Hydrogen exchange in a six-membered ring transition state. Evidence for a stepwise McLafferty rearrangement

Mass spectra of 2-methyl-5-chloro-3-N-ethyl-benzothiazolium iodide and labeled counter-parts are discussed with particular emphasis on the loss of ethylene from the moiety produced by thermal elimination of HI from this salt. This process demonstrates the stepwise nature of a McLafferty rearrangement involving a hydrogen transfer to an olefinic carbon.     

Determination of domoic acid toxins in shellfish by biosense ASP ELISA--a direct competitive enzyme-linked immunosorbent assay: collaborative study

A collaborative study was conducted on the Biosense amnesic shellfish poisoning (ASP) enzyme-linked immunosorbent assay (ELISA) for the determination of domoic acid (DA) toxins in shellfish in order to obtain interlaboratory validation data for the method. In addition, a method comparison study was performed to evaluate the ASP ELISA as an alternative to the current liquid chromatography (LC) reference method for DA determination. The study material comprised 16 shellfish samples, including blue mussels, Pacific oysters, and king scallops, spiked with contaminated mussel homogenates to contain 0.1-20 mg DA/kg shellfish flesh. The shellfish samples were extracted with 50% aqueous methanol, and the supernatants were directly analyzed. Sixteen participating laboratories in 10 countries reported data from the ASP ELISA, and 4 of these laboratories also reported data from instrumental LC analysis. The participating laboratories achieved interlaboratory precision estimates for the 8 Youden paired shellfish samples in the range of 10-20% for RSD(r) (mean 14.8 +/- 4%), and 13-29% for RSDR (mean 22.7 +/- 6%). The precision estimates for the ELISA data did not show a strong dependence on the DA concentration in the study samples, and the overall precision achieved was within the acceptable range of the Horwitz guideline with HorRat values ranging from 1.1 to 2.4 (mean HorRat 1.7 +/- 0.5). The analysis of shellfish samples spiked with certified reference material (CRM)-ASP-MUS-b gave recoveries in the range of 88-122%, with an average recovery of 104 +/- 10%. The estimate on method accuracy was supported by a correlation slope of 1.015 (R2 = 0.992) for the determined versus the expected DA values. Furthermore, the correlation of the ASP ELISA results with those for the instrumental LC analyses of the same sample extracts gave a correlation slope of 1.29 (R2 = 0.984). This indicates some overestimation of DA levels in shellfish by the ELISA, but it is also a result of apparent low recoveries for the LC methods. This interlaboratory study demonstrates that the ASP ELISA is suitable for the routine determination and monitoring of DA toxins in shellfish, and that it offers a rapid and cost-effective methodology with high sample throughput.     

Pyrene‐cored covalent organic polymers by thiophene‐based isomers,their gas adsorption,and photophysical properties

Two new pyrene‐cored covalent organic polymers (COPs), CK‐COP‐1 and CK‐COP‐2 , were synthesized via the one‐step polymerization of two thiophene‐based isomers, 1,3,6,8‐tetra(thiophene‐2‐yl) pyrene ( L₁ ) and 1,3,6,8‐tetra(thiophene‐3‐yl) pyrene ( L₂ ). The resulting pyrene‐cored COPs exhibit rather different surface areas of 54 m² g^?1 and 615 m²g^?1 for CK‐COP‐1 and CK‐COP‐2 , respectively. The CO₂ uptake capacities of CK‐COP‐1 and CK‐COP‐2 also show different values of 2.85 and 9.73 wt % at 273 K, respectively. Furthermore, CK‐COP‐2 offers not only a larger CO₂ adsorption capacity but also a better CO₂/CH₄ selectivity at 273 K compared with CK‐COP‐1 . CK‐COP‐1 and CK‐COP‐2 also exhibit considerable differences in their photophysical property. The different structure and properties of CK‐COPs could be attributed to the isomer effect of their corresponding thiophene‐based monomers. © 2017 Authors. Journal of Polymer Science Part A: Polymer Chemistry Published by Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 2383–2389     

Sponge‐Like Behaviour in Isoreticular Cu(Gly‐His‐X) Peptide‐Based Porous Materials

We report two isoreticular 3D peptide‐based porous frameworks formed by coordination of the tripeptides Gly‐L ‐His‐Gly and Gly‐L ‐His‐L ‐Lys to Cu^II which display sponge‐like behaviour. These porous materials undergo structural collapse upon evacuation that can be reversed by exposure to water vapour, which permits recovery of the original open channel structure. This is further confirmed by sorption studies that reveal that both solids exhibit selective sorption of H₂O while CO₂ adsorption does not result in recovery of the original structures. We also show how the pendant aliphatic amine chains, present in the framework from the introduction of the lysine amino acid in the peptidic backbone, can be post‐synthetically modified to produce urea‐functionalised networks by following methodologies typically used for metal–organic frameworks built from more rigid “classical” linkers.     

Formation mechanism for a hybrid supramolecular network involving cooperative interactions

A novel mechanism of hybrid assembly of molecules on surfaces is proposed stemming from interactions between molecules and on-surface metal atoms which eventually got trapped inside the network pores. Based on state-of-the-art theoretical calculations, we find that the new mechanism relies on formation of molecule-metal atom pairs which, together with molecules themselves, participate in the assembly growth. Most remarkably, the dissociation of pairs is facilitated by a cooperative interaction involving many molecules. This new mechanism is illustrated on a low coverage Melamine hexagonal network on the Au(111) surface where multiple events of gold atoms trapping via a set of so-called "gate" transitions are found by kinetic Monte Carlo simulations based on transition rates obtained using ab initio density functional theory calculations and the nudged elastic band method. Simulated STM images of gold atoms trapped in the pores of the Melamine network predict that the atoms should appear as bright spots inside Melamine hexagons. No trapping was found at large Melamine coverages, however. These predictions have been supported by preliminary STM experiments which show bright spots inside Melamine hexagons at low Melamine coverages, while empty pores are mostly observed at large coverages. Therefore, we suggest that bright spots sometimes observed in the pores of molecular assemblies on metal surfaces may be attributed to trapped substrate metal atoms. We believe that this type of mechanism could be used for delivering adatom species of desired functionality (e.g., magnetic) into the pores of hydrogen-bonded networks serving as templates for their capture.     

Storage of multiple coherent microwave excitations in an electron spin ensemble

Strong coupling between a microwave photon and electron spins, which could enable a long-lived quantum memory element for superconducting qubits, is possible using a large ensemble of spins. This represents an inefficient use of resources unless multiple photons, or qubits, can be orthogonally stored and retrieved. Here we employ holographic techniques to realize a coherent memory using a pulsed magnetic field gradient and demonstrate the storage and retrieval of up to 100 weak 10?GHz coherent excitations in collective states of an electron spin ensemble. We further show that such collective excitations in the electron spin can then be stored in nuclear spin states, which offer coherence times in excess of seconds.     

Computing ionization states of proteins with a detailed charge model

A convenient computational approach for the calculation of the p K_as of ionizable groups in a protein is described. The method uses detailed models of the charges in both the neutral and ionized form of each ionizable group. A full derivation of the theoretical framework is presented, as are details of its implementation in the UHBD program. Application to four proteins whose crystal structures are known shows that the detailed charge model improves agreement with experimentally determined pK_as when a low protein dielectric constant is assumed, relative to the results with a simpler single-site ionization model. It is also found that use of the detailed charge model increases the sensitivity of the computed pK_as to the details of proton placement. © 1996 by John Wiley & Sons, Inc.     

Surface spectroscopic imaging of PEG-PLA tissue engineering constructs with ToF-SIMS

Poly(lactic acid) (PLA) has been studied extensively in the field of tissue engineering due to its well-established biodegradability and biocompatibility. To improve its non-fouling properties we entrapped poly(ethyl glycol) (PEG), into the surface, which was characterised by a combination of ToF-SIMS and XPS. Cell attachment was subsequently investigated as a function of the amount of entrapped PEG. XPS was instrumental in quantifying the amount of PEG entrapped at higher concentrations, whereas the significantly higher sensitivity and superior imaging capabilities of ToF-SIMS enabled a full characterisation of the PEG distribution at the low concentrations required for the desired non-fouling properties, which were well below the XPS quantification limit. It was found that pronounced segregation effects leading to the formation of PEG-enriched domains ranging between ca. 5 and several tens of microns across were present at all PEG concentrations investigated.