Interaction with the Surrounding Water Plays a Key Role in Determining the Aggregation Propensity of Proteins

Understanding the molecular determinants of the relative propensities of proteins to aggregate in a cellular environment is a central issue for treating protein‐aggregation diseases and developing peptide‐based therapeutics. Despite the expectation that protein aggregation can largely be attributed to direct protein–protein interactions, a crucial role the surrounding water in determining the aggregation propensity of proteins both in vitro and in vivo was identified. The overall protein hydrophobicity, defined solely by the hydration free energy of a protein in its monomeric state sampling its equilibrium structures, was shown to be the predominant determinant of protein aggregation propensity in aqueous solution. Striking discrimination of positively and negatively charged residues by the surrounding water was also found. This effect depends on the protein net charge and plays a crucial role in regulating the solubility of the protein. These results pave the way for the design of aggregation‐resistant proteins as biotherapeutics.     

Structure‐Based Design of Inhibitors of the Aspartic Protease Endothiapepsin by Exploiting Dynamic Combinatorial Chemistry

Structure‐based design (SBD) can be used for the design and/or optimization of new inhibitors for a biological target. Whereas de novo SBD is rarely used, most reports on SBD are dealing with the optimization of an initial hit. Dynamic combinatorial chemistry (DCC) has emerged as a powerful strategy to identify bioactive ligands given that it enables the target to direct the synthesis of its strongest binder. We have designed a library of potential inhibitors (acylhydrazones) generated from five aldehydes and five hydrazides and used DCC to identify the best binder(s). After addition of the aspartic protease endothiapepsin, we characterized the protein‐bound library member(s) by saturation‐transfer difference NMR spectroscopy. Cocrystallization experiments validated the predicted binding mode of the two most potent inhibitors, thus demonstrating that the combination of de novo SBD and DCC constitutes an efficient starting point for hit identification and optimization.     

Turning Tryptophanase into Odor‐Generating Biosensors

An odor‐based sensor system that exploits the metabolic enzyme tryptophanase (TPase) as the key component is reported. This enzyme is able to convert an odorless substrate like S‐methyl‐L ‐cysteine or L ‐tryptophan into the odorous products methyl mercaptan or indole. To make a biosensor, TPase was biotinylated so that it could be coupled with a molecular recognition element, such as an antibody, to develop an ELISA‐like assay. This method was used for the detection of an antibody present in nM concentrations by the human nose. TPase can also be combined with the enzyme pyridoxal kinase (PKase) for use in a coupled assay to detect adenosine 5′‐triphosphate (ATP). When ATP is present in the low μM concentration range, the coupled enzymatic system generates an odor that is easily detectable by the human nose. Biotinylated TPase can be combined with various biotin‐labeled molecular recognition elements, thereby enabling a broad range of applications for this odor‐based reporting system.     

Nucleophilic Reactivity of a Copper(II)–Superoxide Complex

Metal‐bound superoxide intermediates are often implicated as electrophilic oxidants in dioxygen‐activating metalloenzymes. In the nonheme iron α‐ketoglutarate dependent oxygenases and pterin‐dependent hydroxylases, however, Fe^III–superoxide intermediates are postulated to react by nucleophilic attack on electrophilic carbon atoms. By reacting a Cu^II–superoxide complex ( 1 ) with acyl chloride substrates, we have found that a metal–superoxide complex can be a very reactive nucleophile. Furthermore, 1 was found to be an efficient nucleophilic deformylating reagent, capable of Baeyer–Villiger oxidation of a number of aldehyde substrates. The observed nucleophilic chemistry represents a new domain for metal–superoxide reactivity. Our observations provide support for the postulated role of metal–superoxide intermediates in nonheme iron α‐ketoglutarate dependent and pterin‐dependent enzymes.     

Aromatic Homologation by Non‐Chelate‐Assisted RhIII‐Catalyzed CH Functionalization of Arenes with Alkynes

Larger condensed arenes are of interest owing to their electro‐ and photochemical properties. An efficient synthesis is the catalyzed aromatic annulation of a smaller arene with two alkyne molecules. Besides difunctionalized starting materials, directed C? H functionalization can be used for such aromatic homologation. However, thus far the requirement of either pre‐functionalized substrates or suitable directing groups were limiting this approach. Herein, we describe a rhodium(III)‐catalyzed method allowing the use of completely unbiased arenes and internal alkynes. The reaction works best with copper(II) 2‐ethylhexanoate and decabromodiphenyl ether as the oxidant combination. This aromatic annulation tolerates a variety of functional groups and delivers homologated condensed arenes. Aside from simple benzenes, naphthalenes and higher condensed arenes provide access to highly substituted and highly soluble acenes structures having important electronic and photophysical properties.     

Metal‐Triflate‐Catalyzed Synthesis of Polycyclic Tertiary Alcohols by Cyclization of γ‐Allenic Ketones

It has been established that bismuth(III) triflate catalyzes the cyclization of γ‐allenic ketones under mild reaction conditions. This reaction allows the selective formation of polycyclic tertiary alcohols from cyclic ketone derivatives. The resulting dienols can engage in stereoselective cycloadditions to efficiently afford complex polycyclic systems.     

Hexafluoroantimonic Acid Catalysis: Formal [3+2+2] Cycloaddition of Aziridines with Two Alkynes

A practical method for the synthesis of azepine derivatives, a typical seven‐membered heterocyclic ring system, was developed and involves the use of hexafluoroantimonic acid to catalyze a formal [3+2+2] cycloaddition of aziridines with two alkynes. This method was applicable to two of the same or different terminal alkynes for the [3+2+2] cycloaddition with unactivated aziridines, and furnished the corresponding azepine derivatives in good yields with good levels of chemo‐ and regioselectivity. The mechanism was also discussed according to the results of the in situ HRMS and ¹H NMR analysis.     

Isocyanide‐Based Multicomponent Reactions: Concise Synthesis of Spirocyclic Oxindoles with Molecular Complexity by Using a [1,5]‐Hydrogen Shift as the Key Step

A concise multicomponent reaction of isocyanide, α‐substituted allenoate, and methyleneindolinone has been disclosed. This protocol provides a fast and straightforward approach to synthesize unusual tricyclic oxindoles in an efficient and atom‐economic manner. Mechanistically, the present cycloaddition may proceed through a cascade sequence involving double Michael addition, double cyclization, double [1,5]‐hydrogen shift, and group migration. The introduction of a special alkyl group to the allenoate is believed to play a key role in the cascade reaction. This method also features a broad substrate scope, which is particularly useful for the delivery of a large number of compounds.     

Mechanistic Insights into the Pd‐Catalyzed Direct Amination of Allyl Alcohols: Evidence for an Outer‐Sphere Mechanism Involving a Palladium Hydride Intermediate

The mechanism of direct amination of allyl alcohol by a palladium triphenylphosphite complex has been explored. Labelling studies show that the reaction proceeds through a π‐allylpalladium intermediate. A second‐order dependence of reaction rate on allyl alcohol concentration was observed. Kinetic isotope effect studies and ESI‐MS studies are in agreement with a reaction proceeding through a palladium hydride intermediate in which both O–H bond and C–O bond cleavages are involved in rate‐determining steps. A stereochemical study supports an outer‐sphere nucleophilic attack of the π‐allylpalladium intermediate giving complete chiral transfer from starting material to product.     

The Benzylperoxyl Radical as a Source of Hydroxyl and Phenyl Radicals

The benzyl radical ( 1 ) is a key intermediate in the combustion and tropospheric oxidation of toluene. Because of its relevance, the reaction of 1 with molecular oxygen was investigated by matrix‐isolation IR and EPR spectroscopy as well as computational methods. The primary reaction product of 1 and O₂ is the benzylperoxyl radical ( 2 ), which exists in several conformers that can easily interconvert even at cryogenic temperatures. Photolysis of radical 2 at 365 nm results in a formal [1,3]‐H migration and subsequent cleavage of the O?O bond to produce a hydrogen‐bonded complex between the hydroxyl radical and benzaldehyde ( 4 ). Prolonged photolysis produces the benzoyl radical ( 5 ) and water, which finally yield the phenyl radical ( 7 ), CO, and H₂O. Thus, via a sequence of exothermic reactions 1 is transformed into radicals of even higher reactivity, such as OH and 7 . Our results have implications for the development of models for the highly complicated process of combustion of aromatic compounds.