembly mechanisms on the molecular level is key to designing nanomaterials with desired physical and chemical properties. Here, we demonstrate how molecular modifications of bovine serum albumin (BSA) with fluorescein isothiocyanate (FITC) can affect MOF crystal size, morphology, and encapsulation efficiency. Final crystal properties are characterized using scanning electron microscopy (SEM), powder X-ray diffraction (PXRD), fluorescent microscopy, and fluorescence spectroscopy. To probe MOF self-assembly, in situ experiments were performed using cryogenic transmission electron microscopy (cryo-TEM) and X-ray diffraction (XRD). Biophysical characterization of BSA and FITC-BSA was performed using ζ potential, mass spectrometry, circular dichroism studies, fluorescence spectroscopy, and Fourier transform infrared (FTIR) spectroscopy. The combined data reveal that protein folding and stability within amorphous precursors are contributing factors in the rate, extent, and mechanism of crystallization. Thus, our results suggest molecular modifications as promising methods for fine-tuning protein@MOFs' nucleation and growth.

Aqueous RAFT Synthesis of Low Molecular Weight Anionic Polymers for Determination of Structure/Binding Interactions with Gliadin.

Gliadin, a component of gluten and a known epitope, is implicated in celiac disease (CeD) and results in an inflammatory response in CeD patients when consumed. Acrylamide-based polyelectrolytes are employed as models to determine the effect of molecular weight and pendent group on non-covalent interaction modes with gliadin in vitro. Poly(sodium 2-acrylamido-2-methylpropane sulfonate) and poly(sodium 3-methylpropyl-3-butanoate) are synthesized via aqueous reversible addition fragmentation chain transfer (aRAFT) polymerization and characterized by gel permeation chromatography-multiangle laser light scattering. The polymer/gliadin blends are examined via circular dichroism, zeta potential measurements, 8-anilinonaphthalene-1-sulfonic acid fluorescence spectroscopy, and dynamic light scattering. Acrylamide polymers containing strong anionic pendent groups have a profound effect on gliadin secondary structure and solution behavior below the isoelectric point, while polymers containing hydrophobic character only have a minor impact. The polymers have little effect on gliadin secondary structure and solution behavior at the isoelectric point.

The Droserasin 1 PSI: A Membrane-Interacting Antimicrobial Peptide from the Carnivorous Plant Drosera capensis.

The Droserasins, aspartic proteases from the carnivorous plant Drosera capensis, contain a 100-residue plant-specific insert (PSI) that is post-translationally cleaved and independently acts as an antimicrobial peptide. PSIs are of interest not only for their inhibition of microbial growth, but also because they modify the size of lipid vesicles and strongly interact with biological membranes. PSIs may therefore be useful for modulating lipid systems in NMR studies of membrane proteins. Here we present the expression and biophysical characterization of the Droserasin 1 PSI (D1 PSI.) This peptide is monomeric in solution and maintains its primarily α -helical secondary structure over a wide range of temperatures and pH values, even under conditions where its three disulfide bonds are reduced. Vesicle fusion assays indicate that the D1 PSI strongly interacts with bacterial and fungal lipids at pH 5 and lower, consistent with the physiological pH of D. capensis mucilage. It binds lipids with a variety of head groups, highlighting its versatility as a potential stabilizer for lipid nanodiscs. Solid-state NMR spectra collected at a field strength of 36 T, using a unique series-connected hybrid magnet, indicate that the peptide is folded and strongly bound to the membrane. Molecular dynamics simulations indicate that the peptide is stable as either a monomer or a dimer in a lipid bilayer. Both the monomer and the dimer allow the passage of water through the membrane, albeit at different rates.