Menger algebras of n-place interior operations

Algebraic properties of n-place interior operations on a fixed set are described. Conditions under which a Menger algebra of rank n can be represented by n-place interior operations are found.     

A comparative DFT study of substrates and products of industrial enzyme nitrile hydratase

Nitrile hydratase (NHase) is a metalloenzyme used in industrial biotechnology for a large scale production of common chemicals. NHases convert nitriles to the corresponding amides. Although the structures of some forms of NHases containing nonheme low spin Fe(III) or low spin noncorrinoid Co(III) are known, neither a catalytic mechanism nor the reasons of high selectivity towards aromatic ligands are recognized. Optimized geometries, molecular electrostatic potential maps and infrared spectra of commercially important aromatic substrates of the NHase (nicotinonitrile, o‐, m‐, p‐methylbenzonitrile) and the corresponding products (nicotinamide, o‐, m‐, p‐methylbenzamid) were investigated using the density functional theory method with the B3LYP functional and the 6‐31G(d,p) basis set. Calculated hypothetical intrinsic reaction paths indicate that benzimidic acids may be involved as intermediates. This study elucidates differences in the electronic properties of substrates and products of NHases, provides an insight into the molecular basis of the catalytic reaction and helps to explain varying enzymatic activities of microbial NHases. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Luminescence from γ-irradiated humic acid

This study was conducted to investigate the ultraweak delayed radiochemiluminescence (RCL) spectra, kinetics and spectroscopic properties of humic acids (HAs) after γ-radiation exposure (absorbed doses of 1−10 kGy, Co-60) in model systems.

The kinetics and spectral distribution of RCL (340–650 nm) were measured using the single photon counting (SPC) method and cut-off filters.

The intensity of fluorescence (λ_ex=390, 440, 490 and 540 nm) covering the spectral range 400–580 nm was heavily dependent on the λ_ex and slightly increased with the absorbed dose of γ-radiation.

Absorption spectra (the range 240−800 nm) and color coefficients E_2.6/4 and E_4/6 of irradiated solutions indicated that post-radiative degradation/polymerization processes take place in the HA, changing their macromolecule size or properties.

Comparison of FTIR spectra and elemental analysis proved an increased O and decreased C atoms in irradiated samples. The data indicate on the radiolysis-induced degradation of native HA into fulvic-like acids with higher hydrophilicity and lower molecular size.     

Global Hopf bifurcation of differential equations with threshold type state-dependent delay

We develop global Hopf bifurcation theory of differential equations with state-dependent delay using the S¹$S^1$-equivariant degree and investigate a two-degree-of-freedom mechanical model of turning processes. For the model of turning processes we show that the extreme points of each vibration component of the non-constant periodic solutions can be embedded into a manifold with explicit algebraic expression. This observation enables us to establish analytical upper and lower bounds of the amplitudes of the periodic solutions in terms of the system parameters and to exclude certain periods. Using the achieved global bifurcation theory we reveal that if the relative frequency between the natural frequency and the turning frequency varies in a certain interval, then generically every bifurcated continuum of periodic solutions must terminate at a bifurcation point. This termination means that the underlying system with parameters in the stability region near the vertical asymptotes of the stability lobes is less subject to chatter. In the process, several sufficient conditions for the non-existence of non-constant periodic solutions are also obtained.     

Sensing abilities of materials prepared by sol–gel technology

The advantages of the sol–gel technology are undoubtedly simplicity and versatility. It enables to obtain for example oxides in the form of layers, powders, monoliths or fibers. These materials can be successfully applied for sensing purposes due to their properties such as transparency, porosity, and high surface areas. In this article, the basis of operation of mainly optical and semiconductor sensors are presented. A brief overview of various kinds of sensors is submitted. The utility of optical fibers and planar waveguides in these systems is discussed. The paper contains also some results obtained by the authors in the field of thin film-based sensors.     

Avoidable structures, I: Finite ordered sets, semilattices and lattices

We find all finite unavoidable ordered sets, finite unavoidable semilattices and finite unavoidable lattices. While working on this paper, the second and third authors were supported by US NSF grant DMS-0604065. The second author was also supported by the Grant Agency of the Czech Republic, grant #201/05/0002 and by the institutional grant MSM0021620839 financed by MSMT.     

Fluorescence resonance energy transfer in a non-conjugated system of CdSe quantum dots/zinc-phthalocyanine

Energy transfer between CdSe quantum dots (QDs) as donors and Zn phthalocyanine (Zn-Pc) molecules as acceptors was studied using steady-state photoluminescence and time-correlated single photon counting techniques. With the latter technique it is evaluated that the lifetime of Zn-Pc emission increases from 4 ns to ca. 30 ns on 460 nm excitation in the presence of the QDs. The concomitant decrease in the lifetime of the QDs emission (from 23.5 to 18.4 ns) indicates that the excitation of Zn-Pc occurs not only through reabsorption but also through fluorescence resonance energy transfer.     

Materials processing with a tightly focused femtosecond laser vortex pulse

In this Letter we present the first (to our knowledge) demonstration of material modification using tightly focused single femtosecond laser vortex pulses. Double-charge femtosecond vortices were synthesized with a polarization-singularity beam converter based on light propagation in a uniaxial anisotropic medium and then focused using moderate- and high-NA optics (viz., NA=0.45 and 0.9) to ablate fused silica and soda-lime glass. By controlling the pulse energy, we consistently machine micrometer-size ring-shaped structures with <100nm uniform groove thickness.