Mechanisms of metastable N2 and Kr electron stimulated desorption from N2---Kr double layer and mixed films

Desorption of metastable particles from layered and mixed films, composed of N₂ and Kr, is induced by the impact of 6–50 eV monoenergetic electrons. From yield functions and time-of-flight analysis of the metastable particles emanating from these films, N*₂ and Kr* are identified as the desorbing species. Basic mechanisms responsible for their desorption are discussed. It is suggested that the desorption of Kr* arises from dissociation of transitory [Kr·N₂]* excimers. The desorption of N*₂ can arise from cavity expulsion, intramolecular vibrational energy transfer (with or without prior electronic excitation energy transfer from Kr excitions to N₂) and the dissociation of [Kr·N₂]* excimers.     

Replacement of Water Molecules in a Phosphate Binding Site by Furanoside‐Appended lin‐Benzoguanine Ligands of tRNA‐Guanine Transglycosylase (TGT)

The enzyme tRNA‐guanine transglycosylase has been identified as a drug target for the foodborne illness shigellosis. A key challenge in structure‐based design for this enzyme is the filling of the polar ribose‐34 pocket. Herein, we describe a novel series of ligands consisting of furanoside‐appended lin‐benzoguanines. They were designed to replace a conserved water cluster and differ by the functional groups at C(2) and C(3) of the furanosyl moiety being either OH or OMe. The unfavorable desolvation of Asp102 and Asp280, which are located close to the ribose‐34 pocket, had a significant impact on binding affinity. While the enzyme has tRNA as its natural substrate, X‐ray co‐crystal structures revealed that the furanosyl moieties of the ligands are not accommodated in the tRNA ribose‐34 site, but at the location of the adjacent phosphate group. A remarkable similarity of the position of the oxygen atoms in these two structures suggests furanosides as a potential phosphate isoster.     

Simple one-pot process for the bioresolution of tertiary amino ester protic ionic liquids using subtilisin

An efficient hydrolase-catalyzed bioresolution of tertiary amino ester protic ionic liquids has been demonstrated. Protic ionic liquids have been prepared in one step from the corresponding tertiary amino alcohols by treatment with butyric anhydride. After bioresolution, unreacted esters can be easily separated from the corresponding alcohols by extraction with hexane. Bioresolution of quinuclidin-3-yl butyrate has been performed with excellent selectivity.     

Semi-classical orbits in a caterpillar like Sinaï billiard

The transport properties of a caterpillar-like Sinai billiard are investigated in the ballistic regime. The experimental kinetic coefficients exhibit pronounced features, characteristic of this kind of billiard. A Monte-Carlo style simulation of the semi-classical electron motion is performed in order to model the transmission probabilities T_ij of an ideal caterpillar structure. By visualising the spatial charge density distributions, the main features in the transmission probabilities are related to special electron orbits which dominate the electron dynamics at a given B. Finally, Poincaré sections are calculated close to the commensurability condition. The appearance of stability islands surrounded by the stochastic sea characterises the presence of regular electron skipping orbits involved in the ballistic transport phenomena.     

Electron stimulated desorption of anionic fragments from films of pure and electron-irradiated thiophene

The electron stimulated desorption (ESD) of anions is used to explore the effects of electron irradiation on a thiophene film and we report measurements for electron impact on multilayer thiophene condensed on a polycrystalline platinum substrate. Below 22 eV and at low electron dose, desorbed anions include H- (the dominant signal) as well as S-, CH2-, SH- and SCH2-. Yield functions show that anions are desorbed both by dissociative electron attachment (DEA) with resonances observed at 9.5, 11, and 16 eV, and for energies >13 eV, by dipolar dissociation (DD). An increase in the S- signal from electron irradiated (beam-damaged) thiophene films and the appearance of a new DEA resonance in the S- yield function at 6 eV are linked to rupture of the thiophene ring and the formation of sulfur-terminated products within the film. The threshold energy for ring rupture is 5 eV. The desorption of new anions such as C4H3S- (Thiophene-H)- is also observed from electron irradiated films and these likely arise from the decomposition of large radiation product molecules synthesized in the film. The yield functions of H-, S-, SH-, (Thiophene-H)-, and (Thiophene+H)- anions from irradiated thiophene films that have been annealed to 300 K, each exhibit a single resonant feature centered around 5.1 eV, suggesting that all signals derive from DEA to the same molecular radiation product. In contrast, only H- and S- are observed to desorb from films of 2-2-bithiophene and no resonance is seen below approximately 10 eV in the anion yield functions. These data suggest that electron irradiation causes formation of ring-opened oligomers, and that closed-ring or 'classical" oligomers, (similar to bithiophene) if formed, contribute little to the ESD of anions.     

Strong effect of resonant impurities on Landau-level quantization

We investigate experimentally the effect of a random distribution of nitrogen (N) impurities on the Landau-level spectrum of a GaAs quantum well. Our magnetotunneling study reveals complex and nonequally spaced Landau levels and a quenching of the Landau states at a well-defined bias and electron energy which is resonant with that of the N atoms. Analysis of the magnetic field dependence of the tunnel current into the Landau levels of the well also provides quantitative information about the nonresonant component of the N-related scattering potential.     

Low-energy electrons and X-ray irradiation effects on plasma-polymerized allylamine bioactive coatings for stents

Radiation used in biomedical applications causes chemical changes to biomedical materials. This work is an ex situ simulation of the influence of low-energy electron (LEE) impact and X-ray irradiation on the chemical properties of plasma-polymerized allylamine (PPA) bioactive and biocompatible stent coatings. Preliminary X-ray photoelectron spectroscopy (XPS) results show that PPA coatings oxidize in contact with ambient air by the detection of C-O and CO bonds which are typical of polymer oxidation. Chemical changes after LEE and X-ray irradiation are mainly a loss of oxygen, assuming a surface deoxidizing and not a complete destruction of the surface. XPS survey analyses show that the amine groups remain stable during irradiation. LEE impact measurements by TOF mass spectrometry show that the main ionic losses are H⁻ ions. It appears that CN groups are stable under irradiation and we observe a loss of hydrogen and oxygen as the main chemical modifications. In conclusion, these results suggest that PPA coatings are stable under biomedical radiation, and they can therefore be used for bioactive and biocompatible stent coatings.