As nanotechnology floods application areas like medicine, electronics, water remediation, space and textiles, just to name a few, some nanomaterials remain in the spotlight due to their fantastic ...properties and their incredible potential. Such is the case of the 2D, transparent, flexible, strong, carbon-based nanomaterial called graphene. Graphene consists of sp2 hybridized carbon arranged in a flat network packed in a honey-comb pattern, having thus mono-atomic thickness. Its isolation in 2004 opened the door to numerous investigations and its research is funded each year by governments, industries and academia worldwide. Due to its non-hydrophilic nature, some applications represent a challenge (particularly biological and medical applications), thus an oxygen/hydrogen-functionalized hydrophilic version of it has lately gained popularity, its name is graphene oxide. This document aims to review the synthesis methods of graphene, graphene oxide and reduced graphene oxide. A revision of the most important top-down and bottom-up methods is presented, focusing on chemical vapor deposition for the growth of graphene and the wet-chemical methods for the synthesis of graphene oxide and the reduction techniques available for reduced graphene oxide. We conclude by analyzing the current situation of graphene and graphene oxide production and the challenges that need to be tackled in order to meet the short-term demand of these nanomaterials for the promised applications.
Alarming: Multiple sources of errors in DFT energetics of CC bond‐forming reactions were investigated by evaluating structural transformations in Diels–Alder reactions: conversion of π into σ bonds ...and changes in conjugation, hyperconjugation, and branching interactions. A startling overestimation of the π to σ bond conversion is found with most methods, a central problem to all reactions involving addition of π bonds (electrocyclic processes, ene, aldol).
De Novo Computational Design of Retro-Aldol Enzymes Jiang, Lin; Althoff, Eric A.; Clemente, Fernando R. ...
Science (American Association for the Advancement of Science),
03/2008, Volume:
319, Issue:
5868
Journal Article
Peer reviewed
Open access
The creation of enzymes capable of catalyzing any desired chemical reaction is a grand challenge for computational protein design. Using new algorithms that rely on hashing techniques to construct ...active sites for multistep reactions, we designed retro-aldolases that use four different catalytic motifs to catalyze the breaking of a carbon-carbon bond in a nonnatural substrate. Of the 72 designs that were experimentally characterized, 32, spanning a range of protein folds, had detectable retroaldolase activity. Designs that used an explicit water molecule to mediate proton shuffling were significantly more successful, with rate accelerations of up to four orders of magnitude and multiple turnovers, than those involving charged side-chain networks. The atomic accuracy of the design process was confirmed by the x-ray crystal structure of active designs embedded in two protein scaffolds, both of which were nearly superimposable on the design model.
Computational studies have led to models to understand some classic and contemporary asymmetric reactions involving organocatalysts. The Hajos−Parrish−Eder−Sauer−Wiechert reaction and intermolecular ...aldol reactions as well as Mannich reactions and oxyaminations catalyzed by proline and other amino acids, and Diels−Alder reactions catalyzed by MacMillan's chiral amine organocatalysts have been studied with density functional theory. Quantitative predictions for several new catalysts and reactions are provided.
In the present work, electrical measurements using in situ transmission electron microscopy (TEM) on pentagonal silver nanowires were performed. Electrical biasing was applied to individual nanowires ...with and without simultaneous in situ TEM mechanical deformation. The response of the ohmic resistance was measured in the I-V curves. A reduction in the break voltage and the resistance was measured, when the nanowires were subjected to a bending deformation. In situ electric measurements on both, with and without deformation, show a typical semiconductor behavior. Surface scattering of electrons in the nanowires and movement of dislocations act as the main causes of the electrical properties reported herein. In this way, the determination of the surface morphology was carried out by using off-axis electron holography followed by a phase reconstruction and structural modeling. The high Miller-index facets were determined to be the (533) stepped surface plane on all five longitudinal sides of the nanowires. Additionally, due to electrical saturation, a breakdown of the nanowires was observed during the in situ electrical measurements without mechanical deformation.
Graphical abstract
The effects of different amino acid catalysts and substrate substituents on the stereoselectivity of the title reactions have been studied with the aid of density functional theory methods. ...Experimental data available in the literature have been compiled. B3LYP/6-31G(d) calculations match the general experimental trends and provide useful insights into the origins of the variations in stereoselectivities. Acyclic primary amino acids allow a greater conformational flexibility in the aldol transition states compared with proline. This makes them poorer enantioselective catalysts with triketone substrates with a methyl ketone side chain. The steric repulsion upon substitution at the terminal methyl group increases the energy difference between anti- and syn-chairs with primary amino acid catalysts and, consequently, the stereoselectivities. Proline, in contrast, is a poor catalyst for the latter reactions because the substituent's steric bulkiness raises the activation energy of the favored C−C bond-forming pathway.
A comparison of previously proposed models of the CC bond‐forming step of the title reaction with density functional methods indicate that the most favored one involves an enamine intermediate ...undergoing a concerted aldol cyclization with proton transfer from the proline carboxylic acid group (see structure). This step is equal in energy to the intramolecular deprotonation leading to the enamine, and both are partially rate‐determining steps.
The archetypical proline-catalyzed intramolecular aldol reaction, the Hajos-Parrish-Eder-Sauer-Wiechert reaction, has served as a model reaction for the mechanistic study of the ever-growing class of ...proline-catalyzed conversions. Experimental measurements of the 13C kinetic isotope effects for this reaction show conclusively that carbon−carbon bond formation is not rate-limiting.
The design of active sites has been carried out using quantum mechanical calculations to predict the rate-determining transition state of a desired reaction in presence of the optimal arrangement of ...catalytic functional groups (theozyme). Eleven versatile reaction targets were chosen, including hydrolysis, dehydration, isomerization, aldol, and Diels−Alder reactions. For each of the targets, the predicted mechanism and the rate-determining transition state (TS) of the uncatalyzed reaction in water is presented. For the rate-determining TS, a catalytic site was designed using naturalistic catalytic units followed by an estimation of the rate acceleration provided by a reoptimization of the catalytic site. Finally, the geometries of the sites were compared to the X-ray structures of related natural enzymes. Recent advances in computational algorithms and power, coupled with successes in computational protein design, have provided a powerful context for undertaking such an endeavor. We propose that theozymes are excellent candidates to serve as the active site models for design processes.
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