Solvent Influence on Cellulose 1,4‐β‐Glycosidic Bond Cleavage: A Molecular Dynamics and Metadynamics Study

We explore the influence of two solvents, namely water and the ionic liquid 1‐ethyl‐3‐methylimidazolium acetate (EmimAc), on the conformations of two cellulose models (cellobiose and a chain of 40 glucose units) and the solvent impact on glycosidic bond cleavage by acid hydrolysis by using molecular dynamics and metadynamics simulations. We investigate the rotation around the glycosidic bond and ring puckering, as well as the anomeric effect and hydrogen bonds, in order to gauge the effect on the hydrolysis mechanism. We find that EmimAc eases hydrolysis through stronger solvent–cellulose interactions, which break structural and electronic barriers to hydrolysis. Our results indicate that hydrolysis in cellulose chains should start from the ends and not in the centre of the chain, which is less accessible to solvent.     

Temperature dependence of hyperfine fields in intermediate valent140CeLa and140CeLu

The hyperfine fields for the systems¹⁴⁰CeLa and¹⁴⁰CeLu have been determined by time differential perturbed angular correlations (TDPAC) in the temperature range 4.2 K≤T≤330 K. Magnitude and temperature dependence of the paramagnetic enhancement factor β indicate that cerium in either system is of intermediate valence. This behaviour is discussed in terms of a theory proposed by Ramakrishnan et al. [1–3].     

Image Formation in Low Vacuum SEM

The role of positive gaseous ions in the formation of secondary electron images in low vacuum scanning electron microscopes is discussed. This paper describes the charging processes and related effects that occur during high vacuum imaging of insulators and then discusses the influence of ions on those processes. The ions are responsible for a number of phenomena, including distortion of the electric field above and below the specimen surface due to space charge, removal of excess negative charge from the specimen, alteration of the specimen surface barrier, and scavenging/filtering of the secondary electron emission. The resulting electron-specimen-ion interactions can give rise to interesting contrast effects that are unique to this class of instruments.     

Monitoring genomic alterations with a panel of oligonucleotide probes specific for various simple repeat motifs

Germline and somatic instability of the human genome was studied, using synthetic oligonucleotides specific for simple repeat motifs. The following probes were used: (GTG)5, (GACA)4, (GATA)4, (CT)8, (TTAGGG)3, (GT)8, (GAA)6 and (GGAT)4. Each of them is unique with respect to the target regions recognized in the genome. Thus compilation of the various fingerprint data provides a complex map of the genome (and its deviations). While the fingerprints of differentiated somatic tissues never showed any alterations, in tumor tissues (namely gliomas) many changes could be detected. Most of the latter reflect secondary karyological aberrations. In nearly one third of the gliomas, drastically amplified and apparently monomorphic DNA fragments were identified. This marker should make it possible to deal with causal pathogenetic mechanisms as well as novel diagnostic strategies.     

Synchronization Analysis of Coupled Noncoherent Oscillators

We present two different approaches to detect and quantify phase synchronization in the case of coupled non-phase coherent oscillators. The first one is based on the general idea of curvature of an arbitrary curve. The second one is based on recurrences of the trajectory in phase space. We illustrate both methods in the paradigmatic example of the R?ssler system in the funnel regime. We show that the second method is applicable even in the case of noisy data. Furthermore, we extend the second approach to the application of chains of coupled systems, which allows us to detect easily clusters of synchronized oscillators. In order to illustrate the applicability of this approach, we show the results of the algorithm applied to experimental data from a population of 64 electrochemical oscillators.     

High-efficiency erbium ion luminescence in silicon nanocrystal systems

The photoluminescence spectra and kinetics of both erbium-doped and undoped multilayer structures of quasi-ordered silicon nanocrystals in a silicon dioxide matrix were studied. It was shown that the optical excitation energy of silicon nanocrystals 2–3 nm in size can be practically completely transferred to Er³⁺ ions in the oxide surrounding the nanocrystals, with its subsequent radiation at 1.5 μm. Possible reasons for the high excitation efficiency of the Er³⁺ ions are discussed, and the conclusion is drawn that the F?rster mechanism is dominant in the energy transfer processes occurring in these structures. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 1, 2004, pp. 105–109. Original Russian Text Copyright ? 2004 by Kashkarov, Kamenev, Lisachenko, Shalygina, Timoshenko, Schmidt, Heitmann, Zacharias.     

Laser spectroscopy of desorbing molecules

In this article the application of tunable dye lasers to desorption phenomena is illuminated. These lasers provide radiation continuously tunable from 105 nm in the vacuum ultraviolet to about 10 m in the mid-IR. By employing either laser induced fluorescence (LIF) or resonance enhanced multiphoton ionization (REMPI) spectroscopy almost all diatomic and many polyatomic molecules can be probed with the sensitivity required to detect desorbing molecules under UHV conditions. The spectral resolution of the lasers is sufficiently high that rotational state selectivity is achieved. Recent developments permit in addition the velocity distributions of molecules to be determined with internal quantum state resolution. Therefore very detailed information about the molecular dynamics has been obtained. In most experiments so far reactive recombinations off surfaces have been investigated. In this paper special emphasis will be given to the recombination of hydrogen on copper and palladium surfaces. For these systems very detailed data about the internal state populations at various surface temperatures have been obtained. The rotational cooling previously observed in molecular beam scattering has also been established for desorption. Strong vibrational excitation has been observed, which in the case of desorption from copper may be associated with the recombination dynamics, whereas for desorption of D₂ from Pd(100) a molecular precursor state might be responsible. By measuring the velocity distribution in each quantum state, the complete energetics of the desorbing molecules has been determined. Some first experiments on laser induced desorption with state selective detection of the desorbing molecules will also be discussed. Finally, making use of the polarization analysis of the signal, alignment effects in the desorption can be observed, permitting observation of molecular dynamics with a magnifying glass.Heisenberg fellow of the Deutsche Forschungsgemeinschaft     

XPS investigations of ink‐jet printed paper

Different ink‐jet printed paper materials were investigated using X‐ray photoelectron spectroscopy (XPS) yielding the elemental composition of the near‐surface region of the papers. We found significant differences with respect to the detected elements and their atomic concentrations in the different inks studied here. Two different groups of inks could be identified by means of a lower ratio of the O and C atomic concentrations and lower concentrations in specific trace elements like Mg, Na and Si. High‐resolution spectra of C 1s and O 1s core levels allowed a detailed determination of the chemical state of the respective elements. On the basis of a detailed deconvolution of these XPS signals, significant differences between all the investigated ink‐jet printed papers were found, thereby allowing their discrimination. The applicability of the measurements and, more generally, the XPS technique for forensic investigations of paper are discussed. Copyright © 2007 John Wiley & Sons, Ltd.