Phenylalanine Ammonia Lyase Catalyzed Synthesis of Amino Acids by an MIO‐Cofactor Independent Pathway

Phenylalanine ammonia lyases (PALs) belong to a family of 4‐methylideneimidazole‐5‐one (MIO) cofactor dependent enzymes which are responsible for the conversion of L ‐phenylalanine into trans‐cinnamic acid in eukaryotic and prokaryotic organisms. Under conditions of high ammonia concentration, this deamination reaction is reversible and hence there is considerable interest in the development of PALs as biocatalysts for the enantioselective synthesis of non‐natural amino acids. Herein the discovery of a previously unobserved competing MIO‐independent reaction pathway, which proceeds in a non‐stereoselective manner and results in the generation of both L ‐ and D ‐phenylalanine derivatives, is described. The mechanism of the MIO‐independent pathway is explored through isotopic‐labeling studies and mutagenesis of key active‐site residues. The results obtained are consistent with amino acid deamination occurring by a stepwise E₁cB elimination mechanism.     

Programmable Polymer‐Based Supramolecular Temperature Sensor with a Memory Function

A new class of polymeric thermometers with a memory function is reported that is based on the supramolecular host–guest interactions of poly(N‐isopropylacrylamide) (PNIPAM) with side‐chain naphthalene guest moieties and the tetracationic macrocycle cyclobis(paraquat‐p‐phenylene) (CBPQT⁴⁺) as the host. This supramolecular thermometer exhibits a memory function for the thermal history of the solution, which arises from the large hysteresis of the thermoresponsive LCST phase transition (LCST=lower critical solution temperature). This hysteresis is based on the formation of a metastable soluble state that consists of the PNIPAM–CBPQT⁴⁺ host–guest complex. When heated above the transition temperature, the polymer collapses, and the host–guest interactions are disrupted, making the polymer more hydrophobic and less soluble in water. Aside from providing fundamental insights into the kinetic control of supramolecular assemblies, the developed thermometer with a memory function might find use in applications spanning the physical and biological sciences.     

On the Formation of a Protic Ionic Liquid in Nature

The practical utility of ionic liquids (ILs) makes the absence (heretofore) of reported examples from nature quite puzzling, given the facility with which nature produces many other types of exotic but utilitarian substances. In that vein, we report here the identification and characterization of a naturally occurring protic IL. It can be formed during confrontations between the ants S. invicta and N. fulva. After being sprayed with alkaloid‐based S. invicta venom, N. fulva detoxifies by grooming with its own venom, formic acid. The mixture is a viscous liquid manifestly different from either of the constituents. Further, we find that the change results as a consequence of formic acid protonation of the N centers of the S. invicta venom alkaloids. The resulting mixed‐cation ammonium formate milieu has properties consistent with its classification as a protic IL.     

A Supramolecular Sorting Hat: Stereocontrol in Metal–Ligand Self‐Assembly by Complementary Hydrogen Bonding

A combination of self‐complementary hydrogen bonding and metal–ligand interactions allows stereocontrol in the self‐assembly of prochiral ligand scaffolds. A unique, non‐tetrahedral M₄L₆ structure is observed upon multicomponent self‐assembly of 2,7‐diaminofluorenol with 2‐formylpyridine and Fe(ClO₄)₂. The stereochemical outcome of the assembly is controlled by self‐complementary hydrogen bonding between both individual ligands and a suitably sized counterion as template. This hydrogen‐bonding‐mediated stereoselective metal–ligand assembly allows the controlled formation of nonsymmetric discrete cage structures from previously unexploited ligand scaffolds.     

Metastable Ionic Diodes Derived from an Amine‐Based Polymer of Intrinsic Microporosity

A highly rigid amine‐based polymer of intrinsic microporosity (PIM), prepared by a polymerization reaction involving the formation of Tröger’s base, is demonstrated to act as an ionic diode with electrolyte‐dependent bistable switchable states.     

Facile CH Bond Formation by Reductive Elimination at a Dinuclear Metal Site

The electronically unsaturated dirhenium complex [Re₂(CO)₈(µ‐AuPPh₃)(µ‐Ph)] ( 1 ) was obtained from the reaction of [Re₂(CO)₈{µ‐η²‐C(H)?C(H)nBu}(µ‐H)] with [Au(PPh₃)Ph]. The bridging {AuPPh₃} group was replaced by a bridging hydrido ligand to yield the unsaturated dirhenium complex [Re₂(CO)₈(µ‐H)(µ‐Ph)] ( 2 ) by reaction of 1 with HSnPh₃. Compound 2 reductively eliminates benzene upon addition of NCMe at 25 °C. The electronic structure of 2 and the mechanism of the reductive elimination of the benzene molecule in its reaction with NCMe were investigated by DFT computational analyses.     

Crowning Proteins: Modulating the Protein Surface Properties using Crown Ethers

Crown ethers are small, cyclic polyethers that have found wide‐spread use in phase‐transfer catalysis and, to a certain degree, in protein chemistry. Crown ethers readily bind metallic and organic cations, including positively charged amino acid side chains. We elucidated the crystal structures of several protein‐crown ether co‐crystals grown in the presence of 18‐crown‐6. We then employed biophysical methods and molecular dynamics simulations to compare these complexes with the corresponding apoproteins and with similar complexes with ring‐shaped low‐molecular‐weight polyethylene glycols. Our studies show that crown ethers can modify protein surface behavior dramatically by stabilizing either intra‐ or intermolecular interactions. Consequently, we propose that crown ethers can be used to modulate a wide variety of protein surface behaviors, such as oligomerization, domain–domain interactions, stabilization in organic solvents, and crystallization.     

How Many Molecules are Required to Obtain a Steady Faradaic Current from Mediated Electron Transfer at a Single Nanoparticle on a Supporting Surface?

We investigate the chronoamperometric noise characteristics of electron‐transfer reactions occurring on single nanoparticles (NPs) and assemblies of well‐separated NPs on a supporting surface. To this end, we combine a formerly described expression for the steady‐state current of a single particle with the shot‐noise model and derive an expression for the signal‐to‐noise ratio as a function of bulk concentration and particle radius. Our findings are supported by random‐walk simulations, which closely match the analytical results.