Proton Shuttle-Assisted Surface Reconstruction toward Nonpolar Facets-Terminated Zinc-Blende CdSe/CdS Core/Shell Quantum Dots

Surface reconstruction can rearrange the surface atoms of a crystal without the need of growth processes and has the potential to synthesize crystals with novel morphologies and facets that cannot be obtained through regular synthesis. However, little is known about the molecular mechanisms of the surface reconstruction process. Here, utilizing surface reconstruction, we report the synthesis of nonpolar facets (110) facets)-terminated dodecahedral zinc-blende CdSe/CdS core/shell quantum dots. The morphology transformation is achieved by first fully exchanging the cadmium carboxylate ligand with oleylamine and then undergoing surface reconstruction. The surface reconstruction-induced morphology transformation is confirmed by transmission electron microscopy and absorption spectroscopy. Details of kinetic experiments and simulation results demonstrated that successful surface reconstruction must be assisted by a proton shuttle. Except for the first report on zinc-blende quantum dots terminated with (110) facets, the surface reconstruction aided by the proton shuttle offers valuable insights for devising methods to regulate the properties of nanocrystals.

Effect of the acyl-group length on the chemoselectivity of the lipase-catalyzed acylation of propranolol-a computational study

The selective N-acylation of 1,2-amino alcohols has been proposed to occur through the proton shuttle mechanism. However, the O-acetylation of propranolol catalyzed by Candida antarctica lipase B is an exception. We investigated the relation between the chemoselectivity of this reaction and the acyl group length. For this purpose, we compared the acyl groups: ethanoyl, butanoyl, octanoyl, and hexadecanoyl. We studied the Michaelis complexes between serine-acylated Candida antarctica lipase B and propranolol, employing a computational approach that involved sampling Michaelis complex conformations through ensemble docking plus consensus scoring and molecular dynamics simulations. The conformations were then classified as near attack conformations for acylation of the amino or hydroxy group. The relative populations of these two classes of conformations were found to be consistent with the experimentally observed chemoselective O-acetylation. We predict that increasing the length of the hydrocarbon chain of the acyl group will cause O-acylation to be unfavorable with respect to N-acylation. The nucleophilic attack of propranolol to the acylated lipase was found to be more favorable through the classical mechanism when compared with the proton shuttle mechanism.

Perturbating Intramolecular Hydrogen Bonds through Substituent Effects or Non-Covalent Interactions

An analysis of the effects induced by F, Cl, and Br-substituents at the α-position of both, the hydroxyl or the amino group for a series of amino-alcohols, HOCH2(CH2)nCH2NH2 (n = 0-5) on the strength and characteristics of their OH···N or NH···O intramolecular hydrogen bonds (IMHBs) was carried out through the use of high-level G4 ab initio calculations. For the parent unsubstituted amino-alcohols, it is found that the strength of the OH···N IMHB goes through a maximum for n = 2, as revealed by the use of appropriate isodesmic reactions, natural bond orbital (NBO) analysis and atoms in molecules (AIM), and non-covalent interaction (NCI) procedures. The corresponding infrared (IR) spectra also reflect the same trends. When the α-position to the hydroxyl group is substituted by halogen atoms, the OH···N IMHB significantly reinforces following the trend H < F < Cl < Br. Conversely, when the substitution takes place at the α-position with respect to the amino group, the result is a weakening of the OH···N IMHB. A totally different scenario is found when the amino-alcohols HOCH2(CH2)nCH2NH2 (n = 0-3) interact with BeF2. Although the presence of the beryllium derivative dramatically increases the strength of the IMHBs, the possibility for the beryllium atom to interact simultaneously with the O and the N atoms of the amino-alcohol leads to the global minimum of the potential energy surface, with the result that the IMHBs are replaced by two beryllium bonds.

Optimization of the Lipase-Catalyzed Selective Amidation of Phenylglycinol

Ceramides and their analogs have a regulatory effect on inflammatory cytokines expression. It was found that a kind of ceramides analog synthesized from phenylglycinol could inhibit the production of cytokine TNF-α. However, two active hydrogen groups are present in the phenylglycinol molecule. It is difficult to control the process without hydroxyl group protection to dominantly produce amide in the traditional chemical synthesis. A selective catalytic the amidation route of phenylglycinol by lipases was investigated in this research. The results indicated that the commercial immobilized lipase Novozym 435 has the best regio-selectivity on the amide group. Based on the experimental results and in silico simulation, it was found that the mechanism of specific N-acyl selectivity of lipase was not only from intramolecular migration and proton shuttle mechanism, but also from the special structure of active site of enzyme. The optimal reaction yield of aromatic amide compound in a solvent-free system with lipase loading of 15 wt% (to the weight of total substrate) reached 89.41 ± 2.8% with very few of byproducts detected (0.21 ± 0.1% ester and 0.64 ± 0.2% diacetylated compound). Compare to other reported works, this work have the advantages such as low enzyme loading, solvent free, and high N-acylation selectivity. Meanwhile, this Novozym 435 lipase based synthesis method has an excellent regio-selectivity on most kinds of amino alcohol compounds. Compared to the chemical method, the enzymatic synthesis exhibited high regio-selectivity, and conversion rates. The method could be a promising alternative strategy for the synthesis of aromatic alkanolamides.

Switched reaction specificity in polyesterases towards amide bond hydrolysis by enzyme engineering

The recalcitrance of plastics like nylon and other polyamides contributes to environmental problems (e.g. microplastics in oceans) and restricts possibilities for recycling. The fact that hitherto discovered amidases (EC 3.5.1. and 3.5.2.) only show no, or low, activity on polyamides currently obstructs biotechnological-assisted depolymerization of man-made materials. In this work, we capitalized on enzyme engineering to enhance the promiscuous amidase activity of polyesterases. Through enzyme design we created a reallocated water network adapted for hydrogen bond formation to synthetic amide backbones for enhanced transition state stabilization in the polyester-hydrolyzing biocatalysts Humicola insolens cutinase and Thermobifida cellulosilytica cutinase 1. This novel concept enabled increased catalytic efficiency towards amide-containing soluble substrates. The afforded enhanced hydrolysis of the amide bond-containing insoluble substrate 3PA 6,6 by designed variants was aligned with improved transition state stabilization identified by molecular dynamics (MD) simulations. Furthermore, the presence of a favorable water-molecule network that interacted with synthetic amides in the variants resulted in a reduced activity on polyethylene terephthalate (PET). Our data demonstrate the potential of using enzyme engineering to improve the amidase activity for polyesterases to act on synthetic amide-containing polymers.

Catalysis by Networks of Cooperative Hydrogen Bonds

The main paradigm of today's chemistry is sustainability. In pursuing sustainability, we need to learn from chemical processes carried out by Nature and realize that Nature does not use either strong acids, or strong bases or fancy reagents to achieve outstanding chemical processes. Instead, enzyme activity leans on the cooperation of several chemical entities to avoid strong acids or bases or to achieve such an apparently simple goal as transferring a proton from an NuH unit to an E unit (NuH + E → Nu–EH). Hydrogen bond catalysis emerged strongly two decades ago in trying to imitate Nature and avoid metal catalysis. Now to mount another step in pursuing the goal of sustainability, the focus is upon cooperativity between the different players involved in catalysis. This chapter looks at the concept of cooperativity and, more specifically, (a) examines the role of cooperative hydrogen bonded arrays of the general type NuH⋯(NuH)n⋯NuH (i.e. intermolecular cooperativity) to facilitate general acid–base catalysis, not only in the solution phase but also under solvent-free and catalyst-free conditions, and, most important, (b) analyzes the capacity of designer chiral organocatalysts displaying intramolecular networks of cooperative hydrogen bonds (NCHBs) to facilitate enantioselective synthesis by bringing conformational rigidity to the catalyst in addition to simultaneously increasing the acidity of key hydrogen atoms so to achieve better complementarity in the highly polarized transition states.

The role of proton shuttling mechanisms in solvent-free and catalyst-free acetalization reactions of imines

Catalyst-free and solvent-free reactions of the type NuH + E → Nu–EH are NuH-catalyzed processes in which Grotthuss-like proton shuttling pays a key role.

Exploring water as building bricks in enzyme engineering

A de novo designed water pattern is used to achieve a 34-fold accelerated promiscuous enzyme catalysis by efficient transition state stabilization.