Valence- and dipole-bound anions of the thymine-water complex: ab initio characterization of the potential energy surfaces

The potential energy surfaces of the neutral and anionic thymine-water complexes are investigated using high-level ab initio calculations. Both dipole-bound (DB) and valence-bound (VB) anionic forms are considered. Four minima and three first-order stationary points are located, and binding energies are computed. All minima, for both anions, are found to be vertically and adiabatically stable. The binding energies are much higher for valence-bound than for dipole-bound anions. Adiabatic electron affinities are in the 66-287 meV range for VB anions and the 4-60 meV range for DB anions, and vertical detachment energies are in the 698-977 meV and 10-70 meV range for VB and DB anions, respectively. For cases where literature data are available, the computed values are in good agreement with previous experimental and theoretical studies. It is observed that electron attachment modifies the shape of the potential energy surfaces of the systems, especially for the valence-bound anions. Moreover, for both anions the size of the energy barrier between the two lowest energy minima is strongly reduced, rendering the coexistence of different structures more probable.     

Prediction of programmed temperature retention times on linked capillary columns of different polarity

Systems formed by serial connection of capillary columns of different polarity were studied with methods previously used to predict the behavior of linked capillary columns under isothermal conditions and to obtain programmed temperature gas chromatography (PTGC) retention times of the individual columns starting from isothermal data. The two calculation methods were simultaneously applied in order to predict PTGC retention times of the series system starting from isothermal data obtained on the two individual columns. Experimental retention values measured using different temperature programs on the individual columns and on the series systems were found to agree with those calculated.     

Techniques for recording reconstituted ion channels

This review describes and discusses techniques useful for monitoring the activity of protein ion channels in vitro. In the first section the biological importance and the classification of ion channels are outlined in order to justify the strong motivation for dealing with this important class of membrane proteins. The expression, reconstitution and integration of recombinant proteins into lipid bilayers are crucial steps to obtain consistent data when working with ion channels. In the second section recording techniques used in research are presented. Since this review focuses on analytical systems bearing reconstituted ion channels the industrial most important patch-clamp techniques of cells are only briefly mentioned. In section three, artificial systems developed in the last decades are described while the emerging technologies using nanostructured supports or microfluidic systems are presented in section four. Finally, the remaining challenges of membrane protein analysis and its potential applications are briefly outlined.     

Unified molecular picture of the surfaces of aqueous acid, base, and salt solutions

The molecular structure of the interfacial regions of aqueous electrolytes is poorly understood, despite its crucial importance in many biological, technological, and atmospheric processes. A long-term controversy pertains between the standard picture of an ion-free surface layer and the strongly ion specific behavior indicating in many cases significant propensities of simple inorganic ions for the interface. Here, we present a unified and consistent view of the structure of the air/solution interface of aqueous electrolytes containing monovalent inorganic ions. Molecular dynamics calculations show that in salt solutions and bases the positively charged ions, such as alkali cations, are repelled from the interface, whereas the anions, such as halides or hydroxide, exhibit a varying surface propensity, correlated primarily with the ion polarizability and size. The behavior of acids is different due to a significant propensity of hydronium cations for the air/solution interface. Therefore, both cations and anions exhibit enhanced concentrations at the surface and, consequently, these acids (unlike bases and salts) reduce the surface tension of water. The results of the simulations are supported by surface selective nonlinear vibrational spectroscopy, which reveals among other things that the hydronium cations are present at the air/solution interface. The ion specific propensities for the air/solution interface have important implications for a whole range of heterogeneous physical and chemical processes, including atmospheric chemistry of aerosols, corrosion processes, and bubble coalescence.     

Systemic Losses Due to Counterparty Risk in a Stylized Banking System

We report a study of a stylized banking cascade model investigating systemic risk caused by counterparty failure using liabilities and assets to define banks’ balance sheet. In our stylized system, banks can be in two states: normally operating or distressed and the state of a bank changes from normally operating to distressed whenever its liabilities are larger than the banks’ assets. The banks are connected through an interbank lending network and, whenever a bank is distressed, its creditor cannot expect the loan from the distressed bank to be repaid, potentially becoming distressed themselves. We solve the problem analytically for a homogeneous system and test the robustness and generality of the results with simulations of more complex systems. We investigate the parameter space and the corresponding distribution of operating banks mapping the conditions under which the whole system is stable or unstable. This allows us to determine how financial stability of a banking system is influenced by regulatory decisions, such as leverage; we discuss the effect of central bank actions, such as quantitative easing and we determine the cost of rescuing a distressed banking system using re-capitalisation. Finally, we estimate the stability of the UK and US banking systems comparing the years 2007 and 2012 by using real data.     

Mathematics of the Neural Response

We propose a natural image representation, the neural response, motivated by the neuroscience of the visual cortex. The inner product defined by the neural response leads to a similarity measure between functions which we call the derived kernel. Based on a hierarchical architecture, we give a recursive definition of the neural response and associated derived kernel. The derived kernel can be used in a variety of application domains such as classification of images, strings of text and genomics data.     

PMMA‐based core‐shell nanoparticles with various PTFE cores

Polytetrafluoroethylene (PTFE) latices with spherical and rod‐like particles in the submicrometer size range, were employed as seeds in the emulsifier‐free methylmethacrylate (MMA) emulsion polymerization to obtain PTFE‐polymethylmethacrylate (PMMA) core‐shell nanoparticles. Stable latices were generally obtained. No residual PTFE was found at the end of the reaction. By appropriately choosing the ratio between MMA and PTFE in the reaction mixture, particles with predetermined size and monodisperse or narrow size distribution were prepared. The high structural regularity of the core‐shell samples allows the preparation of film with a periodic distribution of the cores thus ultimately leading to a well structured 2D colloidal crystal. A very peculiar crystallization behavior was observed because of the PTFE compartmentalization in the composite. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2928–2937, 2009