Biphotonic Ionization of DNA: From Model Studies to Cell

Oxidation reactions triggered by low‐intensity UV photons represent a minor contribution with respect to the overwhelming pyrimidine base dimerization in both isolated and cellular DNA. The situation is totally different when DNA is exposed to high‐intensity UVC radiation under conditions where biphotonic ionization of the four main purine and pyrimidine bases becomes predominant at the expense of singlet excitation processes. The present review article provides a critical survey of the main chemical reactions of the base radical cations thus generated by one‐electron oxidation of nucleic acids in model systems and cells. These include oxidation of the bases with the predominant formation of 8‐oxo‐7,8‐dihydroguanine as the result of preferential hole transfer to guanine bases that act as sinks in isolated and cellular DNA. In addition to hydration, other nucleophilic addition reactions involving the guanine radical cation give rise to intra‐ and interstrand cross‐links together with DNA–protein cross‐links. Information is provided on the utilization of high‐intensity UV laser pulses as molecular biology tools for studying DNA conformational features, nucleic acid–protein interactions and nucleic acid reactivity through DNA–protein cross‐links and DNA footprinting experiments.     

Quasi-Kähler Manifolds with a Pair of Norden Metrics

The basic class of the non-integrable almost complex manifolds with a pair of Norden metrics are considered. The interconnections between corresponding quantities at the transformation between the two Levi-Civita connections are given. A 4-parametric family of 4-dimensional quasi-K?hler manifolds with Norden metric is characterized with respect to the associated Levi-Civita connection.     

Substrate specificity of NovM: implications for novobiocin biosynthesis and glycorandomization

[reaction: see text] In an effort to expand the scope of natural product in vitro glycorandomization (IVG), the substrate specificity of NovM was investigated. A test of four aglycon analogues and over 40 nucleotide sugars revealed NovM has a surprisingly stringent substrate specificity and provided only three new "unnatural" natural products. On the basis of the determined substrate specificity, an alternative to the sugar nucleotide biosynthetic dogma and a cautionary note for the general applicability of IVG are introduced.     

Ligand-based virtual screening by novelty detection with self-organizing maps

We describe a novel method for ligand-based virtual screening, based on utilizing Self-Organizing Maps (SOM) as a novelty detection device. Novelty detection (or one-class classification) refers to the attempt of identifying patterns that do not belong to the space covered by a given data set. In ligand-based virtual screening, chemical structures perceived as novel lie outside the known activity space and can therefore be discarded from further investigation. In this context, the concept of "novel structure" refers to a compound, which is unlikely to share the activity of the query structures. Compounds not perceived as "novel" are suspected to share the activity of the query structures. Nowadays, various databases contain active structures but access to compounds which have been found to be inactive in a biological assay is limited. This work addresses this problem via novelty detection, which does not require proven inactive compounds. The structures are described by spatial autocorrelation functions weighted by atomic physicochemical properties. Different methods for selecting a subset of targets from a larger set are discussed. A comparison with similarity search based on Daylight fingerprints followed by data fusion is presented. The two methods complement each other to a large extent. In a retrospective screening of the WOMBAT database novelty detection with SOM gave enrichment factors between 105 and 462-an improvement over the similarity search based on Daylight fingerprints between 25% and 100%, when the 100 top ranked structures were considered. Novelty detection with SOM is applicable (1) to improve the retrieval of potentially active compounds also in concert with other virtual screening methods; (2) as a library design tool for discarding a large number of compounds, which are unlikely to possess a given biological activity; and (3) for selecting a small number of potentially active compounds from a large data set.     

Influence of ThO<Subscript>2</Subscript> on the photocatalytic activity of TiO<Subscript>2</Subscript>

Microcomposites consisting of TiO₂ (or Ce-doped TiO₂) and ThO₂ (0.5–2% of the TiO₂ mass) are produced by sol-gel synthesis of TiO₂ in presence of ThO₂. X-ray diffraction study reveals the effects of ThO₂ (compared to the ThO₂-free TiO₂, obtained by the same method) on the anatase interplanar distances, crystallites size and phase composition. The photocatalytic tests in presence of the composites under UV irradiation reveal an increase of the Malachite Green degradation rate constant. The effect depends on the Th relative content, temperature of annealing of the catalyst and addition of other doping agent. The highest photocatalytic activity is observed for TiO₂ obtained at 550°C and containing 1% ThO₂. The composite exhibits activity in dark, also. The presence of Ce⁴⁺ ions is not an obligatory requirement for the realization of the ThO₂ effect. The reported results suggest that the radioactivity of the Th and/or its decay products is one of the main factors responsible for the increased photocatalytic activity of TiO₂.      

Conformal Riemannian P-manifolds with connections whose curvature tensors are Riemannian P-tensors

The largest class of Riemannian almost product manifolds, which is closed with respect to the group of the conformal transformations of the Riemannian metric, is the class of the conformal Riemannian P-manifolds. This class is an analogue of the class of the conformal Kähler manifolds in almost Hermitian geometry. The main aim of this work is to obtain properties of manifolds of this class with connections, whose curvature tensors have similar properties as the Kähler tensors in Hermitian geometry.     

Turbid media optical properties derived from the characteristics of propagating laser radiation beams

The possibility is studied to develop a straightforward analytical approach to the determination of the optical properties of liquid turbid media having forward-peaked scattering indicatrices. The approach is based on investigating the in-depth behavior of the radius and the axial intensity of a laser radiation beam propagating through the turbid medium. Based on the small-angle approximation, the detected forward-propagating light power spatial distribution, at relatively small or large optical depths along the beam axis, is obtained asymptotically in analytical form allowing one to derive relatively simple expressions of the extinction, reduced-scattering and absorption coefficients and the anisotropy factor of the medium through the characteristics of the propagating light beam. Preliminary experiments have also been performed, using Intralipid dilutions of different relatively low concentrations and measuring the cross-sectional radial distribution of the detected light power at different depths along the beam axis. The corresponding on-axis detected light power profiles have been measured independently as well. The experimental results are consistent with the analytical expressions obtained that allow one to estimate the optical coefficients and the anisotropy factor of the investigated media on the basis of the measured beam characteristics. The values obtained are near those predicted by other researchers.