Artificial Esterase for Cooperative Catalysis of Ester Hydrolysis at pH 7.

Ester is one of the most prevalent functional groups in natural and man-made products. Natural esterases hydrolyze nonactivated alkyl esters readily but artificial esterases generally use highly activated p-nitrophenyl esters as substrates. We report synthetic esterases constructed through molecular imprinting in cross-linked micelles. The water-soluble nanoparticle catalysts contain a thiouronium cation to mimic the oxyanion hole and a nearby base to assist the hydrolysis. Whereas this catalytic motif readily affords large rate acceleration for the hydrolysis of p-nitrophenyl hexanoate, nonactivated cyclopentyl hexanoate demands catalytic groups that can generate a strong nucleophile (hydroxide) in the active site. The hydroxide is stabilized by the protonated base when the external solution is at pH 7, enabling the hydrolysis of activated and nonactivated esters under neutral conditions.

Supramolecular Regulation of Catalytic Activity in Molecularly Responsive Catalysts.

Some enzymes switch between an open form and a closed form. We report a molecularly tuned catalyst that accommodates a substrate and a signal molecule simultaneously. Binding of the signal molecule helps direct the reactive group of the substrate to the catalytic group and enhances the catalytic activity. Subtle structural changes in either the substrate or the signal molecule are readily detected. The switching mechanism also allows the catalytic reaction to be turned on and off reversibly by specific molecular signals.

Acid-Functionalized Artificial Enzymes with Tunable Selectivity for Vinyl Ether Hydrolysis.

Enzymes often employ catalytic groups with a medium or low intrinsic activity for highly challenging catalytic tasks. In this work, we report nanoparticle catalysts with accurately positioned carboxylic acids through either a covalent or noncovalent imprinting technique. The rationally designed active site allows the catalysis to be highly selective or quite unselective with respect to the substrate. With the proper catalyst, the hydrolysis proceeds smoothly in neutral water or even a slightly basic solution at room temperature.

Site-Selective Catalytic Epoxidation of Alkene with Tunable, Atomic Precision by Molecularly Imprinted Artificial Epoxidases.

Distinction of chemical functionality by their local chemical environment is a skill mastered by enzymes, evident from the selective synthesis, cleavage, and transformation of peptides, nucleic acids, and polysaccharides that abound with the same type of functional groups. In contrast, synthetic catalysts are generally better at differentiating functional groups based on their electronic and steric properties. Here we report artificial epoxidases prepared through molecular imprinting of surface-core doubly cross-linked micelles, followed by efficient functionalization of the imprinted site in the micellar core via photoaffinity labeling. The size and shape of the active sites are tuned by the modularly synthesized templates, with the oxygen-delivering peroxy acid group positioned accurately. These catalysts are used in epoxidation of alkene in water with hydrogen peroxide under mild conditions, without any additional additives. Most importantly, atomic precision is achieved in the catalysis and enables alkenes to be distinguished that differ in the position of the carbon-carbon double bond by a single carbon.

Imprinted Polymeric Nanoparticles as Artificial Enzymes for Ester Hydrolysis at Room Temperature and pH 7.

Natural esterases hydrolyze esters under physiological pHs but chemists often have to use strongly acidic or basic conditions for the same hydrolysis. We report synthetic nanoparticle catalysts that hydrolyze nonactivated alkyl esters at room temperature and neutral pH, with enzyme-like catalytic mechanisms and exquisite substrate selectivity. Unlike natural enzymes that denature easily at elevated temperatures, the synthetic catalysts become more active at higher temperatures.

Selective Hydrolysis of Aryl Esters under Acidic and Neutral Conditions by a Synthetic Aspartic Protease Mimic.

Aspartic proteases use a pair of carboxylic acids to activate water molecules for nucleophilic attack. Here we report a nanoparticle catalyst with a similar catalytic motif capable of generating a hydroxide ion in its active site even under acidic reaction conditions. The synthetic enzyme accelerated the hydrolysis of para-nitrophenyl acetate (PNPA) by 91,000 times and could also hydrolyze nonactivated aryl esters at pH 7. The distance between the two acids and, in particular, the flexibility of the catalytic groups in the active site controlled the catalytic efficiency. The synthetic enzyme readily detected the addition of a single methyl on the acyl group of the substrate, as well as the substitution pattern on the phenyl ring.

Tandem Aldol Reaction from Acetal Mixtures by an Artificial Enzyme with Site-Isolated Acid and Base Functionalities.

Site-isolation of catalysts can enable incompatible catalysts such as acid and base to be used in one pot for enhanced efficiency and other benefits. Although many synthetic platforms have been reported for this purpose, they generally do not possess the exquisite selectivity of site-isolated enzymes in nature. Here we report water-soluble protein-sized nanoparticles with site-isolated acids in the core and amines on the surface. The catalysts were made through molecular imprinting of cross-linked micelles, followed by facile one-step photoaffinity labeling of the imprinted binding site. With a tunable, substrate-specific active site, the bifunctional artificial enzyme catalyzed highly selective tandem cross aldol reaction between acetone and mixtures of isomeric aryl acetals. It could also transform a less reactive substrate over a more reactive one.

Tunable Artificial Enzyme-Cofactor Complex for Selective Hydrolysis of Acetals.

Enzymes frequently use unimpressive functional groups such as weak carboxylic acids for efficient, highly selective catalysis including hydrolysis of acetals and even amides. Much stronger acids generally have to be used for such purposes in synthetic systems. We report here a method to position an acidic group near the acetal oxygen of 2-(4-nitrophenyl)-1,3-dioxolane bound by an artificial enzyme. The hydrolytic activity of the resulting artificial enzyme-cofactor complex was tuned by the number and depth of the active site as well as the hydrophobicity and acidity of the cofactor. The selectivity of the complex was controlled by the size and shape of the active site and enabled less reactive acetals to be hydrolyzed over more reactive ones.

pH-Controlled Nanoparticle Catalysts for Highly Selective Tandem Henry Reaction from Mixtures.

Nature has a remarkable ability to perform selective transformation of complex biological mixtures into desired products using enzymatic catalysts. We report the preparation of nanoparticle catalysts through molecular imprinting within cross-linked micelles. These catalysts were highly selective for their targeted substrates and could selectively hydrolyze less reactive acetals over more reactive ones even under basic conditions. Their catalytic activity and selectivity were tunable through rational postmodification of the active site. These properties enabled the nanoparticle catalysts to produce the desired ß-nitro alcohol from a four-component acetal mixture in a tandem deprotection/Henry reaction that required incompatible acidic and basic catalysts in the two steps.

Discovery of an isocoumarin analogue that modulates neuronal functions via neurotrophin receptor TrkB.

Isocoumarins are lactone ring-containing natural products, are quite abundant in microbes and higher plants, and have been shown to exhibit a broad range of pharmacological properties. However, the molecular mechanism or target of this class of molecules is not known. In this study, we have synthesized 14 isocoumarin derivatives and evaluated for their activity at TrkB receptor in transiently transfected HEK293T cells. We identified 8-hydroxy-3-aryl isocoumarin (1) as a high-affinity agonist at the TrkB receptor. We also demonstrated that isocoumarin 1 activated endogenously TrkB receptor in primary cortical neurons and modulated various markers of synaptic plasticity, and increased dendritic arborization. These results indicate therapeutic potential and molecular target of 8-hydroxy-3-aryl isocoumarin 1 for the treatment of various CNS disorders.