Assembly of π-Stacking Helical Peptides into a Porous and Multivariable Proteomimetic Framework.

The evolution of proteins from simpler, self-assembled peptides provides a powerful blueprint for the design of complex synthetic materials. Previously, peptide-metal frameworks using short sequences (≤3 residues) have shown great promise as proteomimetic materials that exhibit sophisticated capabilities. However, their development has been hindered due to few variable residues and restricted choice of side-chains that are compatible with metal ions. Herein, we developed a noncovalent strategy featuring π-stacking bipyridyl residues to assemble much longer peptides into crystalline frameworks that tolerate even previously incompatible acidic and basic functionalities and allow an unprecedented level of pore variations. Single-crystal X-ray structures are provided for all variants to guide and validate rational design. These materials exhibit hallmark proteomimetic behaviors such as guest-selective induced fit and assembly of multimetallic units. Significantly, we demonstrate facile optimization of the framework design to substantially increase affinity toward a complex organic molecule.

Peptidic Scaffolds Enable Rapid and Multivariate Secondary Sphere Evolution for an Abiotic Metallocatalyst.

Metalloenzymes have benefited from the iterative process of evolution to achieve the precise arrangements of secondary sphere non-covalent interactions that enhance metal-centered catalysis. Iterative synthesis of scaffolds that display complex secondary sphere elements in abiotic systems can be highly challenging and time-intensive. To overcome this synthetic bottleneck, we developed a highly modular and rapid synthetic strategy, leveraging the efficiency of solid-phase peptide synthesis and conformational control afforded by non-canonical residues to construct a ligand platform displaying up to four unique residues of varying electronics and sterics in the secondary coordination sphere. As a proof-of-concept that peptidic secondary sphere can cooperate with the metal complex, we applied this scaffold to a well-known, modestly active C-H oxidizing Fe catalyst to evolve specific non-covalent interactions that is more than double its catalytic activity. Solution-state NMR structures of several catalyst variants suggest that higher activity is correlated with a hydrophobic pocket above the Fe center that may enhance the formation of a catalyst-substrate complex. Above all, we show that peptides are a convenient, highly modular, and structurally defined ligand platform for creating secondary coordination spheres that comprise multiple, diverse functional groups.

A Diuranyl(VI) Complex and Its Application in Electrocatalytic and Photocatalytic Hydrogen Evolution from Neutral Aqueous Medium.

The reaction of equimolar amounts of UO2(OAc)2·2H2O, 2,6-diformyl-4-methylphenol, and N-(hydroxyethyl)ethylenediamine in methanol affords a dinuclear trans-uranyl(VI) complex of the molecular formula [(UO2)2(µ-L)2] (L2- = 2-formyl-4-methyl-6-((2-(2-oxidoethylamino)ethylimino)methyl)phenolate) in 65% yield. Detailed structural elucidation of the complex was performed by using single-crystal X-ray crystallographic and spectroscopic studies. In [(UO2)2(µ-L)2], the metal centers are in edge-shared pentagonal-bipyramidal N2O5 coordination spheres assembled by the two meridional ONNO-donor bridging L2- and two pairs of mutually trans oriented oxo groups. The complex is redox active and displays two successive metal-centered one-electron reductions at Epc = -0.71 and -1.03 V in N,N-dimethylformamide solution. The redox-active complex was used as a heterogeneous catalyst for electrochemical hydrogen evolution from aqueous medium at pH 7 with a turnover frequency (TOF) of 384 h-1 and a Tafel slope of 274 mV dec-1. The Faradaic efficiency of [(UO2)2(µ-L)2] was found to be 84%. Beyond the electrocatalytic response, the [(UO2)2(µ-L)2]-TiO2-N719 composite also exhibited significant heterogeneous photocatalytic hydrogen evolution activity in neutral aqueous medium under visible light and provided a yield of 3439 µmol gcat-1 of H2 in 4 h with a TOF of 172 h-1 and apparent quantum yield (AQY) of 7.6%.

Recognition of steric factor in external association of xanthenocrown-5 and bis-napthalenocrown-6 with bis(benzimidazolium)propane borontetrafluoride.

This is the first report of charge transfer band for crown-axel H-bonding interaction in acetonitrile medium. Monte Carlo simulation established external association of small cavity crown with BBIM-propane dication axels. Resonance energy, binding constant and ratiometric detection of association of xanthenocrown-5 (1) and bis-napthalenocrown-6 (2) with bis-(benzimidazolium)propane borontetrafluoride (3a-3d) in acetonitrile determined the effect of steric factor towards association.

Unraveling the molecular recognition of "three methylene spacer" bis(benzimidazolium) moiety by dibenzo-24-crown-8: pseudorotaxanes under study.

"Three methylene spacer" bis(benzimidazolium) derivatives act as a new template for threading dibenzo-24-crown-8 into [2]pseudorotaxanes. In this Article we sought to unveil the difference in the extent of threading of various "three methylene spacer" bis-benzimidazolium moieties based on differences in aromatic methyl substituent positions and anions through the macrocycle dibenzo-24-crown-8. The temperature of the systems were also varied (low temperatures of 240 K and 253 K), when such interwinding of the thread and crown were not detected at room temperature of 308 K. The presence of such threaded complexes was determined based on (1)H-NMR initially and finally corroborated by high resolution mass spectrometry and DFT calculations. 2D-NMR experiments ((1)H-(1)H-NOESY) proved to be a very important tool in elucidating the interaction present between the components of the pseudorotaxanes. The dethreading/rethreading process was studied. DFT optimized structures suggest lower energy H-bonding orientations and compare the effect of methyl substituents on the axle.