The evolution of travelling waves in generalized Fisher equations via matched asymptotic expansions: Exponential corrections

In this paper we address an initial boundary value problem for a generalized Fisher equation. In particular we develop the matched asymptotic theory presented in Leach and Needham (2000) to consider the correction terms to the asymptotic approach to the wave-front of speed v = v*( 2) as t (time) . We establish the precise form of these corrections, and demonstrate that the rate of approach to the wave-front is algebraic in t when v* = 2 (there being two cases), but exponential in t when v* > 2.     

Homolytic X-H Bond Cleavage at a Gold(III) Hydroxide: Insights into One-Electron Events at Gold

C(sp³)-H and O−H bond breaking steps in the oxidation of 1,4-cyclohexadiene and phenol by a Au(III)-OH complex were studied computationally. The analysis reveals that for both types of bonds the initial X−H cleavage step proceeds via concerted proton coupled electron transfer (cPCET), reflecting electron transfer from the substrate directly to the Au(III) centre and proton transfer to the Au-bound oxygen. This mechanistic picture is distinct from the analogous formal Cu(III)-OH complexes studied by the Tolman group (J. Am. Chem. Soc. 2019 , 141, 17236–17244), which proceed via hydrogen atom transfer (HAT) for C−H bonds and cPCET for O−H bonds. Hence, care should be taken when transferring concepts between Cu−OH and Au−OH species. Furthermore, the ability of Au−OH complexes to perform cPCET suggests further possibilities for one-electron chemistry at the Au centre, for which only limited examples exist.     

Asymmetric Synthesis of Heterocyclic Chloroamines and Aziridines by Enantioselective Protonation of Catalytically Generated Enamines**

We report a method for the synthesis of chiral vicinal chloroamines via asymmetric protonation of catalytically generated prochiral chloroenamines using chiral Brønsted acids. The process is highly enantioselective, with the origin of asymmetry and catalyst substituent effects elucidated by DFT calculations. We show the utility of the method as an approach to the synthesis of a broad range of heterocycle-substituted aziridines by treatment of the chloroamines with base in a one-pot process, as well as the utility of the process to allow access to vicinal diamines.     

Crystallisation of aspirin via simulated pulmonary surfactant monolayers and lung‐specific additives

Pain is a prevalent condition that can have a serious impact upon the socioeconomic function of a population. Numerous methods exist to administer analgesic medication (e.g. aspirin) to the body however inherent drawbacks limit patient acceptability. The inhaled route offers promise to facilitate the administration of medication to the body. Here, we consider the crystallisation behaviour of aspirin, our model therapeutic agent, when in contact with material of relevance to the lung. Thus, our approach aims to better understand the interaction between drug substances and the respiratory tract. Langmuir monolayers composed of a mixed surfactant system were supported on an aqueous subphase containing aspirin (7.5 mg/ml). The surfactant film was compressed to either 5mN/m (i.e. inhalation end point) or 50 mN/m (i.e. exhalation end point), whilst located within a humid environment for 16 h. Standard cooling crystallisation procedures were employed to produce control samples. Antisolvent crystallisation in the presence or absence of lung‐specific additives was conducted. All samples were analysed via scanning electron microscopy and X‐ray diffraction. Drug‐surfactant interactions were confirmed via condensed Langmuir isotherms. Scanning electron microscopy analysis revealed plate‐like morphology. The crystallisation route dictated both the crystal habit and particle size distribution. Dominant reflections were the (100) and (200) aspects. The main modes of interaction were hydrogen bonding, hydrophobic associations, and van der Waals forces. Here, we have demonstrated the potential of antisolvent crystallisation with lung‐specific additives to achieve control over drug crystal morphology. The approach taken can be applied in respirable formulation engineering. Copyright © 2017 John Wiley & Sons, Ltd.     

Targeted particulate adhesion to cellulose surfaces mediated by bifunctional fusion proteins

The adhesion of particles to surfaces is an integral element in many commercial and biological applications. In this article, we report on the direct measurements of protein-mediated deposition and binding of particles to model cellulose surfaces. This system involves a family of heterobifunctional fusion proteins that bind specifically to both a red dye and cellulose. Amine-coated particles were labeled with a red dye, and a fusion protein was attached to these particles at various number densities. The strength of adhesion of a single particle to a cellulose fiber was measured using micropipette manipulation as a function of the specificity of the protein and its surface density and contact time. The frequency and force of adhesion were seen to increase with contact time in fiber experiments. The dynamics of adhesion of the functionalized particles to cellulose-coated glass slides under controlled hydrodynamic flow was explored using a flow chamber for two scenarios: detachment of bound particles and attachment of particles in suspension as a function of the shear rate and surface density of protein. Highly specific adhesion was observed. The critical shear rate for particle detachment was an increasing function of cellulose binding domain (CBD) density on particle surface. A rapid irreversible attachment of particles to cellulose was observed under flow. Using a family of proteins that were divalent for binding either the red dye or cellulose, we found that particle detachment occurred because of the failure of the cellulose-CBD bond. A comparison of fiber binding and particle detachment results suggests that forces of adhesion of particles to cellulose of up to 2 nN can be obtained with this molecular system through multiple interactions. This study, along with the adhesion simulations currently under development, forms the basis of particulate design for specific adhesion applications.     

Macro-cyclic aramids (2n-AZA[2n]paracyclophane-2n-ones):new intermediates for the synthesis of p-aramids

An easy route to a new class of n-azapara[2ⁿ] cyclophane-n-ones, N-substituted p-oriented cyclic aromatic amide (cycloaramid) oligomers, is described. The bulky diamine, N, N′-di-sec-butyl-p-phenylenediamine is condensed with terephthaloyl dichloride at elevated temperature in o-dichlorobenzene to provide a high yield of N, N′-di-sec-butyl-p-phenyleneterephthalamide cyclic oligomers (from dimer to tridecamer). The cyclization is favored by the cis conformation of the N-substituted amide bonds present in the growing chain. Ring opening polymerization of these new cycloaramids to high molecular weight linear polymers can be effected in the melt phase with highly nucleophilic catalysts, such as 1,3-dialkylimidazole-2-thiones especially when an acid cocatalyst is employed. The N-substituted polymers yield crystalline films and fibers with an axial repeat length which can only be explained by an unexpected trans conformation of the amide bonds. The facile synthesis of other macrocyclic amides by the steric control of macrocyclization by N-substitution of the amide bond is also described.     

The Effect of Voltage Bias on the EBIC Contrast Present at Varistor Grain Boundaries

Conductive mode scanning electron microscope (SEM) studies of electrically active grain boundaries in a bismuth + antimony doped zinc oxide based varistor have been carried out using the grain boundary—electron beam induced current (GB-EBIC) configuration. EBIC contrast consistent with negatively charged grain boundary planes flanked by compensating positively charged space-charge regions was found at all the grain boundaries investigated. Grain boundaries showing both type I (symmetric) and type II (asymmetric) EBIC contrast were identified and the effect of an applied voltage bias on the EBIC contrast was studied. It was found that, by applying a small voltage bias of around ±30 mV to a grain boundary showing type I contrast, the EBIC signal on either side of the grain boundary could be suppressed.Equally, when a small voltage bias of appropriate polarity was applied to a grain boundary showing type II contrast, an electrically symmetrical grain boundary barrier structure was restored indicating that these interfaces can show electrical activity in the space-charge regions on both sides of the grain boundary plane and demonstrating a common origin for types I and II contrast.     

Literatur

Ohne Zusammenfassung     

Hexuronic Acid Stereochemistry Determination in Chondroitin Sulfate Glycosaminoglycan Oligosaccharides by Electron Detachment Dissociation

Electron detachment dissociation (EDD) has previously provided stereo-specific product ions that allow for the assignment of the acidic C-5stereochemistry in heparan sulfate glycosaminoglycans (GAGs), but application of the same methodology to an epimer pair in the chondroitin sulfate glycoform class does not provide the same result. A series of experiments have been conducted in which glycosaminoglycan precursor ions are independently activated by electron detachment dissociation (EDD), electron induced dissociation (EID), and negative electron transfer dissociation (NETD) to assign the stereochemistry in chondroitin sulfate (CS) epimers and investigate the mechanisms for product ion formation during EDD in CS glycoforms. This approach allows for the assignment of electronic excitation products formed by EID and detachment products to radical pathways in NETD, both of which occur simultaneously during EDD. The uronic acid stereochemistry in electron detachment spectra produces intensity differences when assigned glycosidic and cross-ring cleavages are compared. The variations in the intensities of the doubly deprotonated (0,2)X(3) and Y(3) ions have been shown to be indicative of CS-A/DS composition during the CID of binary mixtures. These ions can provide insight into the uronic acid composition of binary mixtures in EDD, but the relative abundances, although reproducible, are low compared with those in a CID spectrum acquired on an ion trap. The application of principal component analysis (PCA) presents a multivariate approach to determining the uronic acid stereochemistry spectra of these GAGs by taking advantage of the reproducible peak distributions produced by electron detachment.     

VUV photoionization of acetamide studied by electron/ion coincidence spectroscopy in the 8-24 eV photon energy range

A VUV photoionization study of acetamide was carried out over the 8-24 eV photon energy range using synchrotron radiation and photoelectron/photoion coincidence (PEPICO) spectroscopy. Threshold photoelectron photoion coincidence (TPEPICO) measurements were also made. Photoion yield curves and branching ratios were measured for the parent ion and six fragment ions. The adiabatic ionization energy of acetamide was determined as I.E. (1²A′) = (9.71 ± 0.02) eV, in agreement with an earlier reported photoionization mass spectrometry (PIMS) value. The adiabatic energy of the first excited state of the ion, 1²A″, was determined to be ≈10.1 eV. Assignments of the fragment ions and the pathways of their formation by dissociative photoionization were made. The neutral species lost in the principal dissociative photoionization processes are CH₃, NH₂, NH₃, CO, HCCO and NH₂CO. Heats of formation are derived for all ions detected and are compared with literature values. Some astrophysical implications of these results are discussed.