Electrophilic coordination catalysis: a summary of previous thought and a new angle of analysis

One of the most common, and yet least well understood, enzymatic transformations is proton abstraction from activated carbon acids such as carbonyls. Understanding the mechanism for these proton abstractions is basic to a good understanding of enzyme function. Significant controversy has arisen over the means by which a given enzyme might facilitate these deprotonations. Creating small molecule mimics of enzymes and physical organic studies that model enzymes are good approaches to probing mechanistic enzymology. This Account details a number of molecular recognition and physical organic studies, both from our laboratory and others, dealing with the elucidation of this quandary. Our analysis launches from an examination of the active sites and proposed mechanism of several enzyme-catalyzed deprotonations of carbon acids. This analysis highlights the geometries of the hydrogen bonds found at the enzyme active sites. We find evidence to support pi-oriented hydrogen bonding, rather than lone pair oriented hydrogen bonding. Our observations prompted us to study the stereochemistry of hydrogen bonding that activates carbonyl alpha-carbons to deprotonation. The results from our own thermodynamic, kinetics, and computational studies, all of which are reviewed herein, suggest that an unanticipated level of intermediate stabilization occurs via an electrophilic interaction through the pi-molecular orbital as opposed to traditional lone pair directed coordination. We do not postulate that hydrogen bonding to pi-systems is intrinsically stronger than to lone pairs, but rather that there is a greater change in bond strength during deprotonation when the hydrogen bonds are oriented at the pi-system. Through these studies, we conclude that many enzymes preferentially activate their carbon acid substrates through an electrophilic coordination directed towards the pi-bond of the carbonyl rather than the conventional lone pair directed model.     

Transition Structures of the Electrocyclic Reactions of cis,cis,cis-1,3,5-Cyclooctatriene

The electrocyclic reactions of cis,cis,cis-1,3,5-cyclooctatriene have been studied using ab initio molecular orbital theory. cis,cis,cis-1,3,5-cyclooctatriene can undergo an electrocyclic ring opening in a conrotatory fashion to form cis,cis-1,3,5,7-octatetraene and a disrotatory electrocyclization to form bicyclo[4.2.0]octa-2,4-diene. The transition structures for these electrocyclic reactions have been located. Geometry optimizations employed restricted Hartree-Fock calculations and the 3–21G and 6–31G* basis sets. Electron correlation energies were calculated using second-order, and in some cases fourth-order, Møller-Plesset theory. Scaled RHF/6–31G* force constants were employed in the prediction of secondary deuterium isotope effects for the conrotatory ring opening. The ground state of cis,cis,cis-1,3,5-cyclooctatriene exists in a twist-boat conformation with staggering at the saturated linkage. The transition structure for the conrotatory electrocyclic ring opening to form cis,cis-1,3,5,7-octatetraene has a helical structure, which has implications for the stereoselectivities of ring closure of 1-substituted-cis,cis-1,3,5,7-octatetraenes. The disrotatory transition structure for the electrocyclization to form bicyclo[4.2.0]octa-2,4-diene is strongly distorted from C_s symmetry, in contrast to the transition structure for the disrotatory electrocyclization of cis-1,3,5-hexatriene. This distortion is caused by staggering about the saturated linkage.     

Pore exclusion chromatography-inductively coupled plasma-mass spectrometry for monitoring elements in bacteria: a study on microbial removal of uranium from aqueous solution

The interstitial spaces between spherical particles in a packed column can act as a sieve that passes microorganisms below a certain size. If the bed is a perfusion-type material (containing a binary distribution of large and small pores), colloidal-size microorganisms are subject only to pore exclusion, while all molecules are subject to size exclusion among the various pores. Thus, microorganisms elute first, followed by macromolecules, and then small molecules. Coupling this separation method to an ICP magnetic sector mass spectrometer provides a sensitive, direct means to study the microbial uptake of heavy metals (i.e., uranium) from their surrounding environments. Multiple metal ions can be monitored in the microorganism and in the surrounding solution. In this way, definitive information can be provided for the remediation of radioactive waste sites. The effect of uranium on microbial growth is also discussed.     

Involvement of Lipocalin‐like CghA in Decalin‐Forming Stereoselective Intramolecular [4+2] Cycloaddition

Understanding enzymatic Diels–Alder (DA) reactions that can form complex natural product scaffolds is of considerable interest. Sch 210972 1 , a potential anti‐HIV fungal natural product, contains a decalin core that is proposed to form through a DA reaction. We identified the gene cluster responsible for the biosynthesis of 1 and heterologously reconstituted the biosynthetic pathway in Aspergillus nidulans to characterize the enzymes involved. Most notably, deletion of cghA resulted in a loss of stereoselective decalin core formation, yielding both an endo ( 1 ) and a diastereomeric exo adduct of the proposed DA reaction. Complementation with cghA restored the sole formation of 1 . Density functional theory computation of the proposed DA reaction provided a plausible explanation of the observed pattern of product formation. Based on our study, we propose that lipocalin‐like CghA is responsible for the stereoselective intramolecular [4+2] cycloaddition that forms the decalin core of 1 .     

Stretchable graphene transistors with printed dielectrics and gate electrodes

With the emergence of human interface technology, the development of new applications based on stretchable electronics such as conformal biosensors and rollable displays are required. However, the difficulty in developing semiconducting materials with high stretchability required for such applications has restricted the range of applications of stretchable electronics. Here, we present stretchable, printable, and transparent transistors composed of monolithically patterned graphene films. This material offers excellent mechanical, electrical, and optical properties, capable of use as semiconducting channels as well as the source/drain electrodes. Such monolithic graphene transistors show hole and electron mobilities of 1188 ± 136 and 422 ± 52 cm(2)/(V s), respectively, with stable operation at stretching up to 5% even after 1000 or more cycles.     

An Atomically Thin Optoelectronic Machine Vision Processor

2D semiconductors, especially transition metal dichalcogenide (TMD) monolayers, are extensively studied for electronic and optoelectronic applications. Beyond intensive studies on single transistors and photodetectors, the recent advent of large-area synthesis of these atomically thin layers has paved the way for 2D integrated circuits, such as digital logic circuits and image sensors, achieving an integration level of ≈100 devices thus far. Here, a decisive advance in 2D integrated circuits is reported, where the device integration scale is increased by tenfold and the functional complexity of 2D electronics is propelled to an unprecedented level. Concretely, an analog optoelectronic processor inspired by biological vision is developed, where 32 × 32 = 1024 MoS₂ photosensitive field-effect transistors manifesting persistent photoconductivity (PPC) effects are arranged in a crossbar array. This optoelectronic processor with PPC memory mimics two core functions of human vision: it captures and stores an optical image into electrical data, like the eye and optic nerve chain, and then recognizes this electrical form of the captured image, like the brain, by executing analog in-memory neural net computing. In the highlight demonstration, the MoS₂ FET crossbar array optically images 1000 handwritten digits and electrically recognizes these imaged data with 94% accuracy.     

RACIAL DIFFERENTIALS IN REAL ESTATE ADVERTISING PRACTICES: An Exploratory Case Study

ABSTRACT: This article reports on the print media advertising practices of a major real estate company operating in Milwaukee, Wisconsin, during 1981–1984. Statistical findings suggest that, compared to home sellers in a white neighborhood listing with this company, those in an integrated or black neighborhood could expect significantly: (1) lower frequencies of advertisements and open houses noted in the Sunday metropolitan newspaper; (2) higher probabilities of no properties in their area being advertised or advertised only with “one-line” ads; and (3) lower probability of their area being described favorably in the ad Potential nonracial explanations of these findings appear invalid. The potential motivations for these practices and their segregative consequences are analyzed     

Synaptic linkages between red nucleus cells and limb muscles during a multi-joint motor task

The magnocellular red nucleus (RNm) becomes highly active when a monkey reaches to grasp an object. However, the only spike-triggered averaging studies of the RNm to date have been restricted to a simple wrist tracking paradigm and electromyographic (EMG) measurements of muscles of the forearm. We have now measured EMG signals from a large number of muscles throughout the shoulder, arm, forearm, and hand during a variety of tasks, including unconstrained reaching and grasping movements. Relations between these EMG signals and single-unit activity were assessed by on-line spike-triggered averaging and revealed significant post-spike effects among muscles of the shoulder and proximal arm, as well as intrinsic hand muscles. Although there remained a strong bias toward the extensor muscles of the forearm, as has been shown earlier, these results reinforce the importance of the RNm in the control of coordinated, whole-limb reaching movements.     

Highly Flexible Hybrid CMOS Inverter Based on Si Nanomembrane and Molybdenum Disulfide

2D semiconductor materials are being considered for next generation electronic device application such as thin‐film transistors and complementary metal–oxide–semiconductor (CMOS) circuit due to their unique structural and superior electronics properties. Various approaches have already been taken to fabricate 2D complementary logics circuits. However, those CMOS devices mostly demonstrated based on exfoliated 2D materials show the performance of a single device. In this work, the design and fabrication of a complementary inverter is experimentally reported, based on a chemical vapor deposition MoS₂ n‐type transistor and a Si nanomembrane p‐type transistor on the same substrate. The advantages offered by such CMOS configuration allow to fabricate large area wafer scale integration of high performance Si technology with transition‐metal dichalcogenide materials. The fabricated hetero‐CMOS inverters which are composed of two isolated transistors exhibit a novel high performance air‐stable voltage transfer characteristic with different supply voltages, with a maximum voltage gain of ≈16, and sub‐nano watt power consumption. Moreover, the logic gates have been integrated on a plastic substrate and displayed reliable electrical properties paving a realistic path for the fabrication of flexible/transparent CMOS circuits in 2D electronics.