Pore Restructuring of Peptide Frameworks by Mutations at Distal Packing Residues.

Porous framework materials are highly useful for catalysis, adsorption, and separations. Though they are usually made from inorganic and organic building blocks, recently, folded peptides have been utilized for constructing frameworks, opening up an enormous structure-space for exploration. These peptides assemble in a metal-free fashion using π-stacking, H-bonding, dispersion forces, and the hydrophobic effect. Manipulation of pore-defining H-bonding residues is known to generate new topologies, but the impact of mutations in the hydrophobic packing region facing away from the pores is less obvious. To explore their effects, we synthesized variants of peptide frameworks with mutations in the hydrophobic packing positions and found by single-crystal X-ray crystallography (SC-XRD) that they induce significant changes to the framework pore structure. These structural changes are driven by a need to maximize van der Waals interactions of the nonpolar groups, which are achieved by various mechanisms including helix twisting, chain flipping, chain offsetting, and desymmetrization. Even subtle changes to the van der Waals interface, such as the introduction of a methyl group or isomeric replacement, result in significant pore restructuring. This study shows that the dispersion interactions upholding a peptide material are a rich area for structural engineering.

Reactive high-spin iron(IV)-oxo sites through dioxygen activation in a metal-organic framework.

In nature, nonheme iron enzymes use dioxygen to generate high-spin iron(IV)=O species for a variety of oxygenation reactions. Although synthetic chemists have long sought to mimic this reactivity, the enzyme-like activation of O2 to form high-spin iron(IV) = O species remains an unrealized goal. Here, we report a metal-organic framework featuring iron(II) sites with a local structure similar to that in α-ketoglutarate-dependent dioxygenases. The framework reacts with O2 at low temperatures to form high-spin iron(IV) = O species that are characterized using in situ diffuse reflectance infrared Fourier transform, in situ and variable-field Mössbauer, Fe Kß x-ray emission, and nuclear resonance vibrational spectroscopies. In the presence of O2, the framework is competent for catalytic oxygenation of cyclohexane and the stoichiometric conversion of ethane to ethanol.

Mussel Adhesive-Inspired Proteomimetic Polymer.

Herein, a synthetic polymer proteomimetic is described that reconstitutes the key structural elements and function of mussel adhesive protein. The proteomimetic was prepared via graft-through ring-opening metathesis polymerization of a norbornenyl-peptide monomer. The peptide was derived from the natural underwater glue produced by marine mussels that is composed of a highly repetitive 10 amino acid tandem repeat sequence. The hypothesis was that recapitulation of the repeating unit in this manner would provide a facile route to a nature-inspired adhesive. To this end, the material, in which the arrangement of peptide units was as side chains on a brush polymer rather than in a linear fashion as in the natural protein, was examined and compared to the native protein. Mechanical measurements of adhesion forces between solid surfaces revealed improved adhesion properties over the natural protein, making this strategy attractive for diverse applications. One such application is demonstrated, using the polymers as a surface adhesive for the immobilization of live cells.

Biomimetic O2 adsorption in an iron metal-organic framework for air separation.

Bio-inspired motifs for gas binding and small molecule activation can be used to design more selective adsorbents for gas separation applications. Here, we report an iron metal-organic framework, Fe-BTTri (Fe3[(Fe4Cl)3(BTTri)8]2·18CH3OH, H3BTTri = 1,3,5-tris(1H-1,2,3-triazol-5-yl)benzene), that binds O2 in a manner similar to hemoglobin and therefore results in highly selective O2 binding. As confirmed by gas adsorption studies and Mössbauer and infrared spectroscopy data, the exposed iron sites in the framework reversibly adsorb substantial amounts of O2 at low temperatures by converting between high-spin, square-pyramidal Fe(ii) centers in the activated material to low-spin, octahedral Fe(iii)-superoxide sites upon gas binding. This change in both oxidation state and spin state observed in Fe-BTTri leads to selective and readily reversible O2 binding, with the highest reported O2/N2 selectivity for any iron-based framework.

Chemiresistive Detection of Gaseous Hydrocarbons and Interrogation of Charge Transport in Cu[Ni(2,3-pyrazinedithiolate)2] by Gas Adsorption.

The development of new chemiresistive materials for use in chemical sensors that operate near ambient conditions could potentially reduce the costs of implementation, encouraging their use in new areas. Conductive metal-organic frameworks represent one intriguing class of materials for further investigation in this area, given their vast structural diversity and the specificity of adsorbate interactions afforded by their crystallinity. Here, we re-examine the electronic conductivity of the desolvated and acetonitrile-solvated microporous framework Cu[Ni(pdt)2] (pdt2- = 2,3-pyrazinedithiolate), and find that the conductivity in the pristine material is 200-fold greater than in the solvated state, highlighting the sensitivity of sample conductivity to guest inclusion. Additionally, the desolvated material is demonstrated to selectively adsorb the gaseous hydrocarbons ethane, ethylene, acetylene, propane, propylene, and cis-2-butene at ambient temperature. Investigation of the effect of gas adsorption on conductivity using an in situ measurement cell reveals a chemiresistive response for each adsorbate, and the change in conductivity with adsorbate pressure closely follows an empirical model identical in form to the Langmuir-Freundlich equation. The relative sensitivity of the framework to each adsorbate is, surprisingly, not correlated with binding strength. Instead, the differences in chemiresistive response between adsorbates are found to correlate strongly with gas phase specific heat capacity of the adsorbate. Nanoconfinement effects, manifesting as a relative deviation from the expected chemiresistive response, may influence charge transport in the case of the largest adsorbate considered, cis-2-butene. Time-resolved conductance and adsorption measurements additionally show that the chemiresistive response of the sensor equilibrates on a shorter time scale than gas adsorption, suggesting that interparticle contacts limit conduction through the bulk material and that conductivity at the crystallite surfaces is most responsive to gas adsorption.

Ethylene oligomerization in metal-organic frameworks bearing nickel(ii) 2,2'-bipyridine complexes.

The metal-organic frameworks Zr6O4(OH)4(bpydc)6 (1; bpydc2- = 2,2'-bipyridine-5,5'-dicarboxylate) and Zr6O4(OH)4(bpydc)0.84(bpdc)5.16 (2; bpdc2- = biphenyl-4,4'-dicarboxylate) were readily metalated with Ni(DME)Br2 (DME = dimethoxyethane) to produce the corresponding metalated frameworks 1(NiBr2)6 and 2(NiBr2)0.84. Both nickel(ii)-containing frameworks catalyze the oligomerization of ethylene in the presence of Et2AlCl. In these systems, the pore environment around the active nickel sites significantly influences their selectivity for formation of oligomers over polymer. Specifically, the single-crystal structure of 1(NiBr2)5.64 reveals that surrounding metal-linker complexes enforce a steric environment on each nickel site that causes polymer formation to become favorable. Minimizing this steric congestion by isolating the nickel(ii) bipyridine complexes in the mixed-linker framework 2(NiBr2)0.84 markedly improves both the catalytic activity and selectivity for oligomers. Furthermore, both frameworks give product mixtures that are enriched in shorter olefins (C4-10), leading to deviations from the expected Schulz-Flory distribution of oligomers. Although these deviations indicate possible pore confinement effects on selectivity, control experiments using the nickel-treated biphenyl framework Zr6O4(OH)4(bpdc)6(NiBr2)0.14 (3(NiBr2)0.14) reveal that they likely arise at least in part from the presence of nickel species that are not ligated by bipyridine within 1(NiBr2)5.64 and 2(NiBr2)0.84.