Chemistry, Department of

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Free Radicals Formed by H Atom Addition to Allenes as Determined by Muon Spin Spectroscopy

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2020-12-04
Abstract: 

Allyl and vinyl radicals are important intermediates in diverse areas of chemistry, ranging from combustion to syn-thesis. However, questions remain about the competitive formation of these radicals from allenes. Here we present a study of proto-typical allyl and vinyl radicals formed by H atom addition to allenes. They were studied by forming the analogous muonium ad-ducts, since muonium (Mu) behaves as a light isotope of hydrogen, and muoniated species can be characterized by muon spin spec-troscopy. Two techniques were employed: Transverse-Field Muon Spin Resonance (TF-μSR), and Muon Level Crossing Reso-nance (μLCR), which allow for the measurement of muon hyperfine constants (hfcs) and other nuclear hfcs, respectively, and thus aid identification of the formed radicals. TF-μSR has already been used to determine that two radicals are formed by Mu addition to 1,1-dimethylallene, but μLCR techniques were undeveloped at the time of that study, so assignments were based on ESR data of similar allyl and vinyl radicals. We report here the muon spin spectroscopy of multiple radicals detected from positive muon irradi-ation of 1,1-dimethylallene and 1-methoxyallene in solution. The radicals were identified by comparison of muon and proton hfcs with ESR data and the results of DFT calculations. The conclusion is that muonium (and by extension, the H atom) can add to all three carbons of the allene system, albeit with preference for the central carbon.

Document type: 
Article

Introducing Complex NMR Mixtures at the Undergraduate Level: Isomerization, Separation and Analysis of the Diels-Alder Adducts from the Reaction of Methylcyclopentadiene and Maleic Anhydride (Part II)

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2020-10-26
Abstract: 

This paper introduces undergraduate students to advanced NMR techniques to help elucidate the structures of endo and exo norbornene isomers formed during the Diels-Alder reaction between cyclopentadiene and methylcyclopentadiene with maleic anhydride. The microwave assisted isomerization of the norbornene derivatives resulted in a mixture of endo and exo isomers that were separated and analyzed using advanced NMR spectroscopy techniques, with a focus on the spectral interpretation and unequivocal assignment of the endo and exo stereoisomerism when possible.

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Article
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Adjacent Functional Group Effects on the Assembly of Columnar Liquid Crystals

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2020-04-21
Abstract: 

Although the impact of individual functional groups on the self-assembly of columnar liquid crystal phases has been widely studied, the effect of varying multiple substituents has received much less attention. Herein, we report a series of dibenzo[a,c]phenazines containing an alcohol or ether adjacent to an electron-withdrawing ester or acid. With one exception, these difunctional mesogens form columnar phases. The phase behavior appeared to be dominated by the electron-withdrawing substituent; transition temperatures were similar to derivatives with these groups in isolation. In most instances, the addition of an electron-donating group ortho to an ester or acid suppressed the melting temperature and elevated the clearing temperature, leading to broader liquid crystal thermal ranges. This effect was more pronounced for derivatives functionalized with longer chain hexyloxy groups. These results suggest a potential strategy for controlling the phase ranges of columnar liquid crystals and achieving room temperature mesophases.

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Article
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Structure of Pb(Fe2/3W1/3)O3 Single Crystals with Partial Cation Order

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2020-09-03
Abstract: 

Despite intensive studies on the complex perovskite Pb(Fe2/3W1/3)O3 (PFWO) relaxor, understanding the exact nature of its multifunctional properties has remained a challenge for decades. In this work we report a comprehensive structural study of the PFWO single crystals using a combination of synchrotron X-ray diffraction and high-resolution electron microscopy. The set of {h + ½, k + ½, l + ½} superlattice reflections was observed for the first time based on single-crystal synchrotron X-ray experiments (100–450 K) and transmission electron microscopy investigations, which indicates some kind of B-cation ordering in PFWO which had been thought to be totally disordered. It was found that (1) the crystal structure of PFWO should be described by a partly ordered cubic perovskite (i.e. Fm − 3m), (2) the weak ferromagnetic properties and excess magnetic moment of PFWO can be understood based on non-random distribution of Fe cations between the 4a and 4b sites, and (3) the Pb displacement disorder is present in this material and the cations are probably displaced along the <100> directions. The X-ray diffraction results of this investigation show that partial cation ordering indeed exists in PFWO, which makes it necessary to revisit the generally accepted interpretations of the results obtained up to date. In agreement with X-ray diffraction study the main results of TEM study include: (1) a long range order that can be described with the Fm − 3m symmetry is reliably detected, (2) the coherence length of that long range order is in the order of 1–2 nm and (3) no remarkable chemical inhomogeneity is found in the tested PFWO crystal, excluding the possibility of a compositional ordering arising from substitutional defects in the perovskite structure.

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Article
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Direct Photolithographic Deposition of Color‐Coded Anti‐Counterfeit Patterns with Titania Encapsulated Upconverting Nanoparticles

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2020-07-26
Abstract: 

Creating security labels as anti‐counterfeit measures can require multi‐step methods, clean room processing, and high‐cost equipment. Some labels also have a limited applicability due to the ease of creating a counterfeit. Herein, a photochemical metal‐organic deposition (PMOD) based approach that enables creation of high‐resolution luminescent patterns that retain nanoparticles in transparent metal oxide films is reported. This low‐cost, photoresist‐free process creates high‐resolution patterns of metal oxides without requiring processes such as etching or lift‐off. Upconverting nanoparticles (UCNPs) with tunable red/green or blue emission are prepared by doping Yb3+/Er3+ and Yb3+/Tm3+ into β‐NaYF4 hosts, respectively. Luminescent inks are prepared by suspending UCNPs in solutions with titanium di‐n‐butoxide bis(2‐ethylhexanoate). Customizable luminescent patterns are prepared by casting inks onto substrates, followed by exposure to ultraviolet light through photomasks. Photodecomposition of the titanium precursor yields amorphous oxide films encapsulating the UCNPs. Security labels are prepared by selectively patterning luminescent inks using PMOD. Distinct patterns of red‐green‐blue (RGB) luminescence are discernible only upon excitation with a near‐infrared (NIR) laser. These customizable, anti‐counterfeit labels exhibit the merits of low‐cost, high‐throughput, and simple manufacturing techniques. Yet, the versatility of customizing their emissive properties suggests a practical application as an anti‐counterfeiting measure.

Document type: 
Article

Substrate‐Guided Front‐Face Reaction Revealed by Combined Structural Snapshots and Metadynamics for the Polypeptide N‐Acetylgalactosaminyltransferase 2

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2014-06-20
Abstract: 

The retaining glycosyltransferase GalNAc-T2 is a member of a large family of human polypeptide GalNActransferases that is responsible for the post-translational modification of many cell-surface proteins. By the use of combined structural and computational approaches, we provide the first set of structural snapshots of the enzyme during the catalytic cycle and combine these with quantum-mechanics/molecular-mechanics (QM/MM) metadynamics to unravel the catalytic mechanism of this retaining enzyme at the atomicelectronic level of detail. Our study provides a detailed structural rationale for an ordered bi–bi kinetic mechanism and reveals critical aspects of substrate recognition, which dictate the specificity for acceptor Thr versus Ser residues andenforce a front-face SNi-type reaction in which the substrate Nacetyl sugar substituent coordinates efficient glycosyl transfer.

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Article
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Multivalency To Inhibit and Discriminate Hexosaminidases

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2017-05-26
Abstract: 

A set of multivalent polyhydroxylated acetamidoazepanes based on ethylene glycol, glucoside, or cyclodextrin scaffolds was prepared. The compounds were assessed against plant, mammalian, and therapeutically relevant hexosaminidases. Multimerization was shown to improve the inhibitory potency with synergy, and to fine tune the selectivity profile between related hexosaminidases.

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Article
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A Divergent Synthesis to Generate Targeted Libraries of Inhibitors for Endo-N-Acetylglucosaminidases

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2017-12-18
Abstract: 

Cell active inhibitors of glycoside processing enzymes are valuable research tools that help us understand the physiological roles of this diverse class of enzymes. endo-N-Acetylglucosaminidases have gained increased attention for their important roles in both mammals and human pathogens; however, metabolically stable cell active inhibitors of these enzymes are lacking. Here, we describe a divergent synthetic strategy involving elaboration of a thiazoline core scaffold. We illustrate the potential of this approach by using the copper catalysed azide-alkyne click (CuAAC) reaction, in combination with a suitable catalyst to avoid poisoning by the thiazoline moiety, to generate a targeted panel of candidate inhibitors of endo-N-acetylglucosaminidases and chitinases

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Article
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A Convenient Approach to Stereoisomeric Iminocyclitols: Generation of Potent Brain‐Permeable OGA Inhibitors

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2015-11-06
Abstract: 

Pyrrolidine‐based iminocyclitols are a promising class of glycosidase inhibitors. Reported herein is a convenient epimerization strategy that provides direct access to a range of stereoisomeric iminocyclitol inhibitors of O‐GlcNAcase (OGA), the enzyme responsible for catalyzing removal of O‐GlcNAc from nucleocytoplasmic proteins. Structural details regarding the binding of these inhibitors to a bacterial homologue of OGA reveal the basis for potency. These compounds are orally available and permeate into rodent brain to increase O‐GlcNAc, and should prove useful tools for studying the role of OGA in health and disease.

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Article
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A Selective Inhibitor Gal‐PUGNAc of Human Lysosomal β‐Hexosaminidases Modulates Levels of the Ganglioside GM2 in Neuroblastoma Cells

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2009-01-28
Abstract: 

Gal‐PUGNAc (see picture), a highly selective inhibitor for β‐hexosaminidases HEXA and HEXB is cell‐permeable and modulates the activity of HEXA and HEXB in tissue culture, increasing ganglioside GM2 levels. Gal‐PUGNAc should allow the role of these enzymes to be studied at the cellular level without generating a complex chemical phenotype from concomitant inhibition of O‐GlcNAcase.

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Article
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