Clonal structure and variation in virulence of Salmonella enteritidis isolated from mice, chickens, and humans

The clonal analysis of a diverse collection of Salmonella Enteritidis indicates that most strains belong to a single multilocus genotype (i.e., ET-3) regardless of phage type, geographic origin, or time of isolation that spanned over 2 decades (1978 to 2004). Attachment and invasion assays, however, indicate that, among ET-3 isolates, there is a distinct invasive bacterial subpopulation that is more readily recovered from eggs and clinical cases in humans than from chicken cecal samples. These observations support the hypothesis that the specialized ability of S. Enteritidis to infect the avian reproductive tract and contaminate eggs has been critical in its emergence as a frequent cause of human illness.     

Free-standing metal–organic framework (MOF) monolayers by self-assembly of polymer-grafted nanoparticles

We report a general method for the synthesis of free-standing, self-assembled MOF monolayers (SAMMs) at an air–water interface using polymer-brush coated MOF nanoparticles. UiO-66, UiO-66-NH₂, and MIL-88B-NH₂ were functionalized with a catechol-bound chain-transfer agent (CTA) to graft poly(methyl methacrylate) (PMMA) from the surface of the MOF using reversible addition-fragmentation chain transfer polymerization (RAFT). The polymer-coated MOFs were self-assembled at the air–water interface into monolayer films ∼250 nm thick and capable of self-supporting at a total area of 40 mm². Mixed-particle films were prepared through the assembly of MOF mixtures, while multilayer films were achieved through sequential transfer of the monolayers to a glass slide substrate. This method offers a modular and generalizable route to fabricate thin-films with inherent porosity and sub-micron thickness composed of a variety of MOF particles and functionalities.

We report a general method for the synthesis of free-standing, self-assembled MOF monolayers (SAMMs) at an air–water interface using polymer-brush coated MOF nanoparticles.     

Coherence transfer delay optimisation in PSYCOSY experiments

PSYCOSY is an f1 broadband homonuclear decoupled version of the COSY nuclear magnetic resonance pulse sequence. Here, we investigate by a combination of experimental measurements, spatially distributed spin dynamics simulations, and analytical predictions the coherence evolution delay necessary in PSYCOSY experiments to ensure intensity discrimination in favour of the correlations typically arising from short range (ⁿJ, n ≤ 3) ¹H–¹H couplings and show that, in general, a coherence evolution delay of around 35 ms is optimum.     

HRTEM image simulations for the study of ultrathin gate oxides.

We have performed high resolution transmission electron microscope (HRTEM) image simulations to qualitatively assess the visibility of various structural defects in ultrathin gate oxides of MOSFET devices, and to quantitatively examine the accuracy of HRTEM in performing gate oxide metrology. Structural models contained crystalline defects embedded in an amorphous 16-A-thick gate oxide. Simulated images were calculated for structures viewed in cross section. Defect visibility was assessed as a function of specimen thickness and defect morphology, composition, size, and orientation. Defect morphologies included asperities lying on the substrate surface, as well as "bridging" defects connecting the substrate to the gate electrode. Measurements of gate oxide thickness extracted from simulated images were compared to actual dimensions in the model structure to assess TEM accuracy for metrology. The effects of specimen tilt, specimen thickness, objective lens defocus, and coefficient of spherical aberration (Cs) on measurement accuracy were explored for nominal 10-A gate oxide thickness. Results from this work suggest that accurate metrology of ultrathin gate oxides (i.e., limited to several percent error) is feasible on a consistent basis only by using a Cs-corrected microscope. However, fundamental limitations remain for characterizing defects in gate oxides using HRTEM, even with the new generation of Cs-corrected microscopes.     

Bond alternation, polarizability, and resonance detuning in methine dyes

We derive structure-property relationships for methine ("Brooker") dyes relating the color of the dye and its symmetric parents to its bond alternation in the ground state and also to the dipole properties associated with its low-lying charge-resonance (or charge-transfer) transition. We calibrate and test these relationships on an array of different protonation states of the green fluorescent protein chromophore motif (an asymmetric halochromic methine dye) and its symmetric parent dyes. The relationships rely on the assumption that the diabatic states that define the Platt model for methine dye color [J. R. Platt, J. Chem. Phys. 25, 80 (1956)] can also be distinguished by their single-double bond alternation and by their charge localization character. These assumptions are independent of the primary constraint that defines the diabatic states in the Platt model--specifically, the Brooker deviation rule for methine dyes [L. G. S. Brooker, Rev. Mod. Phys. 14, 275 (1942)]. Taking these assumptions, we show that the Platt model offers an alternate route to known structure-property relationships between the bond length alternation and the quadratic nonlinear polarizability β. We show also that the Platt model can be parameterized without the need for synthesis of the symmetric parents of a given dye, using the dipole data obtained through spectroscopic measurements. This suggests that the Platt model parameters may be used as independent variables in free-energy relationships for chromophores whose symmetric parents cannot be synthesized or chromophores strongly bound to biomolecular environments. The latter category includes several recently characterized biomolecular probe constructs. We illustrate these concepts by an analysis of previously reported electroabsorption and second-harmonic generation experiments on green fluorescent proteins.     

Tetrathienoanthracene-based copolymers for efficient solar cells

A series of semiconducting copolymers (PTAT-x) containing extended π-conjugated tetrathienoanthracene units have been synthesized. It was shown that the extended conjugation system enhanced the π-π stacking in the polymer/PC(61)BM blend films and facilitated the charge transport in heterojunction solar cell devices. After structural fine-tuning, the polymer with bulky 2-butyloctyl side chains (PTAT-3) exhibited a PCE of 5.6% when it was blended with PC(61)BM.     

Covalent modification of a metal-organic framework with isocyanates: probing substrate scope and reactivity

Isoreticular metal-organic framework-3 (IRMOF-3) has been postsynthetically modified with isocyanates to generate unprecedented, microporous urea-functionalized frameworks.     

Promoting peptide α-helix formation with dynamic covalent oxime side-chain cross-links

Covalent side-chain cross-linking has been shown to be a viable strategy to control peptide folding. We report here that an oxime side-chain linkage can elicit α-helical folds from peptides in aqueous solution. The bio-orthogonal bridge is formed rapidly under neutral buffered conditions, and the resulting cyclic oximes are capable of dynamic covalent exchange.     

Tandem postsynthetic modification of metal-organic frameworks using an inverse-electron-demand Diels-Alder reaction

A postsynthetic modification (PSM) scheme for metal-organic frameworks (MOFs) has been developed using a tetrazine-based "Click" reaction. It was found that the efficacy of this modification procedure was dependent on the MOF topology and, in the case of an isoreticular MOF (IRMOF) system, required the formation of a mixed-ligand IRMOF with a suitable ratio of 1,4-benzenedicarboxylate (BDC) and an olefin-tagged BDC derivative. On the basis of the versatile use of tetrazine "Click" chemistry in bioconjugate chemistry, we expect that this scheme will prove to be a useful reaction for preparing functionalized materials, including MOFs.